Particle dynamics in dense suspensions flowing through microchannels

Particle dynamics in dense suspensions flowing through microchannels

Introduction

While colloidal hard sphere suspensions are well-known as model fluids, much less is known about their behavior when they are made to flow through microchannels. We have initiated a study into the spheres’ spatial distribution, flow profile and (apparent) diffusive behavior, as a function of the volume fraction (0.01-0.4), and the Péclet number (0 to 40) which indicates the strength of the shear flow as compared to Brownian motion: Pé=6𝜋η0γ

′_𝑎3

𝑘𝑇 . Cross-sectional channel dimensions are taken 30-50 particle diameters,

while also the roughness of the wall is varied. After confocal microscopy measurements, the particles’ positions and displacements are analyzed using (adapted) tracking codes written in IDL language.

50μm

Observations

Conclusions

Concentration profile Apparent diffusivity normal to the flow

 Use of both low- and high viscous solvents allows to address a broad range of Peclet numbers, while keeping the maximum flow velocity low enough for particle tracking.

 Separating the diffusive displacements from the convective ones requires great care, both at high and low Pe.  Velocity profiles are nearly parabolic. Slip is always suppressed at the rough wall.

 The dependence of D_┴ on Pe and the distance relative to the walls still needs to be understood.

Experiments

CSLM: 488 nm (λ_wave) @ 15 mW 100X /oil

objective,10 to 15 fps glass cover slip

PDMS Channel cross-section Pressure head (0-20mm) FITC-labeled silica (2a = 1μm) suspensions in polar solvent y z Rough wall Smoot h wall

1._Methanol + Bromoform (low viscous)

2._Water + Glycerol (high viscous)

both refractive index matching for silica solvent 1 is also density-matching.

flow speeds: 0.1 – 10 µm/s

movies: 500-1000 frames per condition measurables:

1. local particle number density

2. local mean velocity along flow direction 3. Mean Square Displacement normal to flow: <∆y2> = 2 D_┴ t Low Pe High Pe 0 10 20 30 40 50 60 0.0 0.1 0.2 0.3 0.4 Pe = 4.34 Pe = 7.31 Pe= 11.83 Pe= 22.65 Pe= 26.76 Pe= 29.28

Distance from the rough wall (m)

Parti cl e cover age 0 10 20 30 40 50 60 0 2 4 6 8 10

Distance from the rough wall (m)

A verage vel oci ty (  m/ s ) 0 10 20 30 40 50 60 0.0 5.0x10-4 1.0x10-3 1.5x10-3 2.0x10-3 2.5x10-3 3.0x10-3

Distance from the rough wall (m)

MSD

(



m