Onset of Nonlinear Electroosmotic Flow under an AC Electric Field

Abstract

Nonlinearity of electroosmotic flows (EOFs) is ubiquitous and plays a crucial role in ion transport, specimen mixing, electrochemistry reaction, and electric energy storage and utilization. When and how the transition from a linear regime to a nonlinear one occurs is essential for understanding, prohibiting, or utilizing nonlinear EOF. However, due to the lack of reliable experimental instruments with high spatial and temporal resolutions, the investigation of the onset of nonlinear EOF still remains in theory. Herein, we experimentally studied the velocity fluctuations of EOFs driven by an alternating current (AC) electric field via ultrasensitive fluorescent blinking tricks. The linear and nonlinear AC EOFs are successfully identified from both the time trace and energy spectra of velocity fluctuations. The transitional electric field (E A,C ) is determined by both the convection velocity (U) and AC frequency (f f ) as E A,C ∼ f f 0.48−0.027U . We hope the current investigation could be essential in the development of both theory and applications of nonlinear EOFs.

Numerical Simulation of the Influence of Non-Uniform ζ Potential on Interfacial Flow

“…It is a constant in a chemically uniform interface, determined by material properties and irrelevant to the flow conditions. Similar as and , as is decreased from mV [ 28 ], mV [ 44 ], mV [ 22 ], to mV [ 20 ], ( Figure 11 ) and its gradient increase apparently.…”

“…As mentioned above, in order to investigate the effect of non-uniform ζ potential distribution on interfacial flow, a mathematical model of ζ potential distribution (as plotted in Figure 2 ) was established to characterize the ζ potential distribution in the solid–liquid interface at the bottom of the microchannel only (as shown in Figure 1 ) based on the recent report of Meng et al [ 38 ], as where is the classical ζ potential value at the interface of water-fused silica [ 44 ]. The slope represents how quickly the ζ potential increases in the streamwise direction (with ).…”

“…where 𝜁 is the classical ζ potential value at the interface of water-fused silica [44]. T could be calculated accordingly by Equations ( 3), ( 4), (11) and (12).…”

Transition Routes of Electrokinetic Flow in a Divergent Microchannel with Bending Walls

“…At last, the fluorescent signal was detected by a single photon counter (Hamamatsu H7421, Hamamatsu, Shizuoka, Japan). The spatial resolution of the confocal microscope is ~180 nm [ 31 ].…”