Evaluation of Optical Forces for the Laser Trapping of Microbubbles. 2nd Report. Transverse Trapping Force.

Abstract

Transverse optical force for the laser trapping of microbubbles is evaluated experimentally and theoretically. We simulate the trajectories when a trapped bubble escapes from the trapping region in the uniform flow field. It is shown that the simulation is in good agreement with experimental results. The trajectories of an escaping bubble are significantly dependent on the bubble location where it is trapped. The trajectories are classified into two types. When a bubble is trapped below the location of the peak of the transverse optical force, the trapped bubble escapes from the laser trap after it is repelled downward by the optical force ; this trajectory can be seen for relatively smaller bubbles. In this case, the transverse optical force increases nearly in proportion to the square of a bubble radius. On the other hand, when a bubble is trapped above the location of the peak of the transverse optical force, the bubble escapes from the laser trap without being repelled downward by the laser beam.