Microstructures prepared via inkjet printing and embossing techniques

(1)

Microstructures prepared via inkjet printing and embossing

techniques

Citation for published version (APA):

Perelaer, J. (2009). Microstructures prepared via inkjet printing and embossing techniques. Technische Universiteit Eindhoven. https://doi.org/10.6100/IR640389

DOI:

10.6100/IR640389

Document status and date: Published: 01/01/2009

Document Version:

Publisher’s PDF, also known as Version of Record (includes final page, issue and volume numbers)

Please check the document version of this publication:

• A submitted manuscript is the version of the article upon submission and before peer-review. There can be important differences between the submitted version and the official published version of record. People interested in the research are advised to contact the author for the final version of the publication, or visit the DOI to the publisher's website.

• The final author version and the galley proof are versions of the publication after peer review.

• The final published version features the final layout of the paper including the volume, issue and page numbers.

Link to publication

General rights

Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights. • Users may download and print one copy of any publication from the public portal for the purpose of private study or research. • You may not further distribute the material or use it for any profit-making activity or commercial gain

• You may freely distribute the URL identifying the publication in the public portal.

If the publication is distributed under the terms of Article 25fa of the Dutch Copyright Act, indicated by the “Taverne” license above, please follow below link for the End User Agreement:

www.tue.nl/taverne

Take down policy

If you believe that this document breaches copyright please contact us at: openaccess@tue.nl

(2)

Microstructures Prepared via Inkjet Printing

and Embossing Techniques

PROEFSCHRIFT

ter verkrijging van de graad van doctor aan de Technische Universiteit Eindhoven, op gezag van de Rector Magnificus, prof.dr.ir. C.J. van Duijn, voor een commissie

aangewezen door het College voor Promoties in het openbaar te verdedigen op donderdag 5 maart 2009 om 16.00 uur

door Jolke Perelaer

(3)

prof.dr. U.S. Schubert Copromotor:

dr. P.J. Smith

Microstructures Prepared via Inkjet Printing and Embossing Techniques / by Jolke Perelaer The research described in this thesis forms part of the research programme of the Dutch Polymer Institute (DPI, P.O.Box 902, 5600 MB, Eindhoven), Technology Area High Throughput

Experimentation, DPI project #546 Technische Universiteit Eindhoven, 2009

A catalogue record is available from the Eindhoven University of Technology Library Proefschrift, ISBN: 978-90-386-1529-5

Cover design by Jolke Perelaer and Felice Kroworsch

Printed at PrintPartners Ipskamp, Enschede, The Netherlands

(4)

Microstructures Prepared via Inkjet Printing

and Embossing Techniques

Kerncomissie:

Prof. Dr. U. S. Schubert (Eindhoven University of Technology) Dr. P. J. Smith (University of Freiburg)

Prof. Dr. R. R. Baumann (Chemnitz University of Technology) Prof. Dr. S. Magdassi (Hebrew University of Jerusalem) Prof. Dr. D. J. Broer (Eindhoven University of Technology) Overige commissieleden:

Prof. Dr. V. Subramanian (University of California, Berkeley) Dr. A. W. M. de Laat (Philips Applied Technologies)

(5)

(6)

“... omdat er brood ligt soms bij de Gedächtniskirche, soms op het Alexanderplein”

Klein Orkest, Over de Muur (1984)

“Met diploma's en een yuppie pak, heb je misschien meer kans, maar een b(r)oer op klompen blijft ook in balans”

(7)

1

Inkjet printing: from graphical arts to a

state-of-the art research technique

Abstract

Inkjet printing is a nascent technology that is developing from only printing text and graphics into a major topic of scientific research and R&D, where it can be used as a highly reproducible non-contact patterning technique to print at high speeds either small or large areas with high quality features. Inkjet printing is an additive technique, which requires only small amounts of functional materials, which can vary from a simple polymer solution to advanced nanoparticle dispersions. The latter form of ink has been investigated more and more during the last few years, in order to produce conductive features that require a reduced amount of processing steps. The present chapter provides a literature survey that describes the history and recent achievements in the inkjet printing field, as an introduction to the main research topic of this thesis.

(13)

1.1 A historical overview

The origin of inkjet printing goes back to the eighteenth century when Abbé Nollet published his experiments on the effect of static electricity on a stream of droplets in 1749.[1] Almost a century later, in 1833, Felix Savart discovered the basics for the technique used in modern inkjet printers: an acoustic energy can break up a laminar flow-jet into a train of droplets.[2] It was, however, only in 1858 that the first practical inkjet device was invented by William Thomson, later known as Lord Kelvin.[3] This machine was called the Siphon

recorder, shown in Figure 1.1a, and was used for automatic recordings of telegraph

messages.[4]

(a) (b)

Figure 1.1 The Siphon recorder (a) was the world’s first practical inkjet device and was invented by William

Thomson (b) in 1858. Reprinted from ref. [4].

The Belgian physicist Joseph Plateau and the English physicist Lord Rayleigh studied the break-up of liquid streams and are, therefore, seen as the founders of modern inkjet printing technology. The break-up of a liquid jet takes place because the surface energy of a liquid sphere is smaller than that of a cylinder, while having the same volume – see Figure 1.2.[5]

Figure 1.2 Break-up of a laminar flow-jet into a train of droplets, because of Rayleigh-Plateau instability.

(14)

When applying an acoustic energy, the frequency of the mechanical vibrations is approximately equal to the spontaneous formation rate. Subsequently, the drop-formation process is synchronised by the forced mechanical vibration and therefore produces ink drops of uniform mass. Lord Rayleigh published several papers on the instability and varicosity of jets,[6-9] where he calculated a characteristic wavelength λ for a fluid stream and jet orifice diameter d given by:

d 443 . 4 = λ (1.1)

The numerical value was later slightly corrected to 4.508.[10,11] Plateau derived a relationship between jet diameter and droplet size.[12] However, it took another 50 years before the first design of a continuous inkjet printer, based on Rayleigh’s findings, was filed as a patent by Rune Elmqvist.[13] He developed the first inkjet electrocardiogram printer that was marketed under the name Mingograf by Elema-Schönander in Sweden and Oscillomink by Siemens in Germany.[14]

In the beginning of the 1960s, two continuous inkjet (CIJ) systems were developed simultaneously, with a difference only in function of the electrical driving signals.[15,16]_The

first system was developed by Richard Sweet at Stanford University. He made a high frequency oscillograph, where droplets were formed at a rate of 100 kHz and controlled with respect to their direction by the electrical signal.[17-19] Later, in 1968, the A. B. Dick Company elaborated upon Sweet’s invention to produce a device that was used for character printing and named it the Videojet 9600: this was the first commercial continuous inkjet printer. In parallel at the Lund Institute of Technology in Sweden, Hertz et al. had developed a similar system where an electrical signal was used to disperse the droplets into a mist, which enables frequencies up to 500 kHz.[20,21] However, since their technique used a narrower nozzle diameter, 10 µm versus 50 µm, the chance of nozzle clogging was greater.[22]

Instead of firing droplets in a continuous method, it is also possible to produce droplets when required, hence an impulse jet, or better known as drop-on-demand (DoD). In the late 1940s, Clarence Hansell invented the DoD device, at the Radio Corporation of America (RCA).[23] Figure 1.3 shows the schematics of his invention, which was never developed into a commercial product at that time. It took until 1971 when Casio Company released the model

(15)

Figure 1.3 Schematic drawing of the first drop-on-demand piezoelectric device, patented in 1950 by Hansell.

