Buoyancy and Marangoni effects in an evaporating drop

Experiments and numerical simulations are carried out to study Marangoni and buoyancy effects in a hanging evaporating drop. The liquids investigated are n-octane, which exhibits Marangoni effect, and water, which does not exhibit thermal Marangoni effect. The disk sustaining the drop (diameter of a few millimeters) is held at a constant temperature. A temperature difference arises in the droplet as a consequence of the energy exchange with the ambient and of the evaporative cooling. In the presence of surface tension gradients (Marangoni effect), convective flows are established, and small surface temperature differences are measured at the drop-ambient interface. When the thermal Marangoni effect is absent (as in the water droplet), the surface temperature is stratified, and much larger surface temperature differences are established over the drop surface. The velocity field inside the droplet is evaluated by monitoring the motion of tracers within the drop, in the meridian plane, using a charge-coupled device (CCD) camera. The surface temperature distribution is detected by an infrared camera. The average evaporation rate is measured by computing the liquid volume change, which is done by analyzing the digitized CCD image of the interface shape. The experimental observations are compared with the simulations obtained by a numerical code, solving the axisymmetric steady Navier-Stokes equations taking into account the presence of Marangoni shear stresses and evaporation cooling at the liquid-air interface. A good agreement is found between experimental and numerical results.