Recent progress on the investigation of phase change and interfacial conditions in microsystems

Abstract

Paper presented at the 6th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 30 June - 2 July, 2008.

Evaporation of liquids is a fundamental phenomenon pertaining to a wide range of industrial and biological processes. In this paper we present recent results on evaporation of liquids and interfacial phenomena in the case of two configurations: menisci in micro-channels and sessile wetting droplets . Interfacial temperature is a key factor in the phase change process. The access to the interface temperature at the micro-scale has been a challenging task. Recently Ward and Duan [22] have investigated the cooling effect resulting from the evaporation of water in a reduced pressure environment by using micro-thermocouples near the interface. They show an increase in the cooling effect with the increase in the evaporation mass flux. They also show an important experimental result that is in contrast with classical kinetic theory and non-equilibrium thermodynamics. The temperature in the vapour phase is higher than in the liquid phase. The authors also discussed the fundamental question about the interfacial conditions during phase change. Indeed, as instrumentation has developed, it has become possible to make measurements of the temperature within one-mean-free path of the interface of water as it evaporates steadily, and these measurements have not supported the prediction from classical kinetic theory that the interfacial vapour temperature less than the interfacial liquid temperature. In a first part of the paper, we present data from an experimental study that has been performed to investigate the evaporation of a liquid in a capillary microchannel. The phase change has been found to induce convection patterns in the liquid phase below the meniscus interface. The liquid convective structure has been revealed using m-PIV technique. When extra heating is supplied to the system, the convection pattern is altered and eventually reversed depending on the relative position of the heating element with respect to the liquid-vapour interface. An IR thermography is used to measure temperature gradients generated by the heater along the capillary wall and of the interface. This allowed us to investigate the relation between the temperature gradients applied along the wall and the convection taking place in the liquid under the thermocapillary stress hence generated. In the second part of the paper we investigate the complexity of the evaporative process of wetting drops by means of IR thermography. The obtained data for volatile sessile drops clearly show that there are phenomena at work which, whilst invisible to the naked eye, may have a great importance in many evaporation dependent areas. The naturally occurring thermal instabilities (wave like thermal fluctuations) shown by many investigated working fluids are clearly distinct from each other, and can also be manipulated by altering the evaporation parameters such as substrate material and substrate temperature. What is also interesting to note is that whilst these waves have been observed for these relatively volatile liquids, there appears to be no such behaviour in pure water droplets. The visual observations presented in this paper form the basis for which a full systematic analysis of the wave behaviour can be achieved. Wave number, frequency, velocity, and amplitude are all parameter which can be measured and then used to characterise the behaviour of each fluid. The above described phenomena are entirely self-generated by the phase change process.