Reprinted from ref [23].

Despite the fact that the basis of thermal inkjet (TIJ) DoD devices in the form of the

sudden stream printer had already been developed in 1965 at the Sperry Rand Company,[24]

this idea was picked up much later by the Canon company when in 1979 they filed the patent for the first thermal inkjet printhead.[25-28] Simultaneously, Hewlett-Packard independently developed a similar technology that was first filed in 1981.[29] Thermal inkjet printers are actuated by a water vapour bubble, hence their name bubble jet. The bubble is created by a thermal transducer that heats the ink above its boiling point and, thereby, causes a local expansion of the ink, resulting in droplet formation. The location of the thermal transducer can be either at the top of the reservoir – as used by HP – or at its side, which is the technique Canon uses.

At the beginning of the 1970s the piezoelectric inkjet (PIJ) DoD system was developed.[30] At the Philips laboratories in Hamburg printers operating on the DoD principle were the subject of investigation for several years.[31,32]_{In 1981 the P2131 printhead was developed for}

the Philips P2000T microcomputer, which had a Z80 microprocessor running at 2.5 MHz. Later the inkjet activities of Philips in Hamburg were continued under the spin-off company Microdrop Technologies.[33] The first piezoelectric DoD printer on the market was the serial character printer Siemens PT80 in 1977.

Four different modes for droplet generation by means of a piezoelectric device were developed in the 1970s, which are summarised in Figure 1.4, and further explained below.[34]

(16)

(d) Shear mode (c) Push mode

(b) Bend mode (a) Squeeze mode

Figure 1.4 Different piezoelectric drop-on-demand technologies. Reprinted from ref. [34].

Firstly, the squeeze method, invented by Steven Zoltan,[35] uses a hollow tube of piezoelectric material, that squeezes the ink chamber upon an applied voltage (Figure 1.4a). The squeeze method is nowadays also used in Microdrop printing devices.[36]_{Secondly, the}

bend-mode (Figure 1.4b) uses the bending of a wall of the ink chamber as method for droplet

ejection and was discovered simultaneously by Stemme[37,38]_{of the Chalmers University in}

Sweden and Kyser et al. of the Silonics company in the USA.[39,40]_{This technique is used for}

example in Tektronix and Epson printers. The third mode is the pushing method by Howkins (Figure 1.4c),[41] where a piezoelectric element pushes against an ink chamber wall to expel droplets, and is nowadays used in Trident, Brother and Epson printers. Finally, Fishbeck et al. proposed the shear-mode (Figure 1.4d),[42] where the electric field is designed to be perpendicular to the polarization of the piezo-ceramics. Typical pioneers in shear mode printheads are Xaar and Spectra.[34]

Besides the continuous and drop-on-demand inkjet technique, a third type of inkjet printing is known, which is based on the electrostatic generation of ink droplets.[43,44] The system is weakly pressurised, causing the formation of a convex meniscus of a conductive ink. An electrostatic force, which exceeds the meniscus’ surface tension, is applied between the ink hemisphere and the flat electrode by setting a voltage. Depending on the nature of the electrical potential the system can either be a continuous or drop-on-demand inkjet: the pulse duration determines whether the ejected ink is a continuous stream or a stream of droplets. As a summary of the different inkjet printing technologies, Figure 1.5 schematically represents a classification thereof.

(17)

Continuous Drop-on-Demand Inkjet technology

Undeflected Deflected Unvibrated

Binary Multiple

Acoustic Piezo Electrostatic Thermal

Top shooter Side shooter

Shear Push

Bend Squeeze

Figure 1.5 Classification of inkjet printing technologies, adapted from [45].

Although inkjet printing offers a simple and direct method of electronic controlled writing with many advantages, including high speed production, silent, non-impact and fully electronic operation, inkjet printers failed to be commercially successful in their beginning: print quality as well as reliability and costs were hard to combine in a single printing technique. Whereas CIJ provides high throughput, it also requires high costs to gain good quality. Nowadays this technique is used in lower quality and high speed graphical applications such as textile printing and labelling. On the other hand, PIJ usually provides good quality but lacks high printing velocities: although this can be compensated for by using multi nozzle systems, but this increases the production costs as well. TIJ changed the image of inkjet printing dramatically. Not only could thermal transducers be manufactured in much smaller sizes, since they require a simple resistor instead of a piezoelectric element, but also at lower costs. Therefore, thermal inkjet printers dominate the colour printing market nowadays.[45,46]

In scientific research piezoelectric DoD inkjet systems are mainly used, because of their ability to dispense a wide variety of solvents, whereas thermal DoD printers are more compatible with aqueous solutions.[47,48] Furthermore, the rapid and localised heating of the ink within TIJ induces thermal stress on the ink. Nevertheless, research has been conducted using TIJ printers, for example to form conductive patterns, either by printing the water soluble conjugated polymer poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS),[49,50] or by printing aqueous solutions of conductive multi-walled carbon nanotubes.[51] Furthermore, TIJ has been used to prepare small unilamellar vesicles with good precision.[52]

(18)

1.2 Working principle of piezoelectric printheads

Within a piezoelectric DoD printer a piezo-ceramic plate alters its thickness if an electrical field is applied between its opposite surfaces. Subsequently, a pressure wave is induced into the fluid that causes a droplet to be emitted from the nozzle.[53,54] Fluid is normally retained at the orifice due to surface tension at the fluid/air interface, hence the action of the pressure wave is to overcome this surface tension and to expel a stream of liquid from the orifice. In order to minimise internal energies in the liquid, again due to the surface tension of the liquid, droplets will be formed and droplets are ejected from the nozzle. Typical repetition rates, i.e. print frequencies, of DoD printers are in the range of 0.1 to 30 kHz.

I - Piezo move outwards

II - Negative waves travel outwards

III - Waves reflect and one reverses

IV - Waves arrive at centre when piezo contracts

V - Wave is magnified and drop ejected (a) (b) (c) (d) (e)

Figure 1.6 Schematic representation of the acoustic wave propagation through a glass capillary used in

piezoelectric drop-on-demand devices. A colour version is available on page 173.

Upon setting a voltage the piezoelectric element, which is usually made from lead zirconate titanate, expands (Figure 1.6a) and a pressure wave travels in two directions (Figure 1.6b). When it arrives at the nozzle orifice the pressure wave is conserved, since it is considered as a closed end due to its small diameter compared to the tube diameter, while inversion takes places when arriving at the reservoir, which acts as an open end (Figure 1.6c).[53]_{The moment that both pressure waves arrive at the centre of the nozzle the actuator}

(19)

enhanced compression wave a droplet can be ejected from the nozzle orifice (Figure 1.6e). The propagation and reflection of acoustic pressure waves depend on the printhead design, nozzle orifice diameter, and materials properties. Hence, the desired superposition is typically achieved via adjustments of the electrical signals driving the piezoelectric actuator.

A typical waveform is the so called trapezoidal driving waveform, shown in Figure 1.7,[55] which is composed of voltage rising time tr, dwell time td and voltage falling time tf. It is

important for successful droplet ejection to find a balance between the applied voltage over the piezoelectric element and the duration of the pulse. The latter one is defined as the sum of

tr and td. Due to this balance multiple combinations can be found with good droplet formation.

