Bistable colloidal orientation in polar liquid near a charged wall

by Prof. Yoav Tsori

Dept Of Chemical Engineering, Ben-Gurion University Of The Negev

at Biological and soft-matter physics

Thu, 05 Dec 2019, 12:00
Sacta-Rashi Building for Physics (54), room 207

Abstract

We examine the translation and rotation of an uncharged spheroidal colloid in
polar solvents (water) near a charged flat surface. We solve the nonlinear
Poisson-Boltzmann
equation outside of the colloid in two dimensions for all tilt angles $\theta$
with respect to the surface normal. The colloid's size is assumed to be
comparable to the Debye's length and hence field gradients are essential. The
Maxwell stress tensor, including the ideal gas pressure of ions, is integrated
over the colloid's surface to give the total force and torque on the colloid.
From the torque we calculate the effective angular potential $U_{\rm
eff}(\theta)$. The classical behavior where the colloid tends to align in the
direction perpendicular to the surface (parallel to the field, $\theta=0$) is
retrieved at large colloid-surface distances or small surface potentials. We
find a surprising transition whereby at small separations or large potentials
the colloid aligns parallel to the surface ($\theta=90^\circ$). Moreover, this
colloid orientation is amplified at a finite value of the aspect ratio. This
transition may have important consequences to flow of colloidal suspensions or
as a tool to switch layering of such suspensions near a surface.

Created on 03-11-2019 by Granek, Rony (rgranek)
Updaded on 03-11-2019 by Granek, Rony (rgranek)