The role of colloidal particles on the formulation of emulsions

(1)

The role of colloidal particles on the

formulation of emulsions

Maureen Dinkgreve

University of Amsterdam

Faculteit der Natuurwetenschappen, Wiskunde en Informatica

Van der Waals-Zeeman Institute

Master:

Track:

Daily supervisor:

Group leader:

Date of completion:

Physics

Advanced Matter and Energy

Physics (AMEP)

J. F. Paredes Rojas

Prof. Dr. D. Bonn

December 19 2014

(2)

Van der Waals-Zeeman Institute Sciencepark 904

1098XH Amsterdam The Netherlands

Faculteit der Natuurwetenschappen, Wiskunde en Informatica (FNWI) Van der Waals-Zeeman Institute (WZI)

Master Coordinator: Prof. Dr. P. Schall

Keywords : O/W emulsions, gel, thixotropic emulsions, Pickering emulsions, yield stress, clay, Laponiter, rheology, Confocal Laser Scanning Microscopy.

(3)

Abstract

In the field of research on Pickering emulsions, stability is hardly considered an issue. Pickering emulsions are oil-in-water or water-in-oil emulsions with colloidal particles at the oil-water interface. Particles that stabilize emulsions have a very large stabilization energy; these are located at the interface and prevent the dispersed droplets from coalescence. As Pickering emulsions retain the basic properties of classical, surfactant stabilized, emulsions, they are particularly interesting for many applications within the cosmetic and pharmaceutical industry.

Clay particles are often used to make Pickering emulsions, and emulsions prepared with Laponiter

clay particles are reported to be very stable. In this report, the role of clay particles on the formulation of emulsions is investigated. We show that Laponiter _{stabilized emulsions are stable to coalescence}

but not to shear. The clay particles are visualized by confocal laser scanning microscopy, showing that particles are located at the interface but also aggregate in the continuous aqueous phase. These results suggest that the stabilization of the emulsions is mostly due to gel formation of the clay particles, rather than that the clay is an emulsifier (siting at the interface). This also explains the instability of the emulsions to shear that we observe. As a consequence, it is questioned whether “real” Pickering emulsions with Laponiter _exist.

In addition, thixotropic emulsions are studied, for which the stress at a given flow rate depends on the shear history of the sample. These are emulsion systems with both surfactant and Laponiter

added. It is known that clay induces a hysteresis in the flow curve (stress versus shear rate) of yield stress emulsions; however the exact mechanism causing this has not been demonstrated yet. It has been suggested that the clay induces a depletion interaction, that in turn causes the thixotropic behavior. Studying these systems, we find that these emulsions are very stable compared to emulsions with clay particles only. Using confocal laser scanning microscopy in combination with a rheometer, the flow behavior of the emulsions is visualized during shearing and resting of the sample. Clay particles are found to be located in the continuous aqueous phase. By locating the clay particles in emulsions during shearing, we come one step closer to understanding the mechanism of thixotropy, which is again due to the formation of a Laponiter _{gel in the continuous phase.}

(4)

(5)

CHAPTER

1

Introduction

Emulsions are common materials that are used in foods, cosmetics and pharmaceuticals. Normally, emulsions are stabilized by surfactants, which are amphiphilic molecules that reduce the interfacial ten-sion. As an alternative, colloidal particles can also be used to stabilize emulsions. Emulsions stabilized using colloidal particles are called Pickering emulsions, and they are known for their high stability to coalescence, therefore, play a crucial role in many industrial applications [1].

Clay particles are often used to make Pickering emulsions, it is claimed that emulsions prepared with Laponiter clay particles are very stable [2]. However, the precise role of clay particles on the formulation of emulsions is not fully understood. On the one hand, clay forms a gel when dissolved in water, and has a complex phase diagram. In fact, the phase diagram of Laponiter_{clay particles is still}

under discussion [3]. On the other hand, clay confers thixotropic properties to oil-in-water emulsions: the stress at a given flow rate depends on the shear history of the sample [4]. However the exact mech-anism causing this hysteresis is again not fully understood.

In this Chapter some background information about the formulation of emulsions is described. Important parameters like the stabilization energy, volume fraction, yield stress, and Laponiter _clay

(8)

Introduction

Figure 1.1: Geographical representation of an oil-in-water emulsion [7]. On the left, a classical oil-in-water emulsion stabilized by a surfactant, with an hydrophobic tail (red) and hydrophilic head (green). On the right, a Pickering emulsion stabilized by solid particles, for which the particles are located at the oil-water interface.

1.1 Emulsions

An emulsion is a mixture of two or more liquids that are normally not mixable, but by adding an emulsifier these mixtures can be stabilised. In classical emulsions, emulsifiers are added. Emulsifiers, also known as surface active agents or surfactants, which are amphiphilic compounds that lower the oil-water interfacial tension, consist of a polar (hydrohpilic head) and a nonpolar part (hydrophobic tail). As the hydrophobic tails of the surfactants are favored in the oil, and the hydrophilic heads tend to be in contact with the water, in this way they lower the interfacial tension. Therefore, one liquid can be dispersed (dispersed phase) in another (continuous phase). Depending on the type and concentration of the surfactant an oil-in-water (o/w) or water-in-oil (w/o) emulsion is formed. Besides using surfactants as emulsifier, solid particles can be used to stabilize emulsions. Emulsions stabilized by solid particles only are called Pickering emulsions (named after S.U. Pickering, who first described this phenomenon in 1907 [5], although the effect was first recognized by Walter Ramsden in 1903 [6]). Under the right conditions the particles are localized at the oil-water interface and form a protective layer around the droplets, Figure 1.1.

1.1.1 Stabilization energy

Emulsions are said to be stable when their properties do not change over a certain period of time. However, as emulsions are thermodynamically unstable their properties will change over time; the more slowly the properties change, the more stable the emulsion is. Thermodynamics gives information about the processes taking place during emulsification and kinetics gives information about the rate at which these processes occur. For example, coalescence of oil or water droplets will take place due to a thermodynamic driving force; the time taken by droplets to merge is related to the kinetics. From the second law of thermodynamics, the free energy of formation of emulsions ∆Gf is given by:

(9)

Figure 1.2: (a) Representation of solid particles at an oil-water interface. Showing the position of a spherical particle for a contact angle (measured through the aqueous phase), with corresponding probable positioning of particles on a curved surface. For θ < 90◦_{, a solid stabilized o/w emulsion may}

form (left) and for θ > 90◦, a solid stabilized w/o emulsion may form (right). (b) Single Laponiter

crystal, when in an aqueous solution: negatively charged on the faces and depending on the pH of the solution positively charge on the sides. Chemical formula: N a+0.7_[Si

8M g5.5Li0.3H4O24]−0.7.

where ∆A is the total surface area of the droplets in the emulsion, γ the interfacial tension between the two phases, T the temperature and ∆S the entropy of formation associated with the formation of droplets from the bulk. Emulsions are thermodynamically unstable systems because the free energy ∆Gf is greater than zero. The instability is a result of the large interfacial area of the droplets, it

is energetically favored when both phases are separated. Therefore, ∆Aγ T ∆S, and without any stabilization mechanism (emulsifier) the emulsion will break by coalescence, creaming, flocculation, Ostwald ripening, or a combination of all these processes. By adding a surfactant the interfacial energy is reduced, and a kinetic barrier against coalescence between the droplets is also present, so the system becomes kinetically stable [8].