V oltag e ( V ) Pulse width (µs) tr td tf

Figure 1.7 Typical waveform with voltage and pulse width settings. Reprinted from ref. [55].

The individual settings of voltage and pulse width, however, have a large effect on the droplet. A minimum of voltage is necessary for droplet ejection. Above this threshold value the voltage has a linear influence on the droplet volume, as can be seen in Figure 1.8. The piezoelectric displacement correlates with the applied electric field; a larger voltage amplitude results in larger volume changes of the piezoelectric element in the same amount of time, hence a larger pressure wave is induced, which results in a larger ejected droplet volume due to stronger fluid accelerations.

(20)

90 100 110 120 130 40 60 80 100 120 140 Linear fit (R2_{= 0.94)} Vo lu m e ( pL ) Voltage (V)

Figure 1.8 Influence of the amplitude of the piezoelectric actuator driving signal on the ejected droplet volume.

Data for ethylene glycol, dispensed from a 70 μm nozzle, at a frequency of 100 Hz and a pulse duration of 42 μs.

The pulse width shows an oscillating and more complex behaviour on the droplet volume, as was revealed by Reis et al.[56] The frequency is the rate at which the waveform, depicted in Figure 1.7, is repeated and has a strong influence on the ejected droplet velocity.[57] The frequency affects the acoustic wave speed and thus also depends on the fluid properties, in particular its viscous properties.[58] Obviously, if the frequency is increased more pressure waves propagate through the liquid medium and may cause disturbances, which can have an influence on the droplet formation as well.

A typical droplet formation of ethylene glycol is shown as a sequence of images in Figure 1.9. First, a liquid column of ink is ejected from the nozzle, which will immediately adapt to a spherical shape due to the surface tension of the liquid. The liquid remains attached to the nozzle by a fluid filament, which can form a secondary filament, shown in the third image from the left. Disintegration of the fluid filament starts with the formation of a pinch point behind the primary droplet, followed by the formation of secondary droplets, also known as satellite droplets. Laplace pressure driven fluid drainage from the filament into the droplets finally leads to filament rupture.[59] The as-formed satellite droplet has a lower mass than the main droplet and, consequently, has a larger velocity. Therefore, the satellite can merge with the main droplet, as shown in the image in the middle of Figure 1.9. After merging of the two droplets, the total droplet oscillates and balances into a stable spherical shaped droplet, as depicted in the last image on the right-hand side of Figure 1.9.

(21)

Figure 1.9 Stroboscopic photographs of a typical droplet formation of ethylene glycol from a 70 μm nozzle. The

time interval between two images was 50 μs. The applied voltage and pulse width were 120 V and 42 μs, respectively. The scale bar is 100 µm.

1.3 Fluid dynamics

After a droplet has been successfully formed from an inkjet printer’s nozzle, its journey towards the substrate starts. The time necessary to reach the substrate lies in the order of micro- to milliseconds, depending on the distance between the nozzle and the substrate as well as the droplet velocity.

The main advantage of inkjet printing is its ability to precisely produce droplets with a consistent volume,[56,60] which enables systematic and statistical studies, since many droplets can be printed in sequence that are equally sized.[61] However, the application of inkjet printing of solutions, particularly polymer containing inks, is strongly dependent on ink formulation,[62] the choice of print head and substrate,[63,64] and the rate of solvent evaporation

versus marangoni-flow.[65,66]

A primary consideration for inkjet printing is that liquids must fit the physical and rheological requirements of fluid flow in the printhead, in particular viscosity, in order to be successfully ejected. If the ink is too viscous then a large pressure pulse is needed to generate a droplet. Whereas if the surface tension is too low the printhead will generate attendant satellites as well as the desired droplet, which reduces the resolution of the final as-printed feature. The two main parameters that were varied in this study were the voltage applied across the piezoelectric actuator and the pulse width, the time taken for the voltage to return back to its starting value.

In order to quantify and compare liquid properties, both in-flight and upon impacting the substrate, the dimensionless Reynolds and Weber numbers are important. The Reynolds number is a dimensionless ratio of the inertial forces versus the viscous stress within a droplet:

(22)

η

ρ

dv

=

Re (1.2)

where ρ is the density, d the orifice diameter, v the velocity and η the viscosity of the in-flight droplet. The Weber number is a dimensionless ratio of inertia forces versus the interfacial stress:

γ

ρ

_dv

2

We

=

(1.3)

where γ is the surface tension of the liquid. Both dimensionless numbers have a large influence on the droplet impact and spreading, as shown by Fukai et al.[67] Typically for inkjet print experiments, both the Reynolds and Weber number are relatively low and in the order of 1 – 100, since the velocity and the size of the droplets are relatively small, which usually prevents splashing of the droplets upon impacting the surface.

Fromm’s dimensionless Z-number, which is the inverse of the Ohnesorge number (Oh), has been used to analyze droplet formation[56,68] and can be written as a function of both dimensionless Re and We number:

We d Oh Z ₌ −1 ₌ ₌ Re η γ ρ (1.4)

Fromm predicted that drop formation in DoD systems was only possible for Z greater than 2 and that the droplet volume increases as the value of Z increases. However, Derby et al. refined this prediction to 1 < Z < 10, but seems to be valid only for concentrated wax or paraffin suspensions.[56] In practice, however, the lower limit is determined by the viscosity that dissipates the pressure pulse, whereas the upper limit represents the formation of satellite droplets.[57] Furthermore, systems where the Z-number is much larger than 10 are printable as long as the formed satellites merge with the main droplet. It has been demonstrated that common solvents with a Z-number up to 91 could be successfully inkjet printed.[59]}The main factor that appeared to affect printability was their vapour pressure, with unstable droplets and no droplets being produced for solvents with vapour pressures higher than approximately 13 kPa.

(23)

1.4 Aim and outline of the thesis

Inkjet printing, and in particular drop-on-demand inkjet printing, has developed from only printing text and graphics into a major research topic, which has, for example, been used for manufacturing microelectronic devices. Inkjet printing can compete with lithography techniques, since it places material on demand and in a direct way, which reduces the number of processing steps and the amount of material required. Furthermore, inkjet printing can also be combined with roll-to-roll production. Typical applications can be seen in the field of plastic electronic devices, which are prepared on flexible polymer substrates, including radio frequency identification tags or electrodes for thin-film transistor circuits.

This dissertation is divided into two sections. The first part (Chapters 1 to 4) describes a procedure using inkjet printing to fabricate microstructures onto polymer substrates. The first chapter provides an overview of the history of inkjet printing and discusses the operation of the various types of printers. In the next chapter, the ink’s behaviour in-flight, upon impact with the substrate, and upon drying on the substrate is discussed since these basics must be understood for successful inkjet printing. The third chapter describes the inkjet printing of functional materials, including silver nanoparticle dispersions, with a resolution down to 40 µm. For this purpose, slightly heated substrates in combination with relatively low surface energy substrates were utilised. Nanoparticles smaller than 10 nm have their melting temperature reduced to below 300 °C, as a result of their large surface curvature. After depositing these inks a thermal sintering step is required in order to render the particles conductive, which will be discussed in Chapter 4. Alternative methods for the conversion of nanoparticles into conductive features and the mechanisms involved are also discussed. Polymer substrates can usually not withstand high temperatures and, therefore, require a low temperature during the sintering process. Two new and selective sintering techniques are discussed: exposure to microwave radiation and argon plasma. Both techniques sinter the nanoparticles in a selective way, so that the polymer substrate is not affected. The conductivity after sintering is similar compared to samples that were sintered thermally.

