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.