In the case of Pickering emulsions, considering spherical particles adsorbed at the water/oil interface, a relevant parameter when considering stabilization is the contact angle that the particles have with the interface, θOW (See Figure 1.2a). For hydrophilic particles the contact angle θOW < 90◦ and

oil-in-water emulsions will be formed, conversely for hydrophobic particles, θOW < 90◦, will form water-in-oil

emulsions. The energy required to remove a particle with radius r from an oil-water interface of interfacial tension γOW is given by [9]:

∆E = πr2γOW(1 ± |cosθOW|)2 (1.2)

where θOW is the contact angle and the sign inside the brackets is negative for removal into the water

phase and positive for removal into the oil phase. When the contact angle is approximately 90◦, the energy required to stabilize the system is at its maximum [10]. For example, for polar oils in water with a interfacial tensions γOW = 15mN m−1, r = 10nm and θOW = 90◦ the stabilization energy

∆E ≈ 103_k

BT . In comparison with normal surfactant-stabilized emulsions, having a stabilization

(10)

Introduction

1.2 Laponite

r

clay particles

Clay is a fascinating mineral, present almost everywhere on the Earth. Clays are abundant, inexpensive and environment friendly. Most of natural clays are a heterogeneous mixture of minerals. However, pure clay minerals can be made synthetically, which are ideal for experimental research on colloidal systems such as gels and glass.

The synthetic clay Laponite RD (Laponiter_{from BYK additives Ltd.) used in this research consists}

of colloidal discs, with a diameter of 25 nm and height of 10 nm (figure 1.2b). The faces of the disc are negatively charged when dissolved in aqueous solutions. Depending on the pH of the solution the sides of the disks can be positively charged. Whereas this would imply an electrostatic attraction between the faces and the sides of the discs, a “house of cards” configuration of the Laponiter _{structure has been}

opposed [11]. However, a lot of discussion is going of about the network structure and phase diagram of Laponiter _[₃_{]. Already very dilute suspensions become very viscous, which makes Laponite}r _{an ideal}

thickening agent that is used in a number of applications such as cosmetics and paints.

1.3 Yield stress materials

In the study of emulsions a series of important parameters can be defined, the viscosity of the liquids, the type and concentration of emulsifier (surfactant), temperature and the size and polydispersity of the droplets. Another important parameter is the internal volume fraction (φ), which is the ratio of the volume of the dispersed phase with respect to the total volume of the emulsion. When φ reaches a critical value (φc ≈ 0.64 in the case of emulsions), the system starts to jam [12]. These emulsions

(φ > φc) behave elastically for small deformations and low stresses, and they start to flow above a

critical stress, the yield stress.

Herschel-Bulkley is the most commonly used model to describe yield stress materials [13]:

σ = σy+ K · ˙γn (1.3)

where σ is the shear stress and σythe yield stress, ˙γ is the shear rate and K and n are adjustable model

parameter. Depending on the value of n, three different regimes can be defined. When a stress above the yield stress is applied, the system starts to flow. Then if n = 1 the viscosity remains constant under all applied stresses. When n < 1 the viscosity decreases with increasing shear rate (shear-thinning) and when n > 1 the viscosity increases with shear rate (shear-thickening).

(11)

1.4 Scope of this thesis

This master thesis focuses on the study the role of colloidal particles on the formulation of emulsions. More specifically, the role of Laponiter _{clay particles within emulsions is investigated. To this aim,}

two different emulsion configurations with Laponiterare considered: Thixotropic yield stress emulsions and Pickering emulsions, described in chapter 3 and chapter 4, respectively. This thesis is divided as follows:

Chapter 2: This chapter describes the experimental techniques and the preparation protocols for the materials studied in this thesis.

Chapter 3: In this chapter the properties and behavior of yield stress emulsions are studied. The difference between normal and thixotropic yield stress emulsions and the role of Laponiter_{, causing}

thixotropic behavior, is discussed; Do clay particles induce depletion interaction or do they simply gel in the continuous phase?

Chapter 4: The role of Laponiter _{in emulsions is further investigated in this chapter, where Castor}

oil-in-water and silicone oil-in-water emulsions are stabilized by Laponiter clay particles only. The behavior and stability of Laponiter _{stabilized Pickering emulsions is studied; Is it indeed}

possible to make Pickering emulsions with Laponiter_?

Chapter 5: In addition to the previous chapters, the rescaling of flow curves of yield stress fluids is discussed. The Herschel-Bulkley model is used to describe and predict the flow behavior of several yield stress materials. Within each system the flow curves for different volume fractions are rescaled onto one master curve; What happens to the flow behavior in relation to the volume fraction, when the mechanical properties of the system change?

(12)

(13)

CHAPTER

2

Experimental techniques and materials

2.1 Experimental techniques

The experimental techniques and the corresponding measuring methods used during this study are described in this chapter. The main experiments that were performed are rheological measurements and flow visualization; the former were done using rheometers and the latter by using a confocal laser scanning microscope.

2.1.1 Rheology

Rheology is defined as the study of the deformation of flow and matter [14]. Consider a material between two plates moving in opposite direction, with height h and area A (Figure 2.1).

Figure 2.1: Simplified representation of material flow between two plates, moving in opposite direction with force F, area A, height h and distance d.

(14)

Experimental techniques and materials If a force F is applied, the upper plate moves a distance d with respect to the lower plate, the material will be deformed. Accordingly, we can define the shear stress (σ) as the tangential force F per unit area A:

σ =F

A (2.1)

In addition, the shear strain (γ) is the normalized deformation, a constant of proportionality between h and d:

γ = d

h (2.2)

Differnetiating Eq. 2.2 with respect to time gives the shear rate ( ˙γ) defined as the rate of deformation per time unit:

˙γ = dγ

dt (2.3)

In the case of elastic materials, Hooke’s law describes the stress as a function of the shear elastic modulus G0 and the strain γ:

σ = G0· γ (2.4)

In case of Newtonian fluids [15] a viscous stress arises where the stress is equal to the shear rate multiplied by a constant of proportionality, this constant is the viscosity (η):

σ = η · ˙γ (2.5)

In order to quantify the properties of fluids often flow curves are plot, where the shear stress is plotted as a function of the shear rate. One can distinguish between Newtonian and non-Newtonian fluids. In contrast to Newtonian fluids, non-Newtonian fluid show nonlinear flow behavior and can be shear-thinning or shear-thickening. Here the stress and viscosity are respectively increasing or decreasing with an increasing shear rate (Figure 2.2).

Figure 2.2: Representation of flow curves of different types of fluids: Newtonian, shear thinning and shear thickening.

(15)

2.1.2 Rheological measurements

Rheological measurements are carried out with an Anton Paar MCR 301 rheometer: a shear-stress controlled rheometer, that imposes a torque and measures the angular velocity. A cone-plate geometry was used, where the shear rate is homogeneous as the cone rotates and the plate is fixed, see Figure 2.3. For a cone-plate geometry, the shear rate and shear stress are defined as followed:

˙γ = ω

sin(α) (2.6)

σ = 3M

2πR3_sin2_(α) (2.7)

where ω is the angular frequency, α the angle of the cone, M the torque and R the radius of the cone.

Figure 2.3: Measuring geometry: (a) Cone-plate representation. (b) Anton Paar MCR 301 rheometer.

Steady state measurements

One of the basic rheological measurements is steady shear, where a shear rate (or shear stress) sweep is applied. In our experiments the shear rate is varied from 0.01 s−1 to 100s−1. The results are presented in a flow curve, where shear stress is plotted as a function of shear rate (Figure 2.2). Most experiments are done in steady state, where the duration time of the measurements is significantly larger than

˙γ · t = 1, this means a time for which the deformation γ 1. Oscillatory measurements

Measurements that consist of imposing an oscillatory shear, allow to determine the Storage (G0) and Loss (G00) moduli. G0is a measure of the storage of elastic energy, while G00describes the viscous energy per cycle of deformation [16] . Consider a sinusoidal shear strain γ with an amplitude γ0 and angular

(16)

Experimental techniques and materials amplitude change and phase difference from the strain:

σ(t) = σ0sin(ωt + δ) = γ0[G0(ω)sin(ωt) + G00(ω)cos(ωt)] (2.8)

where δ represents the phase difference between the stress and strain response, G0is the storage modulus and G00 is the loss modulus. In case of a perfectly elastic material, G00 = 0 and δ = 0, whereas for viscous materials G0 = 0 and δ = 90◦. For yield stress materials both G0 and G00 are nonzero and 0◦< δ < 90◦.

