Study of oscillating electroosmotic flows with high temporal and spatial resolution*

In AC electrokinetic (EK) flow where solid-fluid interface exists, oscillating electroosmotic flow (OEOF) is an inevitable flow phenomenon. However, few experimental investigations have been reported on instantaneous velocity of OEOF driven by AC electric field. Here, we studied the near-wall velocity of OEOF by Laser-induced Fluorescence Photobleaching Anemometer (LIFPA). For the first time, an up to 3 kHz velocity response of OEOF had been successfully measured experimentally, even though the oscillating velocity was as low as 600 nm/s. It was found that the oscillating velocity decays with forcing frequency $f_{f}$, as $f_{f}^{-0.66}$. This had never been predicted by any known theoretical investigations. In the investigated range of electric field intensity ($E_{A})$, when $f_{f}$ is below 1 kHz, the linear relation between oscillating velocity and $E_{A}$ was observed. Besides, we also found the bulk flow velocity can significantly affect the oscillating velocity of OEOF. This was also newly observed and implied the bulk flow can affect the formation process of electric double layer. This investigation could be crucial for understanding all OEOF-related phenomena and designing OEOF-based micro/nanofluidics systems.