Laser Manipulation of Bubbles and Evaluation of Its Optical Force

We have developed a laser trapping method to control the bubble position without contact. In this method, we use an empty laser cone obtained by focusing a laser ring. Scanning a laser beam circularly using a pair of galvano mirrors creates the laser ring. This method is adequate to trap a rising bubble because the potential barrier created by the light momentum change due to the reflection and refraction on the bubble surface exists only at the upper surface of the bubble. Bubbles of the order of 10 μm in diameter have been trapped and manipulated successfully using a dry objective lens as well as an immersion objective lens. The laser trapping using the dry objective lens with a large working distance has the advantage that the influence of the lens on the bubble is negligible, as a sufficiently large distance can be maintained between the lens surface and the trapped bubble. We have also evaluated the optical force acting on a bubble both experimentally and theoretically. Trajectories of rising bubbles are predicted by taking the optical force into account. It is shown that the optical force in the lateral direction is weaker than that in the axial direction.