2.1.3 Confocal fluorescent microscope

To visualize the flow behavior of emulsions a Confocal Laser Scanning Microscope (CLSM) (Zeiss Pascal Live) is used . Actually, two CLMSs are used, a normal one for static sample observation and a inverted one in combination with a rheometer (Anton Paar DSR 301) for flow visualization of the emulsions during shearing (Figure 2.4).

Figure 2.4: Confocal Laser Scanning Microscope (CLSM). Zeiss Pascal Live: (a) normal setup used for static sample observation and (b) an inverted version, used in combination with a rheometer to visualize the flow behavior.

The basic principle of Confocal Microscopy is point-by-point illumination of the sample, using fluorescent molecules, and the rejection of out-of-focus light by using a spacial pinhole, that enables to reconstruct a three-dimensional image [17,18]. From this device it is possible to make flow ”movies” of the emulsions to observe the behavior during shear and to localize laponiter _{particles during coalescence, for this the}

(17)

2.2 Materials

2.2.1 Oil-in-water emulsions

The emulsions used in this research are castor oil-in-water and silicone oil-in-water emulsions. The silicone oil-in-water emulsions are index matched to visualize the flow behavior of the emulsions during shearing. We consider three types of emulsions: normal and thixotropic emulsions (discussed in chapter 3), both stabilized with Sodium Dodecyl Sulfate (SDS, from Sigma Aldrich); and Pickering emulsions (discussed in Chapter 4), stabilized with Laponite RD (Laponiter_{, from BYK additives Ltd.) and}

Sodium chloride (NaCl).

Castor oil-in-water emulsion

Castor oil-in-water emulsions are prepared in the following way:

1. Continuous phase: prepared by dissolving 2wt% Laponite RD (Laponiter_{) in 0.1M NaCl (in}

ultra-pure water, Mili-Q r), dispersing by using a high intensity ultrasonic vibarcell processor (Branson solifier 250, tip diameter 5mm), operating at 20kHz and 100W for 30 minutes.

2. Dispersed phase: castor oil (Sigma-Aldrich).

3. Emulsification: the oil is gradually added to the aqueous phase when stirring with a Silverson l5m-a emulsifier that works at 10,000 rpm for 2 minutes. During emulsification the sample is cooled in an ice bath to prevent heating of the sample.

4. Emulsions with lower φ: prepared by diluting the original emulsion (φ = 0.70) with 2wt% Laponiter _{in 0.1M NaCl.}

Silicone oil-in-water emulsion

Silicone oil-in-water transparent emulsions are prepared in the following way:

1. Continuous phase: consists of a solution of 46.2wt% ultra-pure water (Mili-Q r) and 53.8wt% glycerol (99% GC, from Sigma-Aldrich). A molar concentration of 0.1M NaCl is added to the water-glycerol solution and Laponite RD (Laponiter_{) is dissolved in the continuous phase so the}

Laponiter _{concentration is 2wt%. This phase is dyed with 1.4 · 10}−4_{M Rhodamine G6 (Acros,}

99% TLC), at this concentration the dyed is fully adsorbed by the Laponiter _{paricles and no free}

dye is left in the continuous phase.

2. Dispersed phase: silicone oil (Rhodorsil r47 V 500).

3. Emulsification: the oil is gradually added to the aqueous phase when stirring with a Silverson l5m-a emulsifier at 10,000 rpm for 2 minutes. During emulsification the sample is cooled in an ice bath.

4. Centrifugation: in order to remove the air from the samples, these are centrifuged at 2,500 rpm for 15 minutes.

(18)

Experimental techniques and materials Normal and thixotropic emulsions are prepared in a similar way, instead of adding 0.1M NaCl, 1wt% SDS is addedI_{. The aqueous phase of surfactant stabilized emulsions is stirred for at least half an hour}

instead of using ultrasonic. In the case of normal emulsions: 1wt% SDS is dissolved in water with no other components. In the case of thixotropic emulsions: 1wt% SDS and 2wt% Laponiter _{are dissolved}

in water, or in a water-glycerol solution in the case of transparent emulsions. Originally, normal and thixotropic emulsions are prepared at φoil= 0.8, and lower concentrations are diluted form this initial

emulsion with the corresponding continuous aqueous phase.

2.2.2 Different types of surfactant

Emulsions can be mobile or rigid, depending on the interactions of the interfaces [19]. Normally, surfactant-stabilized emulsions are considered to be mobile, however by adding additional compounds a rigid system can be created [20]. Two different formulations are used to create mobile and rigid interactions between droplets:

Mobile: Sodium Dodecyl Sulfate (SDS) is dissolved in ultra-pure water (Mili-Q r) at a concentration of 1wt%.

Rigid: A protein solution composed of Bovine Serum Albumin (BSA) and a cosurfactant Propylene Glycerol Alginate (PGA), both where dissolved in ultra-pure water (Mili-Q r) at a concentration of 0.4wt% for each compound.

Both mobile and rigid emulsions are prepared in a similar way as described before creating Castor oil-in-water emulsions. Notice that the mobile emulsions are similar to the normal yield stress emulsions.

2.2.3 Fluorescent dyes

To visualize the emulsions under the confocal microscope a fluorescent dye is needed. Two types of dyes where used, Nile Red, a hydrophobic dye, and Rhodamine G6, a hydrophilic dye. Nile Red (molecular formula: C20H18N2O2, excitation wavelength is 485nm and emission wavelength is 525nm) is add to the

oil and is used to visualize the oil droplets. While Rhodamine G6 (molecular formula: C28H31N2O7Cl,

excitation wavelength is 532nm and emission wavelength is 555 to 585nm with a maximum at 566nm) is dissolved in the aqueous phase and used to visualize the Laponiter _{particles. At low concentrations}

(1.4 · 10−4M) the dye is completely adsorbed onto the Laponiter_{particles and no free dye is left in the}

aqueous phase.

(19)

(20)

(21)

CHAPTER

3

Normal and thixotropic yield stress

emulsions

One of the most common examples of a yield stress emulsions is mayonnaise, which behaves at first like a solid that responds elastically to low stresses and starts to flow after a certain stress level is exceeded; this stress is called the yield stress. One can distinguish between two types of yield stress emulsions; normal and thixotropic ones. In normal yield stress emulsions the viscosity only depends on the shear rate (or shear stress), whereas in thixoptropic yield stress emulsions the viscosity depends on the shear history of the sample. Over the past 30 years several methods have been developed for measuring yield stresses [21,22,23]. One frequently used method to characterize yield stress materials is to work with two yield stresses, a static and a dynamic yield stress. A static yield stress defines the stress above which the material turns from a solid state to a liquid state, and inversely a dynamic yield stress defines the transition of a liquid state to a solid state. For normal yield stress materials these two yield stresses are the same, in contrast to thixotropic yield stress materials for which the static and dynamic yield stresses are different.

The normal yield stress emulsions in this thesis are Castor oil-in-water or Silicone oil-in-water emul-sions. These emulsions can easily be tuned, by adding clay particles, to thixotropic systems. The rheology of these emulsions has been studied before by Paredes [24]. However, it is still not clear what exact mechanism is causing this thixotropic behavior. Ragouilliaux et al. suggest that clay induces depletion interaction between the dispersed droplets [4]. This depletion enteraction leads to an effective attraction between the emulsions droplets. If this attraction is strong enough, this leads to the forma-tion of a percolated (gel like) structure of droplets, which in turn can explain the thixotropy. However, it is not excluded that the clay is just gelling in the continuous aqueous phase. As a third suggestion, the concept of Pickering emulsions is considered, where particles are located at the oil-water interface.

