Effects of shear and walls on the diffusion of colloids in microchannels

Abstract

Colloidal suspensions flowing through microchannels were studied for the effects of both the shear flow and the proximity of walls on the particles' self-diffusion. Use of hydrostatic pressure to pump micron-sized silica spheres dispersed in water-glycerol mixture through poly(dimethylsiloxane) channels with a cross section of 30×24μm(2), allowed variation in the local Peclet number (Pe) from 0.01 to 50. To obtain the diffusion coefficients, image-time series from a confocal scanning laser microscope were analyzed with a method that, after finding particle trajectories, subtracts the instantaneous advective displacements and subsequently measures the slopes of the mean squared displacement in the flow (x) and shear (y) directions. For dilute suspensions, the thus obtained diffusion coefficients (D(x) and D(y)) are close to the free diffusion coefficient at all shear rates. In concentrated suspensions, a clear increase with the Peclet number (for Pe > 10) is found, that is stronger for D(x) than for D(y). This effect of shear-induced collisions is counteracted by the contribution of walls, which cause a strong local reduction in D(x) and D(y).

Measuring Advection and Diffusion of Colloids in Shear Flow

“…First of all, the data show the expected overall trends: Dx and Dy appear to be equal to each other and independent of the local shear rate (exceptions indicated by triangular symbols are attributed to a wall effect [36]). This suggests that the advection and drifts were adequately subtracted, and that shear-induced hydrodynamic interactions, which would show a y-dependence [36] do not play any role of importance.…”

“…Particle-laden fluids were pumped through by setting a difference in hydrostatic pressure between inlet and outlet (see [36] for details). All suspensions consisted of 1 µm diameter silica spheres with fluorescent cores (see [37,38] for synthesis and characterization details) dispersed in a refractive-index matching mixture of water and glycerol with a viscosity of ≈100 mPa.s.…”

“…In channel flow the MSDs may depend on the y-location, due to possible influences of walls and velocity gradients [36]. Measurement of this dependence is done by calculating for each trajectory i the average y-location <y>I , and using the distribution of the found <y> values as a basis for making a new binning of the Y-range (now optimized for measuring MSDs).…”

Spatially dependent diffusion coefficient as a model for pH sensitive microgel particles in microchannels

“…1 À 10 4 , and these simulations are therefore at the lower end of the experimental scale. 12 In this region, the effects of diffusion and diffusional inhomogeneity are more important than convection, which is the focus of this part of our study. The reduced temperature was T ¼ 1.…”

“…12 A potential consequence of medium inhomogeneity is that the diffusion coefficient of the dispersed particle is not a constant but becomes a space-dependent quantity. The position dependence of diffusivity can also significantly influence properties of the dispersion, such as viscosity and the mixing facility of multicomponent systems.…”