Interfacial crystals

The most commonly observed manifestation of electrokinetic interactions among charge-stabilized colloid is the field-induced formation of colloidal crystals on the electrodes (18). These form once charged colloidal spheres deposit onto the oppositely charged electrode. Once there, the particles distort the local fluxes of simple ions near the electrodes. The resulting structured flows then mediate strong and long-ranged hydrodynamic attractions among the spheres (18). These, in turn, cause the spheres to coalesce into crystalline islands on the electrode.

Such electrohydrodynamically bound crystals also can be induced by an AC field (26). At high enough frequencies, however, the dielectric spheres' induced dipole moments mediate an in-plane repulsion strong enough to and the interfacial crystals dissociate destabilize the interfacial crystals (26).

Field-induced interfacial crystals have been observed in silica spheres, colloidal gold (17), highly charged polystyrene sulfate spheres (17,12,26,19), and even nominally neutral hydroxyl terminated polystyrene spheres, over a wide range of diameters, and on both indium tin oxide (ITO) (17,26,19) and gold electrodes. The threshold voltage for crystallization can be very low if the spheres' diffusion is slow and if their inherent electrostatic and dipolar repulsions are relatively weak.

In our system, we have observed that the threshold voltage for field-induced crystallization depends on the particles' mobility and charge as shown in Fig. 5, but does not depend on the thickness of the cell. Larger spheres crystallize at smaller biases both because they are less diffusive and also because the larger distortions they cause in the surrounding ionic fluxes give rise to stronger attractions. The largest silica spheres we studied, with diameters of 3  $\unit{\mu m}$ crystallized below the 1.23 V redox potential of water. Weakly charged hydroxyl-terminated polystyrene spheres reproducibly crystallize at the lowest voltages of all, presumably because of their substantially weaker electrostatic repulsions.

Figure 5: Threshold voltage for interfacial crystallization for silica, polystyrene sulfate (PS), and neutral polystyrene spheres over a range of particle diameters.