Recently, we built ‘bubble mattresses’ in our laboratory on specially designed and fabricated microfluidic chips. The microfluidic design includes two separate main streams of gas and liquid flow, which are connected by an array of side channels. These side channels mimic the pores of a membrane and they are filled with gas. On the supply of sufficiently small gas pressure, the liquid flows over a mattress of bubbles. As in nature bubbles are merely flat, we also precisely control the curvature of bubble surfaces and their protrusion into the liquid flow.
We measured the velocity fields of water flow over the bubble mattresses. Fluorescent particles of diameter 1 micrometer were dispersed in water during the micro-particle image velocimetry experiments. We measured the influence of bubble curvature on fluid flow. Our measurements show that the most slippery bubble mattresses are obtained when the protrusion angle is in the range of −2 degrees to 12 degrees, corresponding to a 21 percent drag reduction. In a very good agreement with our measurements, our computer simulations suggested a maximum slippage, corresponding to a 23 percent drag reduction at an optimal protrusion angle around 10 degrees.
Our bubble mattresses provide the first high-precision experimental proof that the geometry of the bubbles have a strong influence on the hydrodynamics of the liquid flowing over them. Tunable slippage/drag and the control of flow throughput are of paramount importance for amplified interfacial transport of fluids and particles in both micro- and macro-scale engineering applications.
For more information on our bubble mattresses, please see our recent publication:
http://www.pnas.org/content/early/2013/05/01/1304403110
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