Biomedical Engineering Reference
In-Depth Information
model and predict the evaporation of burning droplet behaviour. Worth mention-
ing also the more traditional sciences, like biology where droplet evaporation has
many practical applications. The rate of droplet evaporation is for example critical
to successfully develop herbicides and fertilisers in order to predict accurate dosing
as well as determining how a droplet behaves upon contacting with a plant surface.
More recently drop wise evaporation has been utilised as a means of “stretching”
DNA molecules, used in DNA and gene analysis.
Because of the growing applications, many attempts have been made to success-
fully model the rate of droplet evaporation. But the complicated interaction of heat
and mass transfer, have so far produced results with limited use. This is for example
the case of the physical phenomenon of phase change that has proved to be histor-
ically one of the most fascinating topics in heat transfer. Experimental work has
intensified over the past fifty years as more sophisticated experimental techniques,
such as high speed imaging and infrared tools, allow better understanding of the
evaporation regimes and their underlying physics. Experimental studies revealed
that the evaporation of a single droplet is a complicated process that results from
the interplay between fluid mechanics, heat transfer and surface chemistry. In this
context, the detailed understanding of the physical and chemical mechanisms in-
volved in the evaporation of heated droplets on a substrate is still in many aspects
an open problem [1-11] in particular for the precise description of the heat/mass
transfers that occur at liquid/vapour and liquid/solid interfaces.
The purpose of this chapter is to provide a review highlighting some challenges
and answers in the kinetics of evaporating droplets in the case of pure droplets of
millimeter size deposited on a heating substrate. This study is further motivated by
the large variety of industrial applications involving droplet evaporation. Such sys-
tems are indeed important in biochemistry [12], new materials development [13,
14], paint industry and printing [15] as well as in innovative nuclear decontami-
nation processes with low effluent volumes. All these systems have in common a
bulk hydrodynamics coupled with interfacial properties. This coupling is actually
one of the difficulties in the description of the mechanisms occurring in the evapo-
ration of fluids and motivated in the past decades extensive fundamental research in
molecular chemistry, fluid mechanics and thermodynamics. Molecules in fluids are
subjected to electrostatic and Van der Waals forces. These interactions are superim-
posed to the ones governing the evolution of atoms within molecules and described
by the time evolution of intra-molecular degrees of freedom. All these contribu-
tions lead to complex chaotic dynamics that appears at the macroscopic scale as
Brownian motion. At liquid/gas interfaces, intermolecular forces are anisotropic in
the normal direction to the interface and the molecules having a sufficient kinetic
energy will overcome Van der Waals attractive forces and escape in the gas phase
surrounding the droplets. The corresponding energy loss of the liquid leads, at the
macroscopic scale, to evaporative cooling that might generate temperature discon-
tinuities at the interface [16]. Finally, it is the balance between mass loss, due to
evaporation, and mass increase, due to the condensation of molecules back on the
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