The second part of this thesis (Chapters 5 and 6) describes a new technique called photo-embossing for the creation of surface relief structures. These structures can be created without any etching steps, but usually have an aspect ratio smaller than 0.05. Chapter 5 describes the

(24)

surface relief structures prepared by hot-embossing are used to further improve the printing resolution down to a few micrometers. Hot-embossing is then applied for the engineering of micro-channels in a polymer substrate and, subsequently, an inkjet printer is used for filling the micro-channels by means of capillary forces with a silver nanoparticle dispersion.

It is concluded that inkjet printing represents a highly suitable technique for the preparation of high-resolution conductive features on or into polymer substrates by tuning the polymer substrate properties as well as the inks. Moreover, new and alternative techniques can be used to improve the sintering time and circumstances. Together with a combination of different embossing techniques this could lead to more continuous overall processing of flexible electronic devices in the future.

1.5 References

1 J. A. Nollet, W. Watson, Philos. Trans. 1749, 46, 368. 2 F. Savart, Ann. Chim. Phys. 1833, 53, 337.

3 W. Thomson, UK Patent 2,147, 1867.

4 http://en.wikipedia.org/wiki/Lord_Kelvin (last accessed: 5th_{January 2009).}

5 E. F. Goedde, M. C. Yuen, J. Fluid Mech. 1970, 40, 495. 6 Lord Rayleigh, Proc. London Math. Soc. 1878, 10, 4.

7 Lord Rayleigh, Proc. London Math. Soc. 1879, 29, 71.

8 Lord Rayleigh, Nature 1891, 44, 249.

9 Lord Rayleigh, Philos. Mag. 1892, 34, 145.

10 H. C. Lee, IBM J. Res. Develop. 1974, 18, 364. 11 D. B. Bogy, Ann. Rev. Fluid. Mech. 1979, 11, 207. 12 J. A. F. Plateau. Philos. Mag. 1856, 12, 286. 13 R. Elmqvist, US Patent 2,566,443, 1951.

14 F. J. Kamphoefner, IEEE T. Electron. Dev. 1972, 19, 584. 15 M. R. Keeling, Phys. Technol. 1981, 12, 196.

16 J. M. Schneider, C. D. Hendricks, Rev. Sci. Instrum. 1964, 35, 1349. 17 R. G. Sweet, R. C. Cumming, US Patent 3,373,437, 1968.

18 R. G. Sweet, Rev. Sci. Instr. 1965, 36, 131. 19 R. G. Sweet, US Patent 3,596,275, 1971.

20 C. H. Hertz, S.-I. Simonsson, US Patent 3,416,153, 1968. 21 C. H. Hertz, S.-I. Simonsson, Med. & Biol. Eng. 1969, 7, 337. 22 J. Heinzl, C. H. Hertz, Adv. Electron. Electron Phys. 1985, 65, 91. 23 C. W. Hansell, US Patent 2,512,743, 1950.

(25)

24 M. Naiman, US Patent 3,179,042, 1965.

25 H. Kobayashi, N. Koumara, S. Ohno, US Patent 4,243,994, 1981.

26 I. Endo, Y. Sato, S. Saito, T. Nakagiri, S. Ohno, GB Patent 2,007,162, 1979. 27 I. Endo, Y. Sato, S. Saito, T. Nakagiri, S. Ohno, US Patent 4,723,129, 1988. 28 I. Endo, Y. Sato, S. Saito, T. Nakagiri, S. Ohno, US Patent 4,740,796, 1988.

29 J. L. Vaught, F. L. Cloutier, D. K. Donald, J. D. Meyer, C. A. Tacklind, H. H. Taub, US Patent 4,490,728, 1984.

30 R. D. Carnahan, S. L. Hou, IEEE Trans. Ind. Appl. 1977, IA-13, 95. 31 M. Döring, Philips Tech. Rev. 1982, 40, 192.

32 J. van Randeraat, R. E. Setterington, Piezoelectric Ceramics, Philips Application-Book, 2nd

Edition, Mullard Ltd., London, 1974.

33 http://www.microdrop.de (last accessed: 5th January 2009). 34 J. Brünahl, A. M. Grishin, Sens. Actuators A 2002, 101, 371. 35 S. I. Zoltan, US Patent 3,683,212/ 1972.

36 http://www.microdrop.de/wDeutsch/technology/microdrop.shtml?navid=28 (last accessed: 5th_{January 2009).}

37 N. G. E. Stemme, US Patent 3,747,120, 1972.

38 E. Stemme, S.-G. Larsson, IEEE Trans. Electron. Dev. 1973, 20, 14. 39 E. L. Kyser, S. B. Sears, US Patent 3,946,398, 1976.

40 E. L. Kyser, L. F. Collins, N. Herbert, J. App. Photographic Engineering 1981, 7, 73. 41 S. D. Howkins, US Patent 4,459,601, 1984.

42 K. H. Fishbeck, A. T. Wright, US Patent 4,584,590, 1986. 43 C. R. Winston, US Patent 3,060,429, 1962.

44 C. R. Winston, US Patent 3,432,844, 1969. 45 H. P. Le, J. Imag. Sci Technol. 1998, 42, 49.

46 H. Kipphan, Handbook of print media: Technologies and manufacturing processes, Springer,

2001.

47 B.-J. de Gans, P. C. Duineveld, U. S. Schubert, Adv. Mater. 2004, 16, 203. 48 P. Calvert, Chem. Mater. 2001, 13, 3299.

49 Y. Yoshioka, G. E. Jabbour, Synth. Met. 2006, 156, 779.

50 B. Ballarin, A. Fraleoni-Morgera, D. Frascaro, S. Marazzita, C. Piana, L. Setti, Synth. Met.

2004, 146, 201.

51 K. Kordás, T. Mustonen, G. Tóth, H. Jantunen, M. Lajunen, C. Soldano, S. Talapatra, S. Kar, R. Vajtai, P. M. Ajayan, Small 2006, 2, 1021.

52 S. Hauschild, U. Lipprandt, A. Rumplecker, U. Borchert, A. Rank, R. Schubert, S. Förster,

Small 2005, 1, 1177.

53 D. B. Bogy, F. E. Talke, IBM J. Res. Develop. 1984, 28, 314. 54 J. F. Dijksman, J. Fluid Mech. 1984, 139, 173.

(26)

57 K. A. M. Seerden, N. Reis, J. R. G. Evans, P. S. Grant, J. W. Halloran, B. Derby, J. Am.

Ceram. Soc. 2001, 84, 2514.

58 C. Ainsley, N. Reis, B. Derby, J. Mater. Sci. 2002, 37, 3155.

59 B.-J. de Gans, E. Kazancioglu, W. Meyer, U. S. Schubert, Macromol. Rapid Commun. 2004,

25, 292.

