TRANSPORT PHENOMENA IN THE IMPACT OF A MOLTEN DROPLET ON A SURFACE: MACROSCOPIC PHENOMENOLOGY AND MICROSCOPIC CONSIDERATIONS PART I: FLUID DYNAMICS
The fundamental problem of a liquid droplet impacting on a surface appears in a wide range of phenomena from nature to technology. It offers many scientific challenges that cover the entire area of transport phenomena. In fluid dynamics, even in the limit of Newtonian liquids, the problem involves a severely deforming free liquid surface that interacts with a gaseous environment, on the one hand, and with a solid surface, on the other hand. Complex wetting, breakup, and re-coalescence phenomena may take place. The associated thermodynamic/heal transfer complexities can be exemplified if one considers the case where the impacting droplet is molten and solidifies upon impact. In addition to the interplay of the three heat transfer modes and thermocapillarity, the solidification may occur under nonequilibrium conditions. Mass transfer phenomena can also be present in the molten droplet impact problem. Adsorption and surfactant presence, for example, can give rise to a dynamic behavior of the surface tension coefficient during the droplet deformation. In the case of a molten metal in an oxidizing environment, the formation of oxides and their impact on the surface tension can affect the process profoundly. The opposite thermodynamic limit of a droplet evaporating upon impact on a surface can also serve to underline the relevant heat and mass transfer challenges, but it is outside the scope of this review article. The purpose of this pair of articles is to review the state of the art of research on the droplet/surface impact problem as it pertains to the deposition of molten droplets, and to identify trends of future research driven by emerging technologies. Part 1 of the review is focused on the fluid dynamics aspects of the problem, in the absence of temperature gradients. Axisymmetric as well as three-dimensional phenomena are discussed assuming incompressible flow, followed by compressibility phenomena, for flat as well as nonflat substrates. Wetting and capillary phenomena arc an important part of the review. The review of the fluid dynamics phenomena in the present paper is not limited to molten droplets, since the majority of these phenomena is common to all fluid droplets impacting on a surface. Part II of the review (Chapter 3) focuses on work relevant to the case where a temperature gradient is present, with as well as without solidification of the droplet material.
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