Acoustic trapping in bubble-bounded micro-cavities

We present a method for controllably producing longitudinal acoustic trapping sites inside microfluidic channels. Air bubbles are injected into a micro-capillary to create bubble-bounded ‘micro-cavities’. A cavity mode is formed that shows controlled longitudinal acoustic trapping between the two air/water interfaces along with the levitation to the centre of the channel that one would expect from a lower order lateral mode. 7 µm and 10 µm microspheres are trapped at the discrete acoustic trapping sites in these micro-cavities. We show this for several lengths of micro-cavity.