60 H. Dong, W. Carr, J. F. Morris, Phys. Fluids 2006, 18, 072102.

61 J. Perelaer, P. J. Smith, C. E. Hendriks, A. M. J. van den Berg, U. S. Schubert, Soft Matter

2008, 4, 1072.

62 A. M. J. van den Berg, A. W. M. de Laat, P. J. Smith, J. Perelaer, U. S. Schubert, J. Mater.

Chem. 2007, 17, 677.

63 J. F. Dijksman, P. C. Duineveld, M. J. J. Hack, A. Pierik, J. Rensen, J.-E. Rubingh, I. Schram, M. M. Vernhout, J. Mater. Chem. 2007, 17, 511.

64 B.-J. de Gans, U. S. Schubert, Macromol. Rapid Commun. 2003, 24, 659. 65 H. Hu, R. G. Larson, Langmuir 2005, 21, 3963.

66 H. Hu, R. G. Larson, J. Phys. Chem. B 2006, 110, 7090.

67 J. Fukai, Z. Zhao, D. Poulikakos, C. M. Megaridis, O. Miyatake, Phys. Fluids A 1993, 5, 2588.

(27)

(28)

2

Ink behaviour in-flight and at the substrate

Abstract

This chapter describes the behaviour of an ink droplet both in-flight as well as upon impinging the substrate. Firstly, an experimental study into the in-flight evaporation and impact of equally-sized inkjet printed droplets that consist of a systematically varied polystyrene concentration in either toluene or butyl acetate is presented. At low polymer concentrations, a linear relationship, which decreased, was observed between the dried droplet diameter and the printing height. Whereas, increased concentrations revealed an initial exponential decay in the dried droplet diameter, which stabilised at greater heights. At higher concentrations and heights, the polymer forms a skin on the surface of the inkjet printed droplet, which causes inhibition of the in-flight evaporation of the solvent.

Secondly, the spreading of inkjet printed droplets on solid dry surfaces of a polystyrene/toluene solution with a varied molar mass of the polymer has been studied. The polymer’s molar mass was varied between 1.5 and 545 kDa, which caused a variation in the viscosity from 0.6 to 1.7 mPa s. The results were compared with theoretical models for droplet spreading and were found to fit with an error between 2 and 20% with the predictions.

Finally, the size-selective segregation of silica based particle populations in drying droplets was studied. It was found that, after drying, smaller particles were located close to the periphery of a droplet, whereas larger particles settle nearer to the centre. It was observed that monodisperse micron-sized particles sediment as close as possible towards the periphery; the actual distance between the location of segregated particles and the contact line increases with increasing particle size.

Parts of this chapter have been and will be published: (1) J. Perelaer, P. J. Smith, M. M. P. Wijnen, E. van den Bosch, R. Eckardt, P. H. J. M. Ketelaars, U. S. Schubert, Macromol. Chem. Phys. 2009, in press; (2) J. Perelaer, P. J. Smith, E. van den Bosch, S. S. C. van Grootel, P. H. J. M. Ketelaars, U. S. Schubert, Macromol. Chem.

Phys. 2009, in press; (3) J. Perelaer, P. J. Smith, C. E. Hendriks, A. M. J. van den Berg, U. S. Schubert, Soft Matter 2008, 4, 1072.

(29)

2.1 Introduction

Inkjet printing has become a major topic in scientific research and has been conducted for various areas of research.[1,2] In particular, inkjet printing has matured as a technique for depositing small amounts of functional materials in a precise and selective way onto a wide range of substrates.[3] For example, inkjet printing is used to produce microelectronic devices, such as conductive antennas for radio frequency identification (RFID) tags,[4-6] organic light-emitting displays (OLED),[7-9] and field-effect transistors (OFET).[10,11] The amount of waste that is generated during processing is drastically reduced due to the selective deposition of materials, which lowers the production costs and favours the environment significantly.

An important topic in understanding the fundamentals of inkjet printing is the impact and spreading behaviour of droplets, which continues to be investigated and has been the topic of studies for over a century.[12,13] The control of droplet deposition is essential in other industrial applications besides inkjet printing, for example spray painting, spray coating and rapid spray cooling of hot surfaces. The type of impact made by an impinging droplet on a surface depends on the surface morphology as well as the liquid properties of the droplet.[14] Upon impact of a droplet on a solid surface four different events can take place: normal deposition, (partial) splashing, receding break-up, or a (partial) rebound.[15,16] The impact has been studied both theoretically, as well as experimentally.[17-21] Due to these studies various impact scenarios of a droplet on a surface are now under control. For example, the rebound of droplets[22]_{can be suppressed by adding small amounts of flexible polymers, whose}

elongation viscosity damps the droplet retraction.[23] Furthermore, it has been found that the splash during the impact of a droplet on a solid surface can be controlled by reducing the surrounding atmosphere pressure.[24,25] When the impact takes place on a liquid surface, for example water, the splashing effect can be minimised either by a hydrophobic layer around the impinging droplet[26] or by perfectly smooth spheres.[27]

The impact and spreading of droplets have been visualised by researchers using high-speed photography or flash techniques. The difference between the two techniques is that the former records a high number of frames of a single experiment, whereas the latter one is a stroboscopic method whereby a sequence of identical droplets are recorded and reconstructed by merging the frames of several identical events.[18,28] Flash photography techniques require that identical droplets, in terms of velocity, shape and volume, are generated. Inkjet printed droplets have been proven to fulfil these requirements.[29,30] However, inkjet printers have

(30)

scarcely been used in impact and spreading studies,[31,32] and relatively large droplets in the order of millimetres have mainly been used.[15,33-35]

Another important topic is the behaviour of liquid droplets on a solid substrate, which is generally well-understood for pure liquids,[36-37] but the behaviour of either a solution or a suspension on a substrate continues to stimulate debate.[38-42] An often seen phenomenon is that droplets of solutions and suspensions dry in such a fashion that the dried material forms a ring. Deegan et al. gave three conditions for ring formation:[39] the solvent meets the substrate at a non-zero contact angle, the solvent evaporates, and the contact line is pinned to its original position. A fourth condition was suggested by Hu and Larson who reported that suppression of the Marangoni flow that results from the latent heat of evaporation is also required.[38] However, Sommer and Rozlosnik have recently provided experimental evidence which they say shows that pinning of the contact line is not necessary for ring formation.[42]

The coffee drop effect is named after the commonly observed dark coloured rings of dried coffee droplets, as shown in Figure 2.1a. Deegan explained the appearance of these and other rings as being caused by a replenishing flow which originates in a drying droplet’s interior and travels towards the substrate-air-liquid interface, i.e. the contact line.[40]

(a)

(b)

Figure 2.1 Photograph of a single dried droplet (a) and a cluster of three droplets (b) of fresh morning coffee

with its typical appearance of ring formation.

Deegan explains the replenishing flow as being a consequence of the evaporation rate being higher at the contact line and the line being pinned. Therefore, as solvent evaporates from the droplet, liquid is carried towards the droplet’s boundary since the contact line is unable to retreat. The higher evaporation rate causes a non-uniform evaporative flux over the surface of the droplet, which Deegan explains is due to the fact that a random walk of an evaporating solvent molecule initiated at the centre of a droplet has a higher chance of re-absorption than the same random walk initiated from the edge, as depicted in Figure 2.2.[40]

(31)

As a consequence of the replenishing flow, liquid and any solute or suspended particles are transported towards the edge of a droplet. Furthermore, when droplets are close to each other, their evaporation behaviour is affected, as can be seen in Figure 2.1b: the outer rim of each droplet is darker coloured than the inner rim, indicating a larger amount of material settled from the liquid. A locally higher vapour pressure at the region where the three droplets meet reduces the evaporation rate and thereby the deposition of solid material, i.e. coffee in this example.

