(358b) Dielectrophoretic Compound Drop Formation Due to Nanodrop Injection at Fluid Contact Lines

Polarization of aqueous conductive liquid drops by an AC field at the interface of an immiscible conductive ionic and electrolye liquid is shown to exhibit significant frequency dependent surface forces such that contact line injection or ejection of nanodrops can occur depending upon the direction of the induced Maxwell force on the interface. Such behavior can be attributed to a singular Maxwell force at the pinned contact line, which reverses direction at a specific cross-over frequency. By using complex permittivities in the interfacial displacement jump to analyze the Laplace equation at a wedge, we show that the cross-over occurs when the permittivity difference is of opposite sign from the conductivity difference of the two phases. At frequencies ω below (above) the inverse charge relaxation times of both phases, conductive (dielectric) polarization dominates at the phase with the higher conductivity (permittivity). The cross-over frequency is shown to be a geometric mean of these two inverse relaxation times and corresponds to a branch point of eigenvalue λ(ω) of the cylindrical harmonics. It represents an extension of the classical Maxwell-Wagner theory for dielectrophoresis from a sphere to a wedge with singular fields.

We experimentally verify these predicted cross-over frequencies for a drop pinned to an insulating surface. By adjusting the frequency and voltage of a far field, we control the magnitude, direction, and rate of the induced interfacial force to form complex compound drops. The frequency is tuned sequentially to cause injection. The injected drops coalesce to form a small number of new interior drops whose number is proportional to the outer drop diameter, an observation consistent with simple interfacial energy considerations.

This technique is extended to a simple AC field assisted hydrodynamic focusing device, whose non-uniform channel wettability produces a flowing rivulet bounded by contact lines, to generate compound droplets of various sizes continuously. By injecting alternating phases of material into a forming drop, successive generations of interior droplets produce a Russian-doll drop with multiple layers. The ability to control contact line dynamics via the applied electric field hence allows great control over compound droplet generation.