(22)

Normal and thixotropic yield stress emulsions In addition to the work of Paredes [24] and Ragouilliaux [4], in this chapter the typical mechanism causing thixotropy in the before mentioned emulsions is studied in more detail.

3.1 Normal yield stress emulsions

Normal oil-in-water emulsions are stabilized with 1 wt% SDS in the continuous phase. Figure 3.1 shows the flow curves at up-and-down shear rate sweep, all curves show a clear plateau and fit the Herschel-Bulkley model well (solid lines). For Silicone oil two curves are shown, one for the normal Silicone oil-in-water emulsion and one for a transparent emulsion (index matched with 53.8wt% glycerol in the aqueous phase, see Appendix). The transparent emulsion has a significant higher yield stress, perhaps due to the relatively high viscosity of glycerol compared to water. Except for the viscosity, the formulation with glycerol is considered to have the same microstructural behavior as the non-transparent emulsions during rheological measurements and observations.

Figure 3.1: Steady state flow curves of Silicone oil- and Castor oil-in-water emulsions (φoil= 0.8). The

grey lines are fits of the flow curves to the Herschel-Bulkley model: for the Castor oil-in-water emulsion 23.96 + 6.85 ˙γ0.49 _{and for the Silicone oil-in-water emulsion 12.17 + 7.66 ˙γ}0.45_{. As for the Silicone oil in}

water emulsions which contains glycerol the yield stress is significantly higher: 69.63 + 29.07 ˙γ0.43_{. On}

the right, confocal microscope image of a transparent Silicone oil-in-water emulsion. Average drop size is 5µm, droplets are not perfectly spherical because of the high volume fraction.

3.2 Thixotropic yield stress emulsions

3.2.1 Flow curves: two different yield stresses

Thixotropic yield stress emulsions are loaded emulsions; a normal yield stress emulsions where a small amount of clay (Laponiter _{RD) is added. Thixotropic emulsions are prepared in the same way as the}

(23)

Figure 3.2: Flow curves of emulsions for different Laponiter _{concentration: Castor oil in water (left)}

and Silicone oil in water (right) emulsions at up-and-down shear rate sweep. Filled symbols correspond to the increasing shear rate sweep and empty symbols correspond to the decreasing shear rate sweep. normal yield stress emulsions, but in addition a small amount of clay (Laponiter _{RD) is added to the}

aqueous phase. Again an up-and-down shear rate sweep is applied, indicating thixotropic behavior, Figure 3.2. The flow curve of the simple emulsions is completely reversible upon increasing and de-creasing shear rate. To the contrary, for the loaded emulsion a hysteresis is observed, which increases with increasing amount of clay. Therefore, these results demonstrate that the unloaded emulsions are simple yield stress materials and when adding clay the emulsions become thixotropic.

Clearly, one can distinguish two yield stresses; a static yield stress, corresponding to higher values, and a dynamic yield stress, which is significantly lower. However, determining the exact value of the dynamic and static yield stress remains an issue since these values strongly depend on the aging time and response of the system to the measuring time - the time in between two consecutive induced shear rate changes. Understanding the continuous water phase and the behavior of Laponiter with and without surfactant is of major importance to understand what happens here.

3.2.2 Attractive and repulsive forces: The role of clay particles

The presence of clay in yield stress emulsions induces thixotropic behavior. Ragouilliaux et al. suggest that clays induce the formation of links between neighboring droplets [4], which leads to flocculation of droplets. This phenomena is visualized by preparing a pure and loaded silicone oil-in-water emulsion, with a fluorescent dyed in the oil (Nile Red). Both samples are diluted with 1wt% SDS in water and visualized with a confocal microscope, Figure 3.3. The samples with Laponiter clay particles show similar rheology as Ragouilliaux et al. observed for emulsions with Bentonite clay particles. For the pure emulsion, the oil droplets are homogeneously dispersed, while for the loaded emulsions droplets aggregate. In the normal emulsions the yield stress is due to repulsion between the dispersed bubbles, however, the yield stress in thixotropic emulsions is not only due to repulsion between the droplets but may also due to attractive forces induced by the clay particles. The aggregation of droplets has therefore been interpreted as being due to the depletion interaction [24].

(24)

Normal and thixotropic yield stress emulsions

Figure 3.3: Diluted silicone oil (dyed with Nile Red) in water emulsions in (a) Normal emulsion and (b) Thixotropic emulsion (1wt% Laponiterin total emulsion). Pictures taken 30 minutes after pre-shearing at ˙γ = 100s−1 for 30 seconds.

However, how the clays induce aggregation of droplets still remains an open question. Three possible situations can be envisioned:

1. Do the clay particles form a continuous gel network around the oil droplets? 2. Do they induce aggregation of droplets via colloidal particle links?

3. Or do they induce depletion an interaction between the droplets?

To address these points, the clay particles inside the thixotropic emulsions are visualized. Laponiter particles are dyed with Rhodamine G6. At low concentrations (1.4 · 10−4M) the dye is completely adsorbed onto the Laponiter particles and no free dye is left in the aqueous phase. Figure 3.4 shows the distribution of Laponiter _{in a silicone oil-in-water emulsion. The clay particles are not located at}

the interface, therefore we can exclude aggregation of droplets via colloidal particle links [5]. Moreover, when the samples are diluted, the clay particles in the aqueous phase form clearly a network surrounding the aggregated droplets. Meanwhile, these diluted samples are observed during shearing and show that the emulsions, when dilute, break at some point, similar to a gel network. We do not observe single droplets moving around and aggregate again. If the clay is inducing a depletion interaction, the droplets should be able to move to find each other and the particles should be able to move around to induce the depletion. This is clearly not the case. Therefore, we conclude that the thixotropic behavior in the emulsions is mainly due to the formation of a Laponiter _{gel in the continuous phase.}

3.3 Type of surfactant

It is known that in emulsions with high surfactant concentration micelles are formed. When the con-centration of surfactant is high enough, not all surfactant molecule can be adsorbed at the interface and free molecules start forming micelles; aggregates of surfactant molecules. This micelle concentration causes an excess of osmotic pressure that pushes the droplets together inducing depletion interaction between the droplets. Therefore, at high surfactant concentration thixotropic behaviour is observed as the dispersed droplets flocculate [25,26,27]. It is possible that in the studied emulsions, there are some

(25)

Figure 3.4: Confocal fluorescent micorscope image of a Thixotropic emulsion: (a) Silicone oil-in-water 80%, with in the aqueous phase 1% SDS and 1% Laponiter _{dyed with Rhodamine G6. (b) Diluted}

sample with 1wt% SDS in water, with a representation of droplets is pointed out in red circles. free surfactant molecules forming micelles that can interact with the clay particles. This could influence the thixotropic behavior of the emulsions.

Also, we need to consider the interaction of Laponiter _{with different surfactants. Three types of}

surfactants are considered. First, Sodium Dodecyl Sulfate (SDS, CH3(CH2)11SO4−N a

+_{), which is an}

anionic surfactant whose polar group is negatively charged. Secondly, Cetyltrimethylammonium bro-mide (CTAB, (C16H33)N (CH3)3Br), which is an anionic surfactant whose polar group is positively

charged. Finally, Tween-80 (Polyoxyethylene(20) sorbitan monooleate), a non-ionic surfactant is con-sidered.

Figure 3.5: Structural formulas of Sodium Dodecyl Sulfate, Cetyltrimethylammonium bromide and Tween-80 as described in the text.

