Orientation of spheroidal colloids near a charged surface

Research output: Chapter in Book/Report/Conference proceedingConference contribution


We look at an uncharged spheroidal colloid in a water near a charged flat surface. We solve the nonlinear Poisson-Boltzmann equation outside of the colloid for various tilt angles θ with respect to the surface. 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 a contribution from the ideal gas of ions, can be integrated along the colloid's surface to give the total force and torque on the colloid. The calculation is for a static colloid but if it were to move translation and rotations would be coupled via the tilt angle. From the torque we calculate the effective angular potential u(θ). The colloid tends to align in the direction perpendicular to the surface (parallel to the field, θ=0) if it is far enough from it. Surprisingly, we find that at short separations or large voltages the colloid will align parallel to the surface (θ=90 degs). Interestingly, colloid orientation parallel to the surface is promoted at a finite value of the eccentricity. Lastly, and this needs to be yet verified, the nonuniform forces on the surface of the colloid seem to amount to a net translational force along the surface although the system is invariant in this direction. Israel Science Foundation Grant No. 56/14.
Original languageEnglish
Title of host publicationAPS March Meeting 2019
StatePublished - 2019


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