Enhanced Electrokinetic Streaming Potential Generation Through Surface Modulation on Microfluidic

The generation of electrical voltages and currents through aqueous electrolyte flow on charged surfaces is of significant interest for insights into surface-fluid interactions as well as for the development of new energy sources. The related phenomena broadly involve the attraction of counterions in the electrolyte to a charged surface forming an electric double layer (EDL). An electrical charge separation in the EDL may be obtained through a pressure difference (<i>D</i><i>P</i>) applied to the electrolyte along the channel length, and results in a streaming potential (<i>V_s</i>). It has been determined that both the surface electrical potential, <i>i.e., </i>the zeta potential (<i>z</i>), as well as the electrolyte slip length (<i>b</i>) contribute to the <i>V_s</i>.<br/>We investigate through detailed experiments and subsequent theoretical analysis, the influence of various patterned surfaces, comprised of grooves, meshes, and pillars, in modulating the electrolyte flow for obtaining large <i>V_s</i>. We have obtained fascinating insights into how such <i>patchy </i>surfaces may be utilized in further increasing the figure of merit (related to the <i>V_s</i>/<i>D</i><i>P</i> ratio) to a record 0.1271 mV/Pa - three-fold larger compared to what was previously obtained, through such investigations. We have probed flow on the patterned surfaces with <i>both </i>impregnated air and electrolyte immiscible oil, <i>i.e., </i>an air-filled surface (<i>AFS</i>) as well as a liquid filled surfaces (<i>LFS</i>), respectively. We note that crucial to obtaining a large <i>V_s </i>is the optimization of the product of the effective slip length (<i>b_eff</i>) and the zeta potential (<i>z</i>). While a larger <i>b_eff </i>may be obtained from the <i>AFS, </i>the effective <i>z</i> is shown to be larger through the use of the <i>LFS. </i>Consequently, the interplay of the <i>z</i> and <i>b_eff</i>, with respect to the underlying patterned surface was considered in detail. On this basis, we posit a modified <i>effective </i>zeta potential which may be of significant use for modeling flow over patterned or non-homogeneous surfaces.<br/>While the use of hydrophobic surfaces was advocated to increase the <i>b</i>, related surfaces are difficult to fabricate and maintain. Much less attention has been paid to methods for enhancing <i>z</i>. We then proposed the use of plasma processing of surfaces to modulate <i>both </i>the <i>b</i> as well as the <i>z</i> over silicon surfaces. Such methodology is shown to result in a larger <i>V_s</i> and a record figure of merit (FOM) of 0.1 mV/Pa – a factor of two larger compared to what was previously obtained. In our work we deployed CF₄ based plasmas for the possibility of creating of a thin fluorocarbon film over the surface, which may increase the hydrophobicity<i>. </i>Moreover, the ions in the plasma would also have a role in increasing the surface charge density and the resultant <i>z</i>. It was then determined experimentally that the latter effect predominates. In contrast, Ar-based plasmas are often preferred for their relatively simple constitution (with only Ar⁺ ions and absence of chemical reactions). However, it was seen that such plasmas were more influential in only roughening the surface with minimal influence on the <i>z</i> or <i>b</i>. Generally, with the estimated <i>b </i>at less than 2 nm, the contribution to the increased <i>V_s </i>is typically small. Consequently, the major contributor to the record <i>V_s </i>values obtained in our study was due to the enlarged <i>z</i>.<br/>Our results have major implications in enhancing the understanding of electrokinetic phenomena with respect to the variation of the electrical and mechanical attributes of surfaces. Further development through deploying hydrophobic surfaces <i>coupled </i>with plasma processing, and related principles outlined in our work, could yield much larger <i>V_s</i> and pave the way for large scale voltage sources. Our two articles, entitled <i>The influence of surface texture on the variation of electrokinetic streaming potentials</i> and <i>The Modulation of Electrokinetic Streaming Potentials of Silicon-Based Surfaces through Plasma-Based Surface Processing</i>, were published at <b><i>Langmuir</i></b>.