Aditya S. Khair

Single particle motion in colloidal dispersions : a simple model for active and nonlinear microrheology

The motion of a single Brownian probe particle subjected to a constant external body force and immersed in a dispersion of colloidal particles is studied with a view to providing a simple model for particle tracking microrheology experiments in the active and nonlinear regime. The non-equilibrium configuration of particles induced by the motion of the probe is calculated to first order in the volume fraction of colloidal particles over the entire range of Pe, accounting for hydrodynamic and excluded volume interactions between the probe and dispersion particles. Here, Pe is the dimensionless external force on the probe, or Peclet number, and is a characteristic measure of the degree to which the equilibrium microstructure of the dispersion is distorted. For small Pe, the microstructure (in a reference frame moving with the probe) is primarily dictated by Brownian diffusion and is approximately fore–aft symmetric about the direction of the external force. In the large Pe limit, advection is dominant except in a thin boundary layer in the compressive region of the flow where it is balanced by Brownian diffusion, leading to a highly non-equilibrium microstructure. The computed microstructure is employed to calculate the average translational velocity of the probe, from which the ‘microviscosity’ of the dispersion may be inferred via application of Stokes drag law. For small departures from equilibrium (Pe), the microviscosity ‘force-thins’ proportional to

The influence of hydrodynamic slip on the electrophoretic mobility of a spherical colloidal particle

\hbox{\it Pe}^2

Fundamental aspects of concentration polarization arising from nonuniform electrokinetic transport

from its Newtonian low-force plateau. For particles with long-range excluded volume interactions, force-thinning persists until a terminal Newtonian plateau is reached in the limit

Surprising consequences of ion conservation in electro-osmosis over a surface charge discontinuity

\hbox{\it Pe}\,{\rightarrow}\,\infty

On the bulk viscosity of suspensions

. In the case of particles with very short-range excluded volume interactions, the force-thinning ceases at

“Microviscoelasticity” of colloidal dispersions

\hbox{\it Pe}\,{\sim}\, O(1)

Microrheology of colloidal dispersions: Shape matters

, at which point the microviscosity attains a minimum value. Beyond

Efficiently accounting for ion correlations in electrokinetic nanofluidic devices using density functional theory

\hbox{\it Pe}\,{\sim}\, O(1)

The influence of inertia and charge relaxation on electrohydrodynamic drop deformation

, the microstructural boundary layer coincides with the lubrication range of hydrodynamic interactions causing the microviscosity to enter a continuous ‘force-thickening’ regime. The qualitative picture of the microviscosity variation with Pe is in good agreement with theoretical and computational investigations on the ‘macroviscosity’ of sheared colloidal dispersions, and, after appropriate scaling, we are able to make a direct quantitative comparison. This suggests that active tracking microrheology is a valuable tool with which to explore the rich nonlinear rheology of complex fluids.

Use of electrochemical impedance spectroscopy to determine double-layer capacitance in doped nonpolar liquids.

Recent theoretical studies have suggested a significant enhancement in electro-osmotic flows over hydrodynamically slipping surfaces, and experiments have indeed measured O(1) enhancements. In this paper, we investigate whether an equivalent effect occurs in the electrophoretic motion of a colloidal particle whose surface exhibits hydrodynamic slip. To this end, we compute the electrophoretic mobility of a uniformly charged spherical particle with slip length λ as a function of the zeta (or surface) potential of the particle ζ and diffuse-layer thickness κ−1. In the case of a thick diffuse layer, κa⪡1 (where a is the particle size), simple arguments show that slip does lead to an O(1) enhancement in the mobility, owing to the reduced viscous drag on the particle. On the other hand, for a thin-diffuse layer κa⪢1, the situation is more complicated. A detailed asymptotic analysis, following the method of O’Brien [J. Colloid Interface Sci. 92, 204 (1983)], reveals that an O(κλ) increase in the mobility occurs...