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(a) First acoustic node visually separates secondary and primary channels; (b) eventually they unite and form one longer channel. The water-assisted femtosecond laser nanomachining directly fabricate subsurface channels of in diameter. (c) Schematics of retarded channel circulation due to the formation of an acoustic node which decreases the driving force generated by bubble expansion. The circulation driving debris extrusion can be described by , where is the resistance to circulation.
(a) Analytic model for the first acoustic node. The gas plug acts as a spring and the water plug at the inlet port as a mass; the mass and equivalent spring constant following Boyle’s law determine the ; is the internal pressure of the gas plug, and and are the lengths of the gas and first water plugs. The in the gas plug is decided by the speed of sound and length of the gas plug; is the specific heat ratio , is the gas constant, and is the temperature of the gas plug. (b) Metastable WGW structure of the internal two phase flow; a large gas plug is formed in the middle of the channel, while the inlet and closed end of the channel are filled with water.
Effect of pressure on the first acoustic node formation. To vary the pressure, a dc motor driven syringe pump was feedback controlled within 1%.
Gas composition of bubble by machining type; water was partially degassed by applying ultrasonic agitation under a vacuum.
Gas composition of bubbles predicted from the node position.
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