So far stable yield stress emulsions were prepared with ionic surfactants (one with SDS and the other with CTAB). However when adding Laponiter_{to the formulation with CTAB, no stable emulsions were}

formed (See Figure 3.6a). Since Laponiteris negatively charged and CTAB positively, it is most likely that the surfactant molecules attach to the clay particles preventing both the clay particles to gel and the surfactant molecules to go to the oil-water interface stabilizing the emulsion. However, the exact formation of the surfactant molecules to the clay particles is not clear yet. Do they form a

(26)

Normal and thixotropic yield stress emulsions monolayer; where the heads of the CTAB molecules are attached to the clay particles therefore forming a hydrophobic layer. Or do they form a double layer; where an additional layer of CTAB is attached to the tails of the first layer forming a hydrophilic layer around the clay particles.

Stable yield stress emulsions were prepared with SDS and Laponiter_{, the results and flow curves}

are shown before (figure 3.2). For the emulsions with Tween-80, we are still in the process of preparing stable emulsions and measuring the rheology. The outcome of this research is important to the further investigation of emulsions with Laponiter _{and the influence of surfactant type.}

Figure 3.6: (a) Castor oil-in-water emulsions with CTAB, on the right normal yield stress emulsions, on the left in addition of Laponiter_{. (b) 10 mM CTAB and 3.5wt% Laponite}r _{in water.}

3.4 Conclusion and discussion

The role of clay particles, causing thixotropic behavior in yield stress emulsions, is discussed and the exact mechanism behind this phenomenon is investigated. Very stable oil-in-water emulsions are formed with SDS and Laponiter_{. From confocal microscope images it is excluded that clay forms a gel-like}

structure and particles are not located at the oil-water interface. If the clay is inducing a depletion interaction, the droplets should be able to move and to find each other and the particles should be able to move around. This is clearly not the case. In conclusion, the thixotropic behavior in the emulsions is due to the formation of a Laponiter _{gel in the continuous phase.}

The next step is to consider the interaction of clays with different types of surfactants. Since SDS and Laponiter _{are both negatively charged, this most likely influences the flow behavior due to a repulsion}

between both compounds. As a comparison a non-ionic surfactant can be used to see if emulsions behave in a similar way or indeed a different behavior is observed. For, a positively charged surfactant no stable emulsions are formed in the presence of Laponiter_{. Probably the surfactant molecules attach}

to the clay particles, so Laponiter does not form a gel in the water and neither does the surfactant go onto the interface to stabilize the emulsion.

Further research on the interaction of Laponiter _{and a non-ionic surfactant (e.g. Tween-80) is}

needed to close the discussion about the mechanism behind clays inducing thixotropic behavior in yield stress emulsions.

(27)

CHAPTER

4

Pickering emulsions

Emulsions are of great importance within the cosmetic and food industry. Most frequently, emulsions are stabilized with surfactants, however, some of them can be toxic to animals, ecosystems or humans [28, 29, 30]. Instead of using surfactants, solid particles can be used to stabilize emulsions. This type of emulsions is called Pickering emulsion; what characterizes these emulsions is that they are stabilized by solid particles only [5,6,31]. An interesting property of Pickering emulsions is their high stabilization energy; particles are adsorbed at the oil-water interface and remain there forming a film, around the dispersed drops impeding coalescence. Most of the time, clay particles are used to stabilize emulsions, and two types of o/w emulsions stabilized by means of clay particles can be distinguished in the literature, namely:

1. Pickering emulsions: stabilized with Laponiter RD [2], where is assumed that the clay particles are located at the oil-water interface.

2. Thixotropic emulsions: stabilized with surfactants, where it is assumed that clay induces a deple-tion interacdeple-tion between the droplets that causes the thixotropic behavior [4].

In both cases, the concentration of clay is rather high and most likely the continuous phase gels, which potentially influences the stability and rheology of the emulsions. In the previous chapter the role of Laponiter _{clay particles in surfactant stabilized emulsions, causing thixotropic behavior, was discussed}

(thixotropic emulsions). This chapter focuses on Laponiter stabilized emulsions, their stability and properties (Pickering emulsions). Binks et al. [2] claim that they have made Laponiter _stabilized

Pickering emulsions and suggest that the Laponiter _{particles are located at the oil-water interface.}

However, they have not proven this last point. In this chapter it is shown where the Laponiter_particles

(28)

Pickering emulsions

4.1 Localization of particles

Confocal fluorescence microscopy is used to visualize both dispersions and emulsions containing Laponiter_,

dyed with Rhodamine G6. The behaviour inside transparent silicone oil-in-water emulsions, during shearing, resting and evaporation, is observed with the confocal microscope in combination with a rheometer. The samples are excited with a laser of 532 nm; a filter at 550nm is used to block the direct laser light, leaving only the emission from the molecules. The Laponiter _{particles are clearly visible}

and oil droplets can be located because of the surrounding clay particles.

Laponiter_{-stabilised silicone oil-in-water emulsions are visualized with a confocal microscope, Figure}

4.1 shows the distribution of Laponiter_{particles in these emulsions. Clearly, clay particles are located}

at the interface, forming a layer around the oil droplets. However, they also appear to form a gel in the aqueous phase; as aggregates of clay particles are formed in between the oil droplets in the continuous aqueous phase.

Figure 4.1: Pickering emulsion: Silicone oil in water with in the aqueous phase 2wt% laponiter_{in 0,1M}

NaCl (laponiter _{dyed with Rhodamine g6). On the right, normal and transparent Silicone oil in water}

emulsion (φ = 0.7), in the transparent emulsion laponiter _{particles are dyed with Rhodamine (pink}

collar).

4.2 Phase diagram

In order to stabilize emulsions with Laponiter _{particles, a small amount of NaCl needs to be added}

[2]. As for the aqueous continuous phase, different concentrations of Laponiter and various concentra-tions of NaCl are tested. Figure 4.2a shows the appearance of castor oil-in-water emulsions 24h after preparation, similar compared to what Binks observed for toluene-in-water. Considering Laponiter concentrations between 1wt% to 2wt%: for low concentrations of NaCl (< 10−2M), the continuous phase forms a gel [3], however, the gelation time is not fast enough to prevent coalescence. At high concentrations of NaCl (>0.1M), flocculation occurs where oil droplets aggregate withing a grey turbid

(29)

Figure 4.2: (a) Representation of Castor oil-in-water emulsions (φ = 0.5) 24h after preparation for different formulation: on top constant NaCl concentration (0.1M) with various laponiter_{concentrations,}

below constant concentration of laponiter _{RD (2wt%) at different concentrations of NaCl.}

(b) Time development of Pickering emulsion: Silicone oil (dyed with Nile Red) in water with 2wt% laponiter _{in 0.1M NaCl. The samples are put on a glass slide at t=0s, the arrow shows the sample 30}

minutes later. On top, in contact with air and on the bottom with evaporation cover.

continuous phase however fast aggregation of many droplets causes coalescence. At higher Laponiter

concentrations (>2.5wt%) the aqueous phase either gels significantly fast or flocculation occurs (at high salt concentrations).

It appears that the only stable formulation for preparing Laponiter _{stabilized emulsions is when}

2wt% of Laponiter in 0.1M NaCl is used as the continuous phase. This exact formulation is very sensi-tive since adding a bit more or less salt destabilizes the emulsion, the same applies to the concentration of Laponiter. The question is then: Why this specific formulation?

4.3 Stability to coalescence and shear

4.3.1 Stability to coalescence

Castor oil-in-water and silicone oil-in-water emulsions are prepared with an aqueous phase consisting of 2wt% Laponiter_{in 0.1M NaCl. Immediately after preparation of the silicone oil-in-water emulsions,}

independently of the oil volume fraction, a layer of oil is formed at the surface, similar to the observations for Toluene-in-water of Ashby and Binks [2]. This layer of oil is not observed in the castor oil emulsions, see figure 4.2a.

