Effect of parameters on redistribution and removal of particles from drop surfaces
Abstract
It was recently shown by us that particles distributed on the surface of a drop can be concentrated at the poles or the equator of the drop by subjecting the latter to a uniform electric field and that such concentrated particles can then be removed from the drop by increasing the electric field intensity. In this paper, we present experimental results for the dependence of the dielectrophoretic force on the parameters of the system such as the particles' and drop's radii and the dielectric properties of the fluids and particles, and define a dimensionless parameter regime for which the technique can work. Specifically, we show that if the drop radius is larger than a critical value, that depends on the physical properties of the drop and ambient fluids and those of the particles, it is not possible to concentrate particles and thus clean the drop of the particles it carries at its surface because the drop breaks or tip-streams at an electric field intensity smaller than that needed for concentrating particles. However, since the dielectrophoretic force varies inversely with the drop radius, the effectiveness of the concentration mechanism increases with decreasing drop size, and therefore the technique (particles concentration followed by drop clean-up or delivery) is guaranteed to work provided the drop radius is sufficiently small. We also show that this concentration method can be used to separate particles experiencing positive