Figure 2.2 The probability of escape of an evaporating molecule is affected by its point of departure. A random

walk initiated at the centre of the drop results in the molecule being reabsorbed so that the final step is not completed (indicated by the dashed line). However, the same random walk initiated from the edge allows the evaporating molecule to escape. Reprinted from ref. [40].

The strength of the replenishing flow was demonstrated by Magdassi et al. who found that a metallic ring, which had formed at room temperature via the evaporation of an aqueous droplet containing silver nanoparticles, exhibited high electrical conductivity (up to 15% of bulk silver).[43] Similarly, colloidal systems consisting of monodisperse microspheres have been investigated for their self-assembly properties upon evaporation of the solvent,[44,45] bio-mimicry patterning,[46] and as photonic bandgap materials.[47] Furthermore, it was reported that in a drying droplet containing a bi-modal particle population of 60 nm and 200 nm sized polystyrene nanoparticles, the larger particles aggregated at the border of the ring, whereas the smaller particles were found as an inner ring nearer to the centre of the dried droplet.[48] This observation is fascinating, but counterintuitive, because it contradicts the classical laws of movement: smaller particles are easier to move than larger particles due to their smaller mass. However, these results suggest that this method could be used as a size separation technique, since particles are segregated according to their size.

(32)

This chapter discusses three fundamental parameters that affect the application of inkjet printing. Firstly, the printing height, which is the distance between the printhead and the substrate, is systematically varied to study the effect on the impression that polymer solutions leave on a substrate. Then, the effect of the molar mass of the dissolved polymer is altered and studied. Finally, suspended micron-sized particles are inkjet printed in order to study their preferred positioning towards the droplets contact line. The purpose of these studies allows an increased interpretation to be made of the results that are obtained by inkjet printing.

2.2 Influence of the printing height

2.2.1 Introduction

The impact of inkjet printed and other dispensed droplets strongly depends on their dimensionless Reynolds and Weber numbers.[15,31] Similarly, printing height also influences droplet impact and spreading. This particular factor, however, has not been investigated so far. As long as the accuracy of droplet positioning is not affected, an increase in height may be a simple way to decrease droplet radius since the droplet, which is primarily composed of a volatile solvent, spends more time in flight and, therefore, has more time to evaporate. This could lead to finer resolution features being produced by inkjet printing. Such an increase in height may even be coupled with the use of a heated substrate, which could further increase the rate of evaporation for certain systems.

Here, the observations of the in-flight evaporation and impact on solid dry substrates of uniformly-sized inkjet printed droplets with various concentrations of polystyrene is described. To reveal information of in-flight evaporation the height between printhead and underlying substrate was systematically increased. After impact on the substrate the droplets evaporate and leave a ‘coffee stained’ ring of dried polymer behind (as will be discussed in further detail in Section 2.3). The ring diameter, measured by means of optical profilometry, was investigated as function of printing height and polymer concentration. An inkjet printer was used to produce a series of equal-sized small droplets dispensed into a 10 × 10 matrix with a droplet interspacing of 1 mm. Using an inkjet printer reduced the error per droplet and significantly increased experimental reproducibility. From these 100 printed droplets, odd shaped droplets were excluded (7%) and from 10 droplets the horizontal and vertical diameters were measured and averaged.

(33)

2.2.2 Experimental

Materials

Butyl acetate was purchased from Fluka and toluene was purchased from Biosolve. All solvents were used as delivered. Polystyrene was purchased from Shell (Styron 648) and its number averaged molar mass was measured to be 90.2 kDa (Mw = 112 kDa, PDI 1.24) by gel

permeation chromatography. Prior to inkjet printing all solutions were ultra-sonicated and filtered with a 5 µm filter (Rezist 30, Whatman GmbH, Dassel, Germany) to prevent nozzle clogging. The contact angle and surface tension were measured using a Dataphysics OCA 30 (Filderstadt, Germany). Typical errors in the surface tension were smaller than 0.2 mN m-1.

Substrates

Commercially available glass substrates of 3 × 1 inch (Marienfeld, Lauda-Königshofen, Germany) were thoroughly cleaned by washing subsequently with soap, water, acetone and isopropanol.[49] After a second rinsing step with isopropanol the glass slides were dried under a flow of air.

Inkjet printing

Inkjet printing was performed using a piezoelectric Autodrop system (Microdrop Technologies, Norderstedt, Germany). This system was fitted with a 70 µm printhead, which can be moved in the z-direction. Droplets were generated using a frequency of 200 Hz. The glass samples were placed onto a heatable platen that can move in x- and y-direction. The Autodrop system has a positioning accuracy of 1 µm and a stroboscopic video camera to investigate and optimise droplet formation. Typical printing settings used a voltage between 150 and 160 V and a pulse width between 100 and 115 µs, which gave an initial in-flight droplet radius of 55 μm for the 1 wt% polystyrene/solvent solutions. All samples were printed at room temperature.

Characterisation

Gel permeation chromatography was measured on a Shimadzu system equipped with a SCK-10A system controller, a LC-SCK-10AD pump, a RID-SCK-10A refractive index detector and a PL gel 5 µm Mixed-D column at 50 °C utilising a chloroform : triethylamine : isopropanol (94:4:2) mixture as eluent at a flow rate of 1 mL min-1. The molar masses were calculated against

(34)

(µSurf, Nanofocus, Germany) using a 20× objective. The droplet profiles were measured using Scanning Probe Image Processor, SPIP software, version 4.1.

Dynamic and kinetic viscosities were measured on an Anton Paar AVMn automated micro-viscometer which is based on the approved and acknowledged rolling/falling ball principle according to DIN 53015 and ISO 12058. The system allows a variable inclination angle of the measurement capillary. Using a 0.900 mm capillary and a gold coated ball with a diameter of 0.794 mm at 20.00 °C, times were determined as the average of 4 determinations per varied angle (30°, 50° and 70°, respectively). Dynamic viscosities were directly calculated by the instruments software.

2.2.3 Results and discussion

To observe the in-flight evaporation, various concentrations of polystyrene in toluene were inkjet printed from different heights. The first experiment was performed from a nominal height of 0 mm, which means a position as close as possible to the substrate. To prevent damage to the nozzle during printing and to prevent smearing out of the printed droplets by the nozzle tip, the printhead was set to a minimal height of approximately 150 µm above the substrate at the starting height. Immediately after ejection from the nozzle, the droplet has to reach equilibrium size and velocity.[50] Therefore, the distance travelled by the droplet over periods of 25 µs was captured and measured using stroboscopic images taken during droplet formation. Figure 2.3a shows an oscillating behaviour of the velocity over time and reveals that the droplet reached equilibrium after 150 µs, with a final velocity of approximately 1 m s-1. In the first 150 microseconds, i.e. when the distance between printhead and substrate is smaller than 0.5 mm, the velocity is between 2.5 and 3.5 m s-1. When a droplet’s impact velocity is larger the spreading of the droplet is also larger due to a corresponding increase of the Re and We numbers, thus leaving a large diameter droplet on the substrate.[17]

Figure 2.3b shows the typical droplet formation for a 2 wt% PS solution in butyl acetate (left) and a 2 wt% PS solution in toluene (right) in stroboscopic images, where the time between each droplet is 100 µs. It can be seen that after approximately four droplets in time the droplet has reached a steady shape, thus equilibrium. Hence, printing should be performed at least from a height where the final equilibrium velocity has been reached. In the experiments the first data point that was recorded with the minimal printing height was, therefore, neglected.