Besides the oily layer for the silicone oil-in-water emulsions, that does not grow significantly in time after preparation, both types of emulsions are stable to coalescence for at least several weeks/months when stored in a properly closed bottle. Unfortunately, the emulsions evaporate very quickly; figure 4.2b shows observations of the evaporation process in the emulsions. In contact with air, fast coalescence is observed in the emulsions. To prevent this an evaporation trap is used during rheological measurements, providing a stable environment for the emulsions.

(30)

Pickering emulsions

Figure 4.3: Castor oil in water emulsion after shearing. In a) Pickering emulsion (2wt% Laponiter _in

0.1M NaCl) and in b) a normal yield stress emulsion (1wt% SDS). (c) Closed bottles containing castor oil-in-water emulsions with 2wt% Laponiter _{in 0.1M NaCl. Before (left) and after shaking 24h later}

(right).

4.3.2 Stability to shear

Unlike the stability to coalescence, when undergoing rheological measurements the emulsions are de-stroyed quite easily. Figure 4.4 shows an up-and-down shear rate sweep performed repeatedly (three times consecutively). The Pickering emulsions show a rapid decrease in stress when performing multiple shear rate sweeps; contrary to what is observed with simple yield stress emulsions, for which the flow curve is perfectly reproducible within short timescales (Figure 3.1). Therefore, the Laponiterstabilized emulsions are unstable to shear as they are destroyed during a simple rheological test.

Figure 4.4: (on the left) Flow curves of Castor oil in water (2wt% Laponiter _{in 0.1M NaCl). Directly}

after samples are prepared the stress is measured by applying increasing (filled symbols) and decreasing (empty symbols) shear rate. Three increasing/decreasing sweeps are done, in the following order (from top to bottom in graph) black, blue and green. (on the right) Storage and Loss modulus, just before and after the shear rate sweep measurements. Experiments carried out at f=1Hz for increasing the strain.

Considering the definition of Pickering emulsions, particles should be located at the interface forming a dense film that protects the droplets from coalescence with each other, therefore, these emulsion are

(31)

very stable. One would expect a similar stability for the Laponiter _{stabilized emulsions. In the case}

of perfect spherical particles the stabilization energy (Eq. 1.2) is of the order of a few hundred kT. However, since Laponiter particles are not spherical but disk-like this formula does not fit perfectly. Moreover, Laponiter_{forms a gel in water solutions and the addition of electrolyte changes the properties}

significantly. As a consequence, the stability of the Laponiter stabilized emulsions mostly depends on the ‘gelling’ of the continuous phase. The experiment to do in fact, is to see whether there is a yield stress also for oil volume fractions φ < 64%.

In addition, Laponiter _{stabilized emulsions with lower salt concentration and lower oil volume}

fraction have been prepared before by Garcia et al. [32]. However it can be discussed if these emulsions are “real” Pickering emulsions. Despite the fact that all ingredients for making Pickering emulsions are present, this is just a dispersion of oil droplets in a Laponiter _gel.

4.4 Role of electrolyte (NaCl)

When Laponiter _{dissolves in water one can distinguish two difference phases: at low concentrations of}

Laponiter_{the system behaves like a gel, where the Laponite}r_{particles tend to aggregate; while at high}

concentrations of Laponiter _{the system reaches a glass state, where the particles are homogeneously}

dispersed. However, a third phase comes forward for Laponiter _{in water dispersions with the addition}

of salt. For very low concentrations of NaCl (<3mM) the system remains unchanged, while for higher concentrations of NaCl the system behaves like an “attractive glass” according to Jabbari et al.[3]. They present a Laponiter _{phase diagram, where they distinguish three different phases (figure 4.5): a Gel}

(0.0-1.0wt% Laponiter_{, 0-3 mM NaCl); a Glass (>2.0wt% laponite}r_{, <5mM NaCl); and an Attractive}

glass (>0.5wt% laponiter_{, >5mM NaCl).}

Figure 4.5: Phase diagram for different concentrations of Laponiter_{and NaCl in water. Three different}

(32)

Pickering emulsions

Figure 4.6: Confocal microscope image of A) 2wt% Laponiter _{in 0.1M NaCl “attractive glass” and B)}

4wt% Laponiter _{in water “glass”. In both images Laponite}r _{is dyed with Rhodamine G6. On the}

right corresponding yield stress samples without dye.

4.5 Conclusion and discussion

Laponiter _{stabilized emulsions are prepared by adding of 0.1M NaCl. These emulsions are stable to}

coalescence for several weeks; however, they are unstable to shear. Mysteriously, this exact amount of salt is needed to stabilize the emulsions, otherwise no stable emulsions were formed. Salt increases the aggregation of Laponiter particles in the continuous phase. As a consequence, an attractive glass is formed. Clay particles are located at the interface but they mostly locate in the continuous aqueous phase. Therefore we conclude that the Laponiter _{stabilized emulsions are no Pickering emulsions (by}

the original definition of S.U. Pickering [5]), but a dispersion of oil droplets in a network. It is questioned whether “real” Pickering emulsions with Laponiter _{exist. As a solution, it has been suggested that}

there are two types of Pickering emulsions [33]:

1. The most general case, solid particles are localized at the interface surrounding the droplets forming a defense layer, providing a high stabilization energy.

2. Particle-particle interaction causing a three-dimensional network of particles, developing a con-tinuous phase surrounding the droplets [33].

It remains remarkable that the point where stable emulsions are formed, is exactly above the phase transition when Laponiter _{forms an attractive gel in the continuous phase. As compared to the results}

described in chapter 3, thixotropic yield stress emulsions with Laponiter and SDS are very stable. Further research is needed focused on how the attractive forces influence the stability of the emulsions.

(33)

CHAPTER

5

Universal rescaling of flow curves of yield

stress fluids

Understanding and predicting the flow behavior of complex fluids is a subject of considerable importance [16]. Most often, a complex fluid is a dispersion of one material in a continuous phase, e.g. suspensions of particles or polymers, foams and emulsions. An interesting characteristic of these systems is that they show a transition between fluid-like and mechanically solid-like states. When the volume fraction of the dispersed phase is higher than some critical value φc, a yield stress emerges; while for low

volume fractions Newtonian flow is observed. The transition from fluid-like to mechanically solid-like behavior, and vice versa, is called the jamming transition and is currently a very popular subject in fluid mechanics [12,34,35,36]. In concentrated systems, where φ > φc, the stress σ is often successfully

described by the Herschel-Bulkley equation: σ = σy+ K ˙γn (Chapter 1.2). But what happens when the

mechanical properties of the system change? For example, emulsions are often used as a model system, with an accurately controllable volume fraction, to study the jamming transition. Paredes et al. [37] show that the mechanical behavior of the emulsion flow curves can be successfully described by the Herschel-Bulkley equation and present that all flow curves for different volume fraction can be rescaled onto one master curve.

In this chapter we show what happens if we change the mechanical properties of the model system. First, the difference between mobile and rigid emulsions is considered. Secondly, the flow curves of two other complex fluids, Carbopol and foams, are examined.

(34)

Universal rescaling of flow curves of yield stress fluids

5.1 Yield stress emulsions

Two types of yield stress emulsions are prepared, one with mobile surfactants at the interface and one system in which the surface layer is rigid. The emulsions used are Castor oil-in-water emulsions. Flow curves are obtained by performing an up-and-down shear rate sweep.

