  In this work, we will describe an experimental setup for a standing--wave
ultrasound trap for air microbubbles in oil. We develop a model for the finite
acoustic beam using the angular spectrum technique, and reconstruct the
pressure field using the General Lorenz-Mie Theory framework, which was
validated using a finite elements method (FEM) simulation. Using Stokes' drag
law, we were able to obtain the radius of the trapped bubbles and estimate the
minimum acoustic force necessary to trap them, which ranged from 3 nN to 780
nN. We also present the force profile as a function of distance for different
bubble that were trapped experimentally, and show that a standing wave formed
by interfering infinite plane waves cannot explain the observed acoustic
trapping of bubbles in 3-D.