(35)

It can be seen that a filament is formed during the start of the formation of the toluene/polystyrene droplet. This behaviour has been observed by other researchers for polymer solutions and is explained as being due to the stretching of the polymer and the non-Newtonian behaviour of the ink.[51-53]

0 100 200 300 400 500 600 700 800 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 D ro p let v e loc ity (m s -1 ) Time (µs) Butyl acetate Toluene (a) (b) 100 µm

Figure 2.3 Velocity as function of time (a) for droplet of a 2 wt% solution of polystyrene in butyl acetate and

toluene from a dispenser nozzle. Photographic images of the droplet formation (b) of 2 wt% PS in butyl acetate (left) and 2 wt% PS in toluene (right). Subsequently, seven images form a multi-layered image, each with a time interval of 100 µs.

The printing height was increased up to 25 mm above the substrate in steps of 1 mm. It should be mentioned that the positioning of the droplets into a matrix at heights above 15 mm is less accurate due to small disturbances in the atmosphere caused by the movement of the platen, whereon the substrate was placed. It was noticed that the matrix becomes less symmetric and at heights above 25 mm several droplets are overlapping. Therefore, printing from these heights is not applicable in practice. However, it was found that jetting at heights of up to 15 mm did not affect droplet placement accuracy for this series of experiments. One

(36)

suggestion to improve the symmetry of the printed matrix is to print at a greater velocity, which makes the droplets less sensitive to air disturbances.[15]

Figure 2.4 shows the measured droplet diameter as a function of printing height with toluene as solvent and representative three-dimensional images of dried droplets that were dispensed from 2, 8 and 18 mm, respectively. All droplets show a rim of polymer material, formed by a replenishing flow during solvent evaporation (the coffee drop effect). The data points in Figure 2.4a were fitted with a linear regression (dotted line) or a first order exponential decay (solid line). The first data point has been excluded from the regression fitting, as explained above. As can be seen, the dried droplet diameter decreases for all concentrations of polymer. The 1 wt% solution shows a linear downwards trend (R2 = 0.97). This trend can be explained by the partial evaporation of the solvent during the descent of the droplet. During the flight of the droplet, the velocity of the droplet is constant, due to a balance between drag and internal forces.[54]_{Therefore, when a droplet travels for a longer}

distance there is more time for evaporation. Upon evaporation the droplets in-flight diameter decreases as well, thus leaving a smaller dried droplet on the surface.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 80 90 100 110 120 130 140 150 160 170 180 190 200 210 220 D rop le t dia m ete r (µ m) Height (mm) 1 wt% PS/toluene 2 wt% PS/toluene Linear fit Exponentional fit

(a)

(b)

Figure 2.4 Dried droplet diameter as a function of nozzle height for solutions of different concentrations of

narrow polydispersity polystyrene in toluene (a). Three-dimensional picture of the dried droplet from a 2 wt% polystyrene/toluene solution (b): from left to right, droplets are shown that were printed from 2, 8 and 18 mm, respectively.

(37)

In contrast, when a 2 wt% solution of polystyrene in toluene was printed the dried droplet diameter does not show a linear downwards trend but an exponential behaviour (R2 = 0.97). This suggests that the evaporation of the toluene is halted at a certain height. The main factors that could decrease the diameter of the dried droplet on the substrate are the in-flight droplet diameter and the overall viscosity of the solution.[17] The in-flight diameter can only be decreased by solvent evaporation, which is significant during the droplet’s free-fall. The viscosity increases upon solvent evaporation, which is similar to the 1 wt% solution, and limits the spreading of the droplet on the surface. Unlike the 1 wt% solution, a higher solute concentration has a greater effect on the viscosity of the solution upon solvent evaporation. It is believed that the polymer forms a skin on the outside of the droplet and evaporation of the solvent is inhibited.[55,56] Furthermore, it was observed that for long hydrocarbon chains there is a stronger retarding effect on the evaporation of the solvent.[57]

It is important to note that both polymer/toluene solutions were inkjet printed with the same settings, in order to obtain a comparable droplet diameter and velocity. Therefore, it is expected that the dried droplet diameters at the start, i.e. the range 1 – 2 mm, would be similar for the two concentrations. However, at a height of 2 mm the droplet diameter for the 2 wt% solution is noticeably smaller than for the droplets produced from the 1 wt% solution (180 µm and 130 µm, respectively). This difference can be explained as being due to a difference in the viscosities (see Table 2.1) of the two solutions; this, in turn, affects the Z-number, which influences the droplet size.[29] The increased viscosity also limits the ability of a droplet to spread on the substrate.[17]

Table 2.1 Typical ink properties for various concentrations (C) of polystyrene in butyl acetate or toluene: surface

tension (γ), viscosity (η), Reynold (Re), Weber (We), Z-number, and contact angle (θ) on the substrate.

Solvent C γ η Re We Z θ (wt%) (mN m-1_{) (mPa}_{s) (-) (-) (-)}_(°) Toluene 0 24.7 0.599 ± 0.006 97.6 2.37 63.4 < 5 Toluene 1 24.3 1.071 ± 0.002 54.6 2.40 35.2 < 5 Toluene 2 24.7 2.048 ± 0.001 28.6 2.37 18.6 < 5 Butyl acetate 0 27.0 0.729 ± 0.004 84.6 2.28 56.0 < 5 Butyl acetate 1 27.1 1.227 ± 0.014 50.3 2.28 33.3 < 5 Butyl acetate 2 27.5 1.729 ± 0.003 35.7 2.24 23.9 < 5 Butyl acetate 5 27.3 5.169 ± 0.018 11.9 2.26 7.92 < 5

(38)

experiment comparable, a solvent with similar Reynolds and Weber number was chosen. Butyl acetate was identified as an appropriate candidate. In order to have comparable print results, the print parameters were set in such a way that the droplet velocity and in-flight diameter (55 μm) were equal to the previous experiment for 1 wt % polystyrene.

Using butyl acetate as the solvent, an experiment identical to that with toluene was performed. Figure 2.5 shows the results for a varied concentration of polystyrene in butyl acetate. Again, all droplets reveal a strong coffee drop effect, similar to the polystyrene/toluene experiments. It can be seen in Figure 2.5a that both the 1 wt% and 2 wt% show a linear decrease in the droplet diameter as height increase (R2 = 0.76 and 0.91, respectively), which is similar to the 1 wt% polystyrene/toluene mixture. However, at a concentration of 5 wt% a similar saturating trend as with the 2 wt% polystyrene/toluene is seen (R2_{= 0.94). Since the same polymer is used for both experiments, the effect seen at}

5 wt% is thought to be due to the solvent properties. The vapour pressure of toluene is 3.79 kPa, whereas butyl acetate has a value of 1.66 kPa. Therefore, since butyl acetate has a lower vapour pressure, less evaporation of the solvent takes place in flight and more time is taken to reach a critical viscosity than with toluene. Therefore, a higher concentration of polymer is required to see the saturation trend.

