Splashing is a phenomenon often observed during liquid droplet impact onto a solid surface. The threshold of splashing is known to be related to droplet size, impact velocity, and physical properties of the liquid, but the mechanisms that initiate splashing are not understood completely. In accordance with the Kelvin-Helmholtz (K-H) instability analysis, recent studies have shown that ambient gas density has a significant effect on the threshold and trajectory of splashing. In this study, the effects of droplet velocity, impact angle, and ambient gas pressure (or density) on the threshold of splashing and the motion of the ambient gas surrounding the droplet were examined. Experimental observations of splashing were carried out with a droplet of 1.7 mm in diameter, while varying droplet velocity, impact angle, and ambient pressure. An empirical correlation was derived using our and other published data to determine the threshold of splashing based on the aforementioned parameters. Also, a numerical simulation using the volume of fluid method was carried out to calculate the gas velocities surrounding the droplet during impact. The results of this model gave supportive evidence that K-H instability is a suitable instability theory that helps explain the splash phenomenon with consideration of the gas motion surrounding the droplet.

High speed video imaging and an inverse heat conduction problem algorithm were used to observe and measure the effect of the angle between the nozzle and surface of a skin phantom on: (a) surface temperature; (b) heat flux q; and (c) overall heat extraction Q during cryogen spray cooling (CSC). A skin phantom containing a fast-response temperature sensor was sprayed with 50 ms cryogen spurts from a commercial nozzle placed 30 mm from the surface. The nozzle was systematically positioned at angles ranging from 5 to 90 degrees (perpendicular) with respect to the phantom surface. It is shown that angles as low as 15 have an insignificant impact on the surface temperature, q and Q. Only exaggerated angles of 5 show up to 10% lower q and 30% lower Q with respect to the maximal values measured when nozzles are aimed perpendicularly. This study proves that the slight angle that many commercial nozzles have does not affect significantly the CSC efficiency.

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