Optical visualization of the velocity distribution in a laser-induced thermocapillary liquid flow

A liquid film of black oil is heated by a cw Gaussian laser beam. This process gives rise to time-increasing, axisymmetric temperature distribution with a maximum in the laser-beam axis. Because the surface tension is decreasing function of temperature, a centrifugal stress gradient and concurrent liquid flow build up in the liquid surface. This phenomenon (known as laser-induced thermocapillary liquid flow) is visualized in the far field of the reflected laser beam, where images microscopic bubbles traveling with the liquid appear as bright spots. Time-resolved experimental records show that the bubbles's images follow rectilinear trajectories at an approximately constant speed. This result is explained by using the theoretical expression of velocity distribution in thermocapillary liquid flow.