0 2 4 6 8 10 12 14 16 18 20 22 24 26 80 100 120 140 160 180 200 220 240 260 280 300 320 340 Dropl et di ameter ( µ m) Height (mm) 1 wt% PS/BuAc 2 wt% PS/BuAc 5 wt% PS/BuAc Linear fit Exponential fit (a)

(b)

Figure 2.5 Dried droplet diameter as a function of nozzle height for solutions of different concentrations of

narrow polydispersity polystyrene in butyl acetate (a). Three-dimensional picture of the dried droplet from a 2 wt% polystyrene/butyl acetate solution (b): from left to right, droplets are shown that were printed from 2, 8

(39)

The dried droplet diameters in the 1 to 2 mm range for the three concentrations of butyl acetate solutions deviate from each other, in the same way as for toluene. Again a difference in viscosity can explain the different spreading behaviour: an increased viscosity limits the droplet spreading on the substrate and also affects the Z-number. Section 2.3 describes further research on the influence of the viscosity on inkjet printed droplets and their impact.

Further observations can be drawn from Figures 2.4 and 2.5. First of all, the droplet diameter at large heights (10 – 20 mm) for both the 2 wt% PS/toluene and the 5 wt% PS/BuAc solutions is comparable. This indicates that the condition of the formation of a skin of precipitated polymer on the surface of the droplet may have been reached, which inhibits further evaporation of the solvent. Secondly, the linear regression of the data for the 1 wt% PS solutions with toluene and butyl acetate revealed that the slope for toluene is one and a half times larger than for butyl acetate. This difference can be attributed to the larger vapour pressure for toluene. This trend can also be seen in Figures 2.4b and 2.5b, where a larger decrease in droplet diameter is seen in the three-dimensional images of the droplet. Finally, the error bars for butyl acetate are larger than for toluene. It was noticed during droplet diameter measurements that the shapes of the droplets formed from the toluene solution were more circular than those formed from butyl acetate, as shown in Figures 2.4b and 2.5b. This may be explained by the small difference in viscosity between toluene (0.560 mPa s) and butyl acetate (0.685 mPa s).[58]

In summary, it was demonstrated that for inkjet printed droplets of a varied concentration of polystyrene, dissolved either in butyl acetate or toluene, the in-flight evaporation of the solvent can be increased by increasing the printing height. This has the advantage of reducing the size of the deposited droplet and thereby increasing the printing resolution. With low polymer concentrations, a linear decrease between dried droplet diameter and printing height was observed; whereas increased concentrations revealed an initial exponential decay in the dried droplet diameter before stabilising at greater heights. An explanation of this observation is that at higher concentration, the polymer forms a skin on the surface of the inkjet printed droplet, which inhibits the in-flight evaporation of the solvent. The influence of this skin on solvent evaporation may be worth bearing in mind when considering the more practical application of printing onto heated substrates regardless of whether one increases the printhead height.

(40)

2.3 The behaviour of inkjet printed droplets of polymer solutions on a dry

solid substrate

2.3.1 Introduction

In this section the observations on the impact and spreading of inkjet printed droplets of a polymer solution is described. The first experiments are conducted with a solution that contains a systematically varied molar mass of linear polystyrene in toluene while maintaining a constant concentration of 1 wt% for each solution. The obtained results are compared with recently described models for spreading of droplets in the open literature. A second set of experiments was done to reveal the effect of the printing height on the droplet spreading, while maintaining a constant vapour pressure for all solutions. According to Raoult’s Law, the vapour pressure decreases with increased solute concentration. Therefore, various concentrations with a varied molar mass of polystyrene in toluene were prepared and inkjet printed from a systematically increased height onto a glass substrate.

An inkjet printer was used in order to produce a series of equal-sized droplets, which allowed the error in the measurements to be reduced and significantly increased experimental reproducibility.

2.3.2 Experimental

Materials

Toluene was purchased from Biosolve and used as delivered. Linear polystyrene standards were purchased from PSS (Polymer Standards Service GmbH, Mainz, Germany). Prior to inkjet printing all solutions were filtered with a 5 µm filter (Rezist 30, Whatman GmbH, Dassel, Germany) to prevent nozzle clogging by dust particles. The contact angle and surface tension were measured using a Dataphysics OCA 30 (Filderstadt, Germany).

Substrates

Commercially available glass substrates of 3 × 1 inch (Marienfeld, Lauda-Königshofen, Germany) were thoroughly cleaned by washing subsequently with soap, water, acetone and isopropanol, respectively. After a second rinsing step with isopropanol the glass slides were dried under a flow of air.

(41)

Inkjet printing

Inkjet printing was performed by using a piezoelectric Autodrop system (Microdrop Technologies, Norderstedt, Germany). This system was fitted with a 70 µm printhead, which can be moved in the z-direction. Droplets were generated using a frequency of 200 Hz. The glass samples were placed onto a heatable platen that can move in x- and y-direction. The Autodrop system has a positioning accuracy of 1 µm and a stroboscopic video camera to investigate and optimise droplet formation. Typical printing settings used a voltage between 75 and 80 V and a pulse width between 23 and 25 µs. All samples were printed at room temperature.

Characterisation

Droplet diameter and velocity were calculated from the stroboscopic images that were captured during droplet formation with the inkjet printer. Droplet profiles were characterised using an optical profilometer (Fogale Zoomsurf, France, white light, magnification 5×).

2.3.3 Results and discussion

2.3.3.1 Molar mass influence on droplet spreading

The influence of the molar mass of polystyrene on the final dried droplet diameter on a glass substrate is here discussed. Glass was chosen since it is wetted very well by toluene, with a contact angle smaller than 5°. This provides a higher contrast between different spreading behaviours.[59] Toluene was chosen, because of its good solubility properties with polystyrene. Polystyrene with a low polydispersity index (PDI < 1.07) has been used and all solutions were made with 1 wt% of polymer. Since standard viscosity measurements require a relatively large amount of materials, we have calculated the dynamic viscosity of each polymer solution using the Staudinger equation for the intrinsic viscosity:

a

KM

= ]

[η (2.1)

where a and K are the parameters tabulated for a variety of polymer-solvent systems.[60,61] Here a value of 0.69 and 0.017 cm3 g-1 were taken for a and Kη, respectively. The solution

Microstructures prepared via inkjet printing and embossing techniques

Microstructures prepared via inkjet printing and embossing

techniques

Microstructures Prepared via Inkjet Printing

and Embossing Techniques

Microstructures Prepared via Inkjet Printing

and Embossing Techniques

Table of contents

1

1

Inkjet printing: from graphical arts to a

state-of-the art research technique

1.1 A historical overview

1.2 Working principle of piezoelectric printheads

1.3 Fluid dynamics

η

ρ

γ

ρ

dv

We

=

1.4 Aim and outline of the thesis

1.5 References

2

2

Ink behaviour in-flight and at the substrate

2.1 Introduction

(a)

(b)

2.2 Influence of the printing height

(a)

(b)

(b)

2.3 The behaviour of inkjet printed droplets of polymer solutions on a dry

solid substrate

_dv