Microfluidics without microfabrication

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Abstract
Microfluidic devices create spatially defined, chemically controlled environments at microscopic dimensions. We demonstrate the formation and control of microscopic hydrodynamic and chemical environments by impinging a low-intensity acoustic oscillation on a cylindrical electrode. The interaction of small-amplitude (≤203 μm), low-frequency (≤515 Hz) fluid oscillations with a submillimeter cylinder creates four microscopic eddies that circulate adjacent to the cylinder. This steady flow is known as acoustic streaming. Because the steady circulation in the eddies has closed streamlines, reagent dosed from the electrode can escape the eddies only by slow molecular diffusion. As a result, reagent dosing rates of 10 nmol/s produce eddy concentrations as high as 8 mM, without a correspondingly large rise in bulk solution composition. Imaging Raman spectroscopy is used to visualize the eddy concentration distribution for various acoustic oscillation conditions, and point Raman spectra are used to quantify eddy compositions. These results, and corresponding numerical simulations, show that each eddy acts as a microchemical trap with size determined by acoustic frequency and the concentration tuned via reagent dosing rate and acoustic amplitude. Low-intensity acoustic streaming flows can serve as microfluidic elements without the need for microfabrication.

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