The dynamics of partially encapsulated microbubbles subjected to ultrasound

Abstract

Acoustically excited microbubbles generate various nonlinear forces that can be leveraged in microscale systems for actuation and manipulation. To obtain optimal performances, bubbles should be characterized; however, so far, they were studied indirectly by measuring downstream phenomena. Here, we present a novel scheme to measure the vibrations of a bubble at the water-air interface using a laser vibrometer and the impinging pressure using a hydrophone. A custom-built optical setup couples the vibrometer to an inverted microscope. The microstructures encapsulating the bubbles are 3D nanoprinted on glass slides, which allows the realization of complex configurations with various polymers. The overall platform enables us to study the dynamics of bubbles with single or multiple interfaces, and their interactions. The measurements are also used to refine the analytical model of the multi-physics system and find optimal operating conditions. The conditions depend on the bubble geometry, boundaries, and excitation source. The measurements reveal that it is easier to excite certain vibration modes when the acoustic wavelength is much larger than the bubble. We demonstrate the controllable motion of 3D nanoprinted flexible structures by harnessing nonlinear forces that are produced by acoustically excited microbubbles. These structures will serve as building blocks for all-mechanical soft microrobots.