Droplet flattening process

For droplets of high surface tension, the droplet flattening process may be governed by the transformation of impact kinetic energy to surface energy. In case that this mechanism dominates, the flattening ratio becomes only dependent on the Weber number, as derived by Madej ski by fitting the numerical results of the full analytical model ... 

Sobolev et al)5111 conducted a series of analytical studies on droplet flattening, and solidification on a surface in thermal spray processes, and recently extended the analytical formulas for the flattening of homogeneous (single-phase) droplets to composite powder particles. Under the condition Re 1, the flattening ratios on smooth and rough surfaces are formulated as ... 

It should be noted that it is difficult to obtain models that can accurately predict thermal contact resistance and rapid solidification parameters, in addition to the difficulties in obtaining thermophysical properties of liquid metals/alloys, especially refractory metals/al-loys. These make the precise numerical modeling of flattening processes of molten metal droplets extremely difficult. Therefore, experimental studies are required. However, the scaling of the experimental results for millimeter-sized droplets to micrometer-sized droplets under rapid solidification conditions seems to be questionable if not impossible,13901 while experimental studies of micrometer-sized droplets under rapid solidification conditions are very difficult, and only inconclusive, sparse and scattered data are available. 

Figure 3.13. Deformation process of a single droplet impinging on a flat surface (Re = 1600, We = 26.7) (a) simulation left), experiment right), and (b) comparison between calculated and measured dimensionless diameter and height ofa flattening droplet. (Photograph Courtesy of Prof. Dr. Jiro Senda at Doshisha University, Japan. Experimental data reprinted with permission from Ref. 334.)...

The coalescence process can be described by two steps. At first, there is a mutual approach of the drops which is controlled by the rheological properties of the continuous (organic) phase (see Figure 4a). Secondly, a flattening of the droplets appears by the formation of a so called "dimple" (see Figure 4b). The decrease in distance d is determined by the rate of flow out of the continuous phase between the droplets (14,15). A thin film is formed which decreases to a certain critical film thickness, dcrit at which point approach stops (16). 

It is often said that we would have to wait as long as the age of the universe for a droplet to spread out. Indeed, since the spreading speed varies as while the droplet is flattening out, it must spread more and more slowly. In consequence, it would take months for a microdroplet to spread spontaneously over several square centimetres, even if the liquid were of very low viscosity. It is thus easy to understand why spreading is forced in industrial processes, so as to cover surfaces more and more quickly (at rates of around the km/min). 

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