  Electroosmotic flow (EOF) is a ubiquitous phenomenon at the solid-liquid
interface when an external electric field is applied. Despite its prevalence,
the characteristics and mechanisms of EOF driven by an alternating current (AC)
electric field, particularly within complex chemical environments, have
remained insufficiently understood, owing primarily to a scarcity of
experimental data. In this investigation, we advance the comprehension of AC
EOF by employing a high-resolution measurement technique - laser-induced
fluorescent photobleaching anemometer (LIFPA). This method allows for precise
empirical characterization of transient velocity of EOF along the electric
double layer (EDL) far from electrode surfaces. We have discerned a distinct
transition in AC EOF behavior - from linear to nonlinear - across a wide
parameter space, such as the velocity of bulk flow, the AC electric field's
frequency and intensity, and the pH of the bulk fluid. Moreover, the transition
within the AC EOF is quantified by the transitional electric field intensity,
$E_{A,C}$, paired with a correlated dimensionless parameter, $Z_{nlc}$. A
power-law relationship between the linear term coefficient $Z_{l}$ and
$Z_{nlc}$ has been established, with the scaling exponents determined by the pH
value of the electrolyte solution. With these findings, we aspire not only to
deepen the understanding of AC EOF transitions but also to establish a robust
model that elucidates the interplay between the electric field and fluid flow
in both linear and nonlinear regimes. This research potentially paves the way
for more predictable and controllable electrokinetic processes in numerous
applications, including micro-/nanofluidic systems, electrochemical reactions,
and beyond.
