## 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)