Oscillating electroosmotic flow in channels and capillaries with modulated wall charge distribution

Electrolyte-filled channels with modulated wall charge distribution subjected to an applied DC electric field form time-independent vortices whose sense of circulation is determined by the field direction [Phys. Rev. Lett. 75, 755 (1995)0031-900710.1103/PhysRevLett.75.755]. In this paper, we show that an electrolyte in a channel or cylindrical capillary subjected to an external alternating (AC) electric field gives rise to various laminar flow structures, including vortices whose sense of circulation changes with the period of oscillation of the applied AC field. The introduction of a period of oscillation lifts certain degeneracies associated with its time-independent counterpart. Although in general the mass flux vanishes, the charge flux is nonzero. The flow is accompanied by a longitudinal (oscillating) advective current that displays hysteresis accompanied by a diverging and negative self-similar conductance that depends on the applied voltage [Nano Lett. 10, 2674 (2010)1530-698410.1021/nl1014734]. We show that this behavior can be interpreted with respect to a "memory retention time" that depends on frequency, viscosity, and the Debye length and could thus form the impetus for investigating control protocols of signal carriers.