Abstract
Utilizing the joint singular natures of electric field and hydrodynamic flow around a sharp nanotip, we report new electrohydrodynamic Landau–Squire-type flows under the actions of alternating current (AC) electric fields, markedly different from the classical Landau–Squire flow generated by pump discharge using nanotubes or nanopores. Making use of the locally diverging electric field prevailing near the conical tip, we are able to generate a diversity of AC electrohydrodynamic flows with the signature of a 1/r point-force-like decay at distance r from the tip. Specifically, we find AC electrothermal jet and Faradaic streaming out of the tip at applied frequencies in the MHz and 102 Hz regimes, respectively. Yet at intermediate frequencies of 1–100 kHz, the jet flow can be reversed to an AC electro-osmotic impinging flow. The characteristics of these AC jet flows are very distinct from AC flows over planar electrodes. For the AC electrothermal jet, we observe experimentally that its speed varies with the driving voltage V as V3, in contrast to the common V4 dependence according to the classical theory reported by Ramos et al. (J. Phys. D: Appl. Phys, vol. 31, 1998, pp. 2338–2353). Additionally, the flow speed does not increase with the solution conductivity as commonly thought. These experimental findings can be rationalized by means of local Joule heating and double layer charging mechanisms in such a way that the nanotip actually becomes a local hotspot charged with heated tangential currents. The measured speed of the AC Faradaic streaming is found to vary as V3/2 logV, which can be interpreted by the local Faradaic leakage in balance with tangential conduction. These unusual flow characteristics signify that a conical electrode geometry may fundamentally alter the features of AC electrohydrodynamic flows. Such peculiar electrohydrodynamic flows may also provide new avenues for expediting molecular sensing or sample transport in prevalent electrochemical or microfluidic applications.
Highlights
When pumping a fluid through a narrow tube into a large reservoir, one will generally observe a jet emerging from the end of the tube
Utilizing the locally diverging electric field prevailing around a sharp conducting nanotip, we discover a new class of Landau–Squire-type flows that can be realized in a purely electrohydrodynamic manner under the actions of ambient alternating current (AC) electric fields
Such a jet-like flow pattern can be reversed into an impinging flow over the tip as a result of the induced AC electro-osmotic flow
Summary
When pumping a fluid through a narrow tube into a large reservoir, one will generally observe a jet emerging from the end of the tube. The typical 1/r velocity field divergence of the LS flow at the discharge tip is a result of the point momentum exerted at the conical tip This 1/r velocity dependence is exactly the signature of the fundamental Stokeslet point-force solution of the Stokes equation under the creeping flow condition (Kim & Karrila 1991). The present work is motivated by a possible issue arising from the use of a sharp metal tip for performing electrochemical STM (ECSTM) probing in an aqueous solution (Itaya & Tomita 1988) In such probing, the excess charges induced by the applied electric field might generate nonlinear electrokinetic flows.
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