Trapping of micron-sized objects at a liquid–air interface

Abstract

The limited working distance of the high numerical aperture microscope objectives used in conventional optical tweezers makes it difficult to trap objects at the liquid-air interface. Since with a weakly focused optical beam the gradient forces are not sufficient to overcome the axial scattering force, we investigated the possibility of the use of surface tension forces generated when the object is pushed against the liquid-air interface by a weakly focused optical beam to balance the axial scattering force. In contrast to the expected trapping of objects at the focal point of the trap beam the objects were observed to get trapped in an annular region about the trap beam. The experimental results and their analysis reveal that, apart from optical and surface tension forces, the laser-induced heating of the interface and the resulting thermocapillary effect are responsible for the observed trapping of objects.