Study of Oscillating Electroosmotic Flows with High Temporal and Spatial Resolution

Abstract

Near-wall velocity of oscillating electroosmotic flow (OEOF) driven by an AC electric field has been investigated using a laser-induced fluorescence photobleaching anemometer (LIFPA). For the first time, an up to 3 kHz velocity response of OEOF has been successfully measured experimentally, even though the oscillating velocity is as low as 600 nm/s. It is found that the oscillating velocity decays with the forcing frequency f as f. In the investigated range of electric field intensity (E), below 1 kHz, the linear relation between oscillating velocity and E is also observed. Because the oscillating velocity at high frequency is very small, the contribution of noise to velocity measurement is significant, and it is discussed in this manuscript. The investigation reveals the instantaneous response of OEOF to the temporal change of electric fields, which exists in almost all AC electrokinetic flows. Furthermore, the experimental observations are important for designing OEOF-based micro/nanofluidics systems.

Transition from periodic to chaotic AC electroosmotic flows near electric double layer

“…This may explain why the flow in their investigation is linearly stable. 46 Nevertheless, in the present investigation, an astonishing chaotic phenomenon has been observed in the ACFEOF as introduced later. The nonlinear terms in Equations ( 1…”

“… U HS is normally small and subsequently leads to a small Re , which means that the influence of inertia and the nonlinear term in Equation ) can be neglected. This may explain why the flow in their investigation is linearly stable 46 . Nevertheless, in the present investigation, an astonishing chaotic phenomenon has been observed in the ACFEOF as introduced later.…”

“…The LIFPA system is developed on the basis of a home‐developed confocal microscope, with 50 mW 405 nm laser as a light source and an Olympus PlanApo 100× NA 1.4 oil immersion lens for high‐resolution measurement. The system is introduced in detail in the paper of Zhao et al 40 The working fluid is deionized water with a 100 μM Coumarin 102 dye solution which is electrically neutral. The spatial resolution of LIFPA is about 203 nm (lateral) and 1 μm (axial) respectively.…”

“…This is also inconsistent with the recent experimental observation that the velocity fluctuation in oscillating EOF is a function of f f . 46 Third, but also the most important, when U HS is used as the characteristic flow velocity and Reynolds number is defined as U HS l s ρ f /μ, the influence of viscosity has been duplicately considered as μ −2 , since U HS is proportional to μ −1 . In other words, if U HS is used as the characteristic flow velocity, the influence of viscosity could be overestimated.…”

“…This will become more interesting if a pressure‐driven flow is present, where the oscillating EOF will interact with the pressure‐driven steady flow. In previous investigations, the velocity response of linearly oscillating AC EOF has been experimentally studied 40 . However, to the best of our knowledge, so far the transient response of ACFEOF on the insulated surface has never been quantitatively and experimentally investigated.…”