TY - JOUR

T1 - Orientation of spheroidal colloids near a charged surface

AU - Tsori, Yoav

PY - 2019

Y1 - 2019

N2 - 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.

AB - 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.

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JO - APS March Meeting 2019

JF - APS March Meeting 2019

ER -