  Microbubbles excited by acoustic fields inside water oscillate, and generate
acoustic radiation forces and drag-induced acoustic streaming. These forces can
be harnessed in various biomedical applications such as targeted drug delivery
and on-chip biomanipulation. The conventional approach for using microbubbles
as actuators is to trap them inside microfabricated cavities. Anisotropic
forces are applied by constraining the interfaces where the air interacts with
water. The existing analytical models derived for spherical bubbles are
incapable of predicting the dynamics of bubbles in such configurations. Here, a
new model for bubbles entrapped inside arbitrarily shaped cavities with
multiple circular openings is developed. The semi-analytical model captures a
more realistic geometry through a solution to an optimization problem. We
challenge the assumption that bubbles should be excited at their first
resonance frequency to optimize their performance. The natural frequencies and
the correlated normal vibration modes are calculated, which are subsequently
used to compute the acoustic streaming patterns and the associated thrust by a
finite element simulation. An experimental platform was built to measure the
deflection of beams loaded by microfabricated bubble actuators, and visualize
the generated streaming patterns. The results highlight the contribution of the
computational model as a design tool for engineering applications.
