Study of Oscillating Electroosmotic Flows with High Temporal and Spatial Resolution Wei Zhao, †,‡ Xin Liu, ‡ Fang Yang, § Kaige Wang, † Jintao Bai, † Rui Qiao,* ,∥ and Guiren Wang* ,‡ † Institute of Photonics and Photon-technology, International Scientific and Technological Cooperation Base of Photoelectric Technology and Functional Materials and Application, Northwest University, 229 North Taibai Road, Xi’an 710069, People’s Republic of China ‡ Department of Mechanical Engineering & Biomedical Engineering Program, University of South Carolina, Columbia, South Carolina 29208, United States § Key Laboratory for Molecular Enzymology and Engineering of Ministry of Education, Jilin University, Changchun 130012, People’s Republic of China ∥ Department of Mechanical Engineering, Virginia Tech, Blacksburg, Virginia 24061, United States * S Supporting Information 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 f as f f −0.66 . In the investigated range of electric field intensity (E A ), below 1 kHz, the linear relation between oscillating velocity and E A 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. I n micro- and nanofluidic systems, due to the large surface-to- volume ratio, electroosmotic flow (EOF) has been widely used to pump fluids and manipulate objects for various applications, such as DNA transport, hybridization and separation in biomedical engineering, and enhancing heat and mass transfer. 1−5 At the early stages, most of the investigations focused on the EOF driven by direct current (DC). The relevant devices have been proven to be effective in driving flows in micro/ nanochannels, which are further used to transport DNA, protein, and cells. However, when the length of channels is long, it normally requires high voltage to generate sufficiently strong electric field to drive the flow. This leads to several drawbacks, such as gas bubble formation due to electrolysis and excessive heating due to electrothermal effects. Relative to DC EOF, the EOF generated by AC electric fields (i.e., AC EOF) has attracted wide interest in the past decade. Compared to DC EOF, AC EOF-based micropumps require lower voltage to pump fluids. This can avoid the generation of microbubbles and make the devices portable. In microfluidics, as early as in 2000, Green et al. 6 reported AC EOF (also known as induced-charge electroosmotic flow, ICEOF) near planar microelectrodes. By monitoring latex tracer particles, the velocity of the flow is experimentally investigated and exhibits apparent dependency on the frequency of the AC field. On the basis of AC EOF, Studer et al. 7 designed a micropump for tunable flow control. Under very low voltages (below 10 V, rms value), a maximum flow speed of 500 μm/s can be achieved at AC electric field of tens of kHz. In the same year, Debesset et al. 8 also designed a micropump for chromatographic application, by using AC EOF. A maximum speed of 50 μm/s was realized. Although the flow speed was an order smaller than that observed by Studer et al., 7 they successfully found that in the investigated parametric region, the flow speed of micropump had approximately linear relation with applied AC voltage and frequency. Later, in 2005, Gagnon and Chang 9 combined an AC EOF with a dielectrophoretic (DEP) flow to achieve fast bacteria detection. The bulk flow velocity due to the AC EOF could be up to 1000 Received: July 27, 2017 Accepted: December 19, 2017 Published: December 19, 2017 Article pubs.acs.org/ac Cite This: Anal. Chem. 2018, 90, 1652-1659 © 2017 American Chemical Society 1652 DOI: 10.1021/acs.analchem.7b02985 Anal. Chem. 2018, 90, 1652−1659