Induced‐charge electrophoresis near a wall

Abstract

Induced-charge electrophoresis (ICEP) has mostly been analyzed for asymmetric particles in an infinite fluid, but channel walls in real systems further break symmetry and lead to dielectrophoresis (DEP) in local field gradients. Zhao and Bau (Langmuir, 23, 4053, 2007) predicted that a metal (ideally polarizable) cylinder is repelled from an insulating wall in a DC field. We revisit this problem with an AC field and show that attraction to the wall sets in at high frequency and leads to an equilibrium distance, where DEP balances ICEP, although, in three dimensions, a metal sphere is repelled from the wall at all frequencies. This conclusion, however, does not apply to asymmetric particles. Consistent with the experiments of Gangwal et al. (Phys. Rev. Lett., 100, 058302, 2008), we show that a metal/insulator Janus particle is always attracted to the wall in an AC field. The Janus particle tends to move toward its insulating end, perpendicular to the field, but ICEP torque rotates this end toward the wall. Under some conditions, the theory predicts steady translation along the wall, perpendicular to the field, at an equilibrium tilt angle around 45 degrees, consistent with the experiments, although improved models are needed for a complete understanding of this phenomenon.

Induced-charge electrokinetic phenomena

“…With non-uniform (but fixed) surface charge, more complex motions are possible as first noted by Anderson [41], such translation perpendicular to a DC field or continuous rotation for non-spherical shapes predicted 3 (a) (b) Figure 2: Experimental observation of induced-charge electrophoresis of metallo-dielectric Janus particles (latex spheres half coated with gold) perpendicular to a uniform AC field (in either direction) by Gangwal et al [29], consistent with theoretical predictions [28,44] by Long and Ajdari [42], although we are not aware of any related experimental observations. For ideally polarizable particles, the present authors [18] first predicted that any broken symmetry in ICEO flow, whether in shape, surface properties (e.g.…”

“…Although symmetric polarizable particles are expected to be repelled from insulating walls [45], the Janus particles were attracted to the surface and observed 4 moving parallel to the surface, very close to it (apparently within a particle diameter). The wall attraction has been attributed to hydrodynamic torque [44], which rotates the (forward facing) dielectric end toward the wall, causing the Janus particle to swim toward it until a collision, and in some cases translate along the wall with a stable tilt angle around 45 degrees. This example shows the rich possibilities of ICEP in confined geometries, which we believe merit further exploration.…”

Electro-orientation and electrorotation of metal nanowires

“…The figure shows a The slip velocity predicted by the standard model is higher than that measured experimentally, sometimes by an order of magnitude [30]. Several reasons for this have been proposed including dielectric coating [31][32][33], counter-ion crowding [34], ion adsorption [32,35], and/or surface roughness [36]. The induced voltage drop across the double layer for our nanorods is of the order of 50 mV, so that ion crowding can be disregarded.…”

Induced charge effects on electrokinetic entry flow

“…28 This flow has a nonlinear quadratic dependence on electric field and often consists of counter-rotating rolls. [29][30][31] It has been extensively studied around conducting surfaces of, for instance, metal electrodes and particles, which can be either electrically activated [32][33][34] or left floating [35][36][37][38][39][40][41][42][43][44][45] as reviewed by Bazant and Squires. 46 ICEO also takes place around an inert but polarizable object where the surface charge is induced by the electric field leaked into the object.…”