5.1.1 Mobile emulsions

First, a relatively simple and common formulation for making mobile emulsions is considered, where SDS is used to stabilize yield stress emulsions. In general most type of surfactant stabilized emulsions are considered as mobile emulsions that have no interaction between the dispersed droplets besides the pressure needed to deform droplets and overcome the yield stress. Furthermore, rigid emulsions will be discussed.

Mobile emulsions were prepared for different volume fractions and flow curves are obtained by per-forming a shear rate sweep (figure 5.1a). All curves above φc, representing the super-critical branch,

are fit with Herschel-Bulkley and from extrapolation is determined that φc ≈ 0.645. In addition, the

branch corresponding to volume fractions below φcrepresents the sub-critical branch [38] (Here we only

focus on the super-critical branch). All curves for different volume fractions are plotted onto one master curve by plotting σ/|∆φ|∆ _{versus ˙γ/|∆φ|}Γ_{, all flow data above φ}

c collapse by fitting ∆ = 2.13 ± 0.11

and Γ = 3.84 ± 0.44 (figure 5.1b) [37].

Figure 5.1: (a) Flow curves of mobile emulsions with different internal volume fractions, showing Herschel-Bulkley fittings for φ > φc, symbols represent different volume fractions of the internal (oil)

phase. (b) Master curve showing collapse of flow curves onto two branches, one for samples with φ > φc

and one for φ < φc, when plotted as σ/|∆φ|∆ versus ˙γ/|∆φ|Γ; the red lines are super-critical and

sub-critical branches representing the Herschel-Bulkley and the Cross fit of the master curve, respectively, both with β = 0.55 and K = 0.87. Black symbols correspond to samples with φ > φc and blue symbols

(35)

5.1.2 Rigid emulsions

In rigid emulsions the interaction between droplets is different than for mobile emulsions. In order to create a rigid system a co-surfactant (PGA) is added to the continuous aqueous phase, creating “rough” surfaces on the droplets and thereby increasing the resistance to deformation. As a consequence, the mechanical properties of the system change and one would expect to see a difference in the flow curves [20].

Figure 5.2: (a) Flow curves of rigid emulsions: Castor oil in water with 0.4wt% BSA and 0.4wt% PGA, for different internal volume fractions. The red lines show the Herschel-Bulkley fittings, with corresponding n = ∆/Γ = 0.53 and K = 6.6. (b) Master curve showing collapse of all flow curves onto one, when plotted as σ/|∆φ|∆_{versus ˙γ/|∆φ|}Γ_.

The obtained flow curves are shown in figure 5.2a. All curves above φc, having a yield stress and

showing shear-thinning behavior, are fitted with Herschel-Bulkley equation with n = 0.53 ± 0.02 and K = 6.6 ± 3.1. From these flow curves is determined that φ = 0.640 ± 0.05, by extrapolating the yield stresses to zero. Surprisingly, all flow curves for different volume fractions can be rescaled onto one master curve, which allows to collapse all flow data by fitting ∆ = 2.10 ± 0.07 and Γ = 3.92 ± 0.18, see figure 5.2b. These results are very similar to those found by Paredes et al. [37].

(36)

Universal rescaling of flow curves of yield stress fluids

5.2 Foams

As a third system, the flow behavior of SDS stabilized foams are investigated. Flow curves are obtained from previous research of S. Marze et al. [39], where they have studied the steady flow of three-dimensional aqueous foams. In addition to their work, all flow curves for different volume fractions are fitted with Herschel-Bulkley, figure 5.3a. The critical volume fraction is determined by linear extrapolation of the yield stresses, resulting in φc = 0.68 ± 0.05. Once again all flow curves can be

rescaled and collapse onto one master curve, when plotted as σ/|∆φ|∆_{versus ˙γ/|∆φ|}Γ_{for corresponding}

∆ = 2.30 and Γ = 3.73.

Figure 5.3: (a) Flow curves for SDS foams for different liquid volume fractions. The red lines show Herschel-Bulkley fittings, with corresponding n = ∆/Γ = 0.44 and K = 3.82. The symbols represent different volume fractions. (b) Master curve showing the collapse of flow curves onto one, when plotted as σ/|∆φ|∆_{versus ˙γ/|∆φ|}Γ_.

5.3 Carbopol

Carbopol ‘gels’ were prepared in a similar way as Paredes describes in his thesis [24]. 2wt% Carbopol (Ultra U10 grade) is mixed with ultra pure water for one hour. Sodium Hydroxide (NaOH, from Sigma-Aldrich) was dissolved in water to obtain a 18wt% NaOH solution, which was used to adjust the pH of the carbopol-water mixture to approximately 7. The resulting mixture was vigorously shaken and left to rest for one day. Samples with lower concentration of carbopol were prepared by diluting the 2wt% mixture with ultra-pure water.

Flow curves are obtained by performing the same rheological setup as for the yield stress emulsions, figure 5.4a. Initially, the flow curves are fitted with the Herschel-Bulkley equation, showing a power law of n = ∆/Γ = 0.43 and corresponding K = 24.7. However, it is hard to determine the exact volume fraction, since φc strongly depends on the pH of the system. Nevertheless, the individual flow curves

can be collapsed onto a master curve by plotting σ/|∆φ|∆_{versus σ/|∆φ|}Γ_{with Γ = 1.35 and ∆ = 0.64,}

(37)

Figure 5.4: (a) Flow curves of Carbopol for different internal volume fractions. The red lines show the Herschel-Bulkley fittings, with corresponding n = ∆/Γ = 0.43 and K = 24.7. (b) Master curve showing collapse of all flow curves onto one, when plotted as σ/|∆φ|∆_{versus ˙γ/|∆φ|}Γ_.

5.4 Conclusions

In order the understand and predict the flow behavior of complex liquids, different systems are inves-tigated and rescaled with a simple power law: the Herschel-Bulkley equation. As indicated before by Paredes, the Herschel-Bulkley model nicely describes the flow behavior of yield stress systems.

We have shown that a rescaled Herschel-Bulkley equation can be used to universally describe the flow behavior for 4 different systems: mobile and rigid emulsions, foams and Carbopol. All of the individual systems are rescaled onto one single master curve by plotting σ/|∆φ|∆ _{versus ˙γ/|∆φ|}Γ_{, the}

fitting parameters are shown in table 5.1. As a remarkable result, all different systems can be rescaled onto one single power law n = 0.49 ± 0.06. In conclusion, the flow behavior of yield stress materials can be described by one universal rescaling; independently of the chemical properties of the system.

System Mobile emulsion Rigid emulsion Foam Carbopol Flow curves φc= 0.645 φc= 0.640 φc= 0.68

K = 0.87 K = 6.57 K = 3.82 K = 24.69 Master curve ∆ = 2.13 ∆ = 2.10 ∆ = 2.30

Γ = 3.84 Γ = 3.92 Γ = 3.73

n = ∆/Γ = 0.55 n = ∆/Γ = 0.53 n = ∆/Γ = 0.44 n = ∆/Γ = 0.43 Table 5.1: Compilation of scaling factors and exponents of the Herschel-Bulkley model, obtained ex-perimentally for different yield stress systems.

(38)

(39)

CHAPTER

6

Outlook

In this master thesis the role of clay particles on formulation of emulsions is studied. Two types of o/w emulsion with Laponiter _{are discussed and further investigated. However, there are still some aspects}

that can be explored in more depth.

6.1 Thixotropic yield stress emulsions

The role of clay particles, causing thixotropic behavior in yield stress emulsions, is discussed. Very stable oil in water emulsions are formed with SDS and Laponiter. From confocal microscope images it is excluded that clay forms a similar structure as in Pickering emulsions, as particles are not located at the oil-water interface. The clay particles are rather located in the continuous phase, and form a gel. We conclude that the thixotropic behavior comes from gelling of the clay in the continuous phase. However, at this point we do not know what influence the interaction between clay particles and surfactant has on the thixotropic behavior.

