Figure 4.

The normalized velocity,${\overrightarrow{v}}^{*}=\overrightarrow{V}/u_{HS}$ (vector plot) and its magnitude, $v*=\left|\overrightarrow{V}\right|/u_{HS}$ (contour plot) with a sinusoidal zeta potential distribution for various nondimensional frequencies: (a) $\Omega =2.5$ $\left(f=160\mathrm{Hz}\right)$, (b) $\Omega =25$ $\left(f=1.6\mathrm{kHz}\right)$, (c) $\Omega =125$ $\left(f=8\mathrm{kHz}\right)$, and (d) $\Omega =250$ $\left(f=16\mathrm{kHz}\right)$ at $\omega t=\pi /2$. All other conditions are the same as those in Figure 3.