Three-dimensional Marangoni cell in self-induced evaporating cooling unveiled by digital holographic microscopy.

A digital holographic microscope has been used to trace the trajectory of a tracer particle inside the liquid phase of an evaporating meniscus formed at the mouth of a 1-mm2 borosilicate tube filled with ethanol. The Marangoni flow cells are generated by the self-induced differential evaporating cooling along the meniscus interface that creates gradients of surface tension which drive the convection. The competition between surface tension and gravity forces along the curved meniscus interface disrupts the symmetry due to surface tension alone. This distorts the shape of the toroidal Marangoni vortex. Thermocapillary instabilities of the evaporating meniscus are reported by analyzing the trajectories of the tracer particle. It is found that the trajectory of the tracer particle makes different three-dimensional loops and every four loops it returns to the first loop. By analyzing several loops it was found that the characteristic frequency of the periodic oscillatory motion is around 0.125 Hz.