Breaking symmetries in induced-charge electro-osmosis and electrophoresis

Abstract

Building on our recent work on induced-charge electro-osmosis (ICEO) and electrophoresis (ICEP), as well as the Russian literature on spherical metal colloids, we examine the rich consequences of broken geometric and field symmetries upon the ICEO flow around conducting bodies. Through a variety of paradigmatic examples involving ideally polarizable (e.g. metal) bodies with thin double layers in weak fields, we demonstrate that spatial asymmetry generally leads to a net pumping of fluid past the body by ICEO, or, in the case of a freely suspended colloidal particle, translation and/or rotation by ICEP. We have chosen model systems that are simple enough to admit analysis, yet which contain the most important broken symmetries. Specifically, we consider (i) symmetrically shaped bodies with inhomogeneous surface properties, (ii) 'nearly symmetric' shapes (using a boundary perturbation scheme), (iii) highly asymmetric bodies composed of two symmetric bodies tethered together, (iv) symmetric conductors in electric-field gradients, and (v) arbitrarily shaped conductors in general non-uniform fields in two dimensions (using complex analysis). In non-uniform fields, ICEO flow and ICEP motion exist in addition to the more familiar dielectrophoretic forces and torques on the bodies (which also vary with the square of the electric field). We treat all of these problems in two and three dimensions, so our study has relevence for both colloids and microfluidics. In the colloidal context, we describe principles to 'design' polarizable particles which rotate to orient themselves and translate steadily in a desired direction in a DC or AC electric field. We also describe 'ICEO spinners' that rotate continuously in AC fields of arbitrary direction, although we show that 'near spheres' with small helical perturbations do not rotate, to leading order in the shape perturbation. In the microfluidic context, strong and steady flows can be driven by small AC potentials applied to systems containing asymmetric structures, which holds promise for portable or implantable self-powered devices. These results build upon and generalize recent studies in AC electro-osmosis (ACEO). Unlike ACEO, however, the inducing surfaces in ICEO can be physically distinct from the driving electrodes, increasing the frequency range and geometries available.

Induced-Charge Electrophoresis of Metallodielectric Particles

“…This unusual phenomenon, which cannot be attributed to DEP, is the first experimental observation of an effect predicted theoretically by Squires and Bazant [7], although we find important differences as well. Using the standard low-voltage model for thin double layers, Squires and Bazant predicted that a metallic sphere with a hemispherical dielectric coating oriented perpendicular to the field will move by ICEP in the direction of its dielectric end at a velocity,…”

“…Bazant and Squires [3,4] conceptually unified this phenomenon with AC electro-osmosis at electrodes, first described by Ramos et al [5] and Ajdari [6], and suggested the term "induced-charge electro-osmosis" (ICEO) to describe all flows resulting from the action of an applied electric field on its own induced diffuse charge near a polarizable surface. They also predicted how broken symmetries could cause polarizable particles to move in electric fields by "induced-charge electrophoresis" (ICEP) [3,7]. …”

“…In what may be the only prior experimental work, Murtsovkin and Mantrov [10] observed that the motion of quartz particles of irregular shapes varies with the square of the applied field, but did not provide a theory. General mobility relations for homogenous non-spherical conducting particles have been derived [11] and ICEP motion has been calculated for arbitrary shape perturbations [7], as well as rod-like spheroidal shapes [12].…”

“…The rotation of the particles to this stable orientation can be caused at least partly by DEP, since it results in the largest induced dipole moment, aligned with the electric field direction. ICEO flows may also contribute, however, since the associated hydrodynamic torque is predicted to rotate the particle to the same orientation as DEP [7]. The particles then moved normal to the applied electric field with their polystyrene hemisphere forward [ Fig.…”

Transport of particles in liquid crystals

“…The nonlinear electrokinetic response was described for particles in an isotropic fluid, first by Murtsovkin et al [178][179][180][181] , and then analysed systematically by Bazant and Squires [182][183][184][185][186] , who predicted that asymmetric particles can show a quadratic dependence of the electrophoretic velocity on the applied electric field, in the so-called induced charge electrophoresis (ICEP).…”