The next step is to check the interaction of clay with different types of surfactants. Since SDS and Laponiter_{are both negatively charged, this most likely influences the flow behavior because of repulsion}

between both compounds. As a comparison a non-ionic surfactant (e.g. Tween-80) can be used to see if emulsions behave in a similar way or indeed a different behavior occurs. This further research might help to enclose the discussion about the mechanism of clays inducing thixotropic behavior in yield stress emulsions.

Already, a positively charged surfactant has been tested, however, no stable emulsions are formed in the presence of Laponiter_{. The surfactant molecules probably attach to the clay particles, so Laponite}r

does not form a gel in the water neither does the surfactant go onto the interface to stabilizing the emulsion. However, again this has not been proven yet and more research is needed.

(40)

Outlook

6.2 Pickering emulsions

The stability of Laponiter _{stabilized emulsions in the addition of salt is discussed. The emulsions are}

stable to coalescnce for several weeks, however, unstable to shear. We have shown that the clay particles are located at the interface and they also form a gel-structure in the continuous phase. It is questioned whether there exist “real” Pickering emulsions with Laponiter_{. In order to further elaborate on this,}

more research is needed.

We propose the following two investigations:

1. silica or latex stabilized Pickering emulsions can be prepared and compared to the Laponiter

stabilized emulsions [40, 41]. Rheological measurements and flow visualization of both systems (real Pickering emulsions versus Laponiter _{emulsions) will provide insights on the difference}

between the systems. In this way, the instability to shear of the Laponiter _{emulsions can be}

studied on shorter time scales.

2. Furthermore, making emulsions with a polymer gel, or hair gel, and investigate the difference of a system with the Laponiter emulsions.

6.3 Laponite

r

phase diagram

One of the problems we faced, when investigating Laponiter _{stabilized emulsions, was the gelling}

of the continuous aqueous phase. Only when the continuous phase forms an attractive glass stable emulsions are formed [3]. Why this exact formulation stabilizes the emulsions is not fully understood yet. Now, it is assumed that because Laponiter forms a strong enough network to contain the oil droplets. However, the attractions in the continuous phase probably also play an important role in the emulsion stabilization. Therefore, surface tension measurements can be done to see how this affects the interfacial tension. Moreover, we propose to look at various concentrations of Laponiter _{in water}

with a confocal microscope. At very low concentrations of Laponiter _{(< 1wt%) it is assumed that}

Laponiter particles form aggregates (so actually does not form a gel but a glass), however, this is never visualized before. Since at low concentration the saturation time is very long (almost 1 year) samples of low concentrations of Laponiter in water should be prepared very carefully and dyed with a non-ionic dye (Rhodamine is charged and therefore can influence the interaction between clay particles). The sample need to saturate for a long time and then very carefully transferred to the confocal microscope to visualize the Laponiter _{distribution.}

(41)

CHAPTER

7

Acknowledgements

One and a half years ago, when I first started a short project in the soft matter group of the WZI, I was convinced that experimental physics was “not my thing”. At that time my project was about constructing simulations, that were to be combined with earlier performed experiments. However, when I got to know the people from the group better and heard all about their projects, my interest in experimental physics grew. After talking to professor Daniel Bonn I was convinced to do my master project in this group as well. With Jos´e as my supervisor I was most lucky to learn how to work in the lab properly and in a very efficient way. He became more than a supervisor, we became friends. And despite the fact that he got a new job outside the University, he was always prepared to help me. Daniel Bonn, being the director of the whole institute, made sure to supervise me as good as possible since Jos´e left. He always gave me inspiration to see things from a different perspective which made my interest in physics grow even more.

I also had all the people from the group to help me. I really started to feel at home and could talk to almost everyone about any issues. I would like to thank you all: Jos´e, for being a great supervisor and friend. Marius, who was the supervisor of my previous project, became also my good friend who was there when I needed help or advise. Odile, who helped me with all my surfactant problems. Nick, Dominik and Janna for all the funny coffee breaks. Chris and Benjamin for making great cappuccino sculptures. Anh who was always there when I needed a hug. And of course all my other colleagues from the group: Julie, Sareh, Sanne, Triet, Henri, Bruce, Bart, Daniel, Mehdi, Mina, Noushine, Rudolf, Peter, Dmitry, Emanuele, Georgios, Karla and Denise.

My special thanks goes to my friends and family, who were most supportive by just listening to me talking all about emulsions, although, most of the time, they had no idea what I was talking about.

Finally, I would like to thank Daniel Bonn for supervising me and not giving up on convincing me to start a PhD with him. I happily look forward to our further collaboration.

(42)

Acknowledgements

(43)

APPENDIX

A

Nederlandstalige samenvatting

Emulsies vervullen een belangrijke rol in de cosmetische en farmaceutische industrie. Ook in vele voedselproducten worden emulsies gebruikt. Bijvoorbeeld mayonaise, een olie-in-water emulsie waarbij een hoge concentratie oliedruppeltjes (de gedispergeerde fase) in het water (de continue fase) vast blijven zitten. Bij een stabiele emulsie scheiden de twee stoffen (olie en water) niet meer, dit resulteert in een troebele vloeistof. Als de volume fractie van de gedispergeerde fase boven een bepaalde kritische waarde komt ontstaat er een yield stress. Yield stress materialen zijn systemen die zich zowel als een vloeistof dan wel als een vaste stof kunnen gedragen. Deze yield stress emulsies (zoals mayonaise) vloeien in eerste instantie niet, bij kleine deformaties gedragen ze zich elastisch. Echter, als er voldoende afschuifspanning wordt uitgeoefend begint het systeem te stromen. Vergelijkbare eigenschappen zien we terug bij cement en verf.

In dit onderzoek zijn yield stress emulsies en de rol van kleideeltjes in deze emulsies bestudeerd. Welke rol spelen kleideeltjes op de formulering van emulsies? Twee olie-in-water emulsies met kleideelt-jes zijn onderzocht. Ten eerste wordt gekeken naar thixotrope emulsies, waar de stress en tijdsafhankelijk is van de afschuifsnelheid en afschuifspanning. Het is bekend dat wanneer kleideeltjes worden toegevoegd aan normale yield stress emulsies, waar de yield stress en viscositeit alleen afhangen van de afschuif-snelheid en afschuifspanning, de emulsies thixotroop worden (tijdsafhankelijk). We onderzoeken welk mechanisme deze hysterese veroorzaakt door met een confocale microscoop naar de emulsies te kijken tijdens reologiemetingen. We concluderen dat de thixotropie grotendeels veroorzaakt wordt doordat de kleideeltjes een netwerk vormen, zoals in een gel.

Verder zijn Pickering emulsies met alleen kleideeltjes onderzocht. In tegenstelling tot klassieke emulsies, die gestabiliseerd worden door middel van oppervlakte actieve stoffen, zijn Pickering emulsies gestabiliseerd door vaste deeltjes die op het druppeloppervlak zitten. Pickering emulsies zijn veel sta-bieler dan klassieke emulsies en het wordt beweerd dat emulsies met Laponiet kleideeltjes ook heel

(44)

Nederlandstalige samenvatting stabiel zijn omdat de klei om de oliedruppels heen zit en zo voorkomt dat deze coalesceren. Wij laten zien dat olie-in-water emulsies met Laponiet kleideeltjes stabiel zijn in rust. Ze blijken echter instabiel te zijn wanneer ze geroerd of geschud worden. Met een confocale microscoop laten we zien dat de kleideeltjes niet alleen op het oppervlak van de druppels zitten, maar ook in de continue fase. Daarom concluderen wij dat dit geen echte Pickering emulsies zijn maar slechts oliedruppels in een gel.

The role of colloidal particles on the formulation of emulsions