Probing radiation forces acting on acoustic bubbles using digital in-line holography.

A digital in-line holography technique (DIH) is applied to measure non-invasively the three-dimensional motion of micrometric bubbles in an acoustic field. In addition to the components of the bubble displacements, DIH also allows the measurement of the temporal evolution of the bubble radii. It is shown that the knowledge of these four variables allow assessing the various forces that the bubbles experience. In the absence of the acoustic field, the buoyancy and drag forces are assessed. When the acoustic field is switched on, the primary radiation force acting on a single bubble is quantified. The order of the magnitude of the force, as well as its evolution along the bubble trajectory, are captured. It is demonstrated that the location of pressure nodes and antinodes in the cavity are recovered when compared with numerical simulations of the acoustic pressure field. When two bubbles nucleated in the cavity are investigated, they can attract and coalesce under the action of the secondary radiation force. DIH technique allows the quantification of this attractive force acting between the bubbles. The evolution of the secondary Bjerknes force with the interbubble distance is shown to be consistent with theoretical models.