Three-Dimensional Marangoni cell in self-induced evaporating cooling unveiled by µ-Particle Image Velocimetry and Digital Holographic Microscopy

µ-Particle Image Velocimetry has successfully been used to map the velocity fields on diametrical horizontal and vertical cross sections of a transparent glass circular tube of 900µm diameter filled with ethanol. 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 1mm square borosilicate tube filled with ethanol. The Marangoni flow cells are due to 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. This is clearly seen in the µ-Particle Image Velocimetry velocity maps. Thermocapillary instabilities of the evaporating meniscus are reported in the present work by analysing the trajectories of a tracer particle. It is found that the trajectory of the tracer makes different shaped three-dimensional loops and every four loops it returns to the first loop. By analysing several loops it was found that the characteristic frequency of the periodic oscillatory motion is around 0.125 Hz.