Shamit Bakshi

Evaporation of a single droplet of a pure liquid in a confined chamber under atmospheric ambient condition is expected to be purely controlled by the rate of diffusion of the vapor into the surrounding. But, it is seen from the experimental results presented in this paper that for several liquids the process is faster than a theoretical estimate of the diffusion-driven process. It is seen from the visualization inside the droplet that these liquids exhibit intense internal circulation during evaporation. From a scaling analysis the temperature variations within the droplet due to surface traction and buoyancy-driven convection during evaporation is estimated. Marangoni and Rayleigh numbers are also obtained from these estimates. The values of these numbers indicate that Marangoni convection aided by buoyancy is probably the reason for the internal circulation induced within the droplet. The average velocity of the internal circulation is measured and is found to compare well with the velocity scale for Marangoni convection. © 2012 Elsevier Ltd.

The effect of the nature of the suspender on heat transfer into an evaporating microliter pendant droplet is studied experimentally and numerically. Experimental studies show that there is an intrusive effect of the suspender in the heat transfer during evaporation of pendant droplets. Evaporation rates of an ethanol droplet (diameter ~ 3mm) are found to be higher when suspended from a flat-tip, hollow metallic needle (steel) than from a quartz fibre. Experiments also show that replacing the metallic needle with a non-conducting (glass) needle, of similar conductivity as the quartz fibre, results in higher evaporation rates for the same initial diameter of the drop under the same atmospheric conditions. This suggests that the heat transfer to the liquid in the hollow portion inside the needle is significant in the case of a suspending needle and has a direct effect on the measured evaporation rates. Steady state simulations of the heat transfer in the evaporating ethanol droplet when suspended from the metallic needle indicate that there is significant heat transfer to the surface of the drop from the suspending needle. The path of heat flow is from the ambient to the liquid inside the needle through the needle wall and then into the droplet. The surface temperature of the droplet for the simulation is obtained from the measured data from the experiments. Simulations are performed using ANSYS Fluent 14.0 in an axisymmetric domain consisting of the needle and the exact shape of the droplet.