Droplet rebound

Depending on the collision velocity, partial rebound of the droplet with lower viscosity from the droplet of a higher viscosity was observed as a new phenomenon as illustrated in Fig. 6.22. This effect is similar to droplet rebound on a hydrophobic surface. The reason behind this is the delay of coalescence as indicated in Fig. 6.20. The surface disrupts about 500 ps after contact, so that the smaller droplet could partially rebound. Most mass of the impacting droplet liquid remains on the larger droplet, but the rebound droplet partially carries also mass from the bigger droplet. 

Fig. 12 shows the spread factors simulated on meshes with different resolutions along with the measurement value of Wachters and Westerling (1966). The spread factor is defined as the radius of the droplet on the solid surface divided by the initial radius of the droplet. Although the convergence is not perfect, the agreement between the experiment and the simulations is relatively good for all resolutions. Consistent with the results of Fig. 11, the effect of the mesh resolution on spread factor becomes notable after 8 ms since the moment of impact, and the coarser resolution tends to yield a slower rebounding process. 

Type R. This mode corresponds to low impinging velocities at any surface temperatures considered (200-400 °C). A droplet spreads as a radial film after impinging on a hot surface. Then, it shrinks and rebounds from the surface without breaking up. Hence, the mode is called R type. 

The aim is to achieve good wetting of the hot mold surface by the emulsion. But good wetting is not sufficient. The projected emulsion droplets that touch the surface will first spread (wet), then partially splash and partially retract and finally rebound. So for optimum efficiency the droplet should have an excellent wetting power but at the same time a limited tendency for retraction and rebound. 

Particles will still collide, but the frequency or the impact of the collisions can be minimised. What happens when the particles do come into close contact The encounters may lead to permanent contact of solid particles or to coalescence of liquid droplets. If they are allowed to continue unchecked, the colloidal system destroys itself through growth of the disperse phase and excessive creaming or sedimentation of the large particles. Whether these collisions result in permanent contact or whether the particles rebound and remain free depends on the forces of interaction, both attractive and repulsive, between the particles, and on the nature of the surface of the particles. 

As already mentioned, in the present study all the collision interactions between the droplets and particles are disregarded. Although two cases of particle-wall interaction are investigated (a) particles hitting walls are escaped from the computational domain, that is, the trajectories of drop-lets/particles are terminated if striking against the chamber walls, and (b) particles can rebound from the walls with restitution coefficients 0.9 (normal) and 0.5 (tangential). 

Besides, it is necessary to be able to predict which regime occurs at droplet impact. Threshold criteria are then defined which establish the boundaries between the four basic outcomes (stick, spread, rebound and disintegration). Particular emphasis is given here to the transition from spread to disintegration, due to its relevance to model the secondary spray generated at spray impact (e.g., [17]). Most criteria make use of the Weber number (e.g., [18]). However, care must be taken to assure that viscous effects are negligible (e.g., [2]), otherwise the Weber number alone does not describe the phenomenon. Prompt splash is then predicted to occur when inertial forces overcome capillary effects, i.e., when ... 

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