Droplet Impact on Supercooled Surfaces

Dry ice, traditionally, has found tremendous application in food storage and blast cleaning. In recent years, there has been renewed interest in exploiting the sublimating vapor of dry ice as a liquid repellent for contactless transport thereby reducing drag. This work investigates the impact of a hexadecane drop on a sublimating layer of dry ice at room temperature. The sublimating CO2 layer formed above the dry ice is compressed as the drop approaches the solid surface. Liquids which have lower specific heat capacity and latent heat comparatively can therefore freeze partially before bouncing at certain low Weber numbers. This unravels a hitherto unknown regime as current literature describes bouncing at low Weber numbers which is immediately followed by sticking and freezing at higher Weber for a given class of fluids. We thus examine the impact of a hexadecane drop below its capillary length on dry ice which exhibits this kind of behavior. The applicability of hexadecane as phase change material (PCM) also makes its use attractive in such a study. As the hexadecane drop spreads on this vapor cushion partial freezing ensures the movement of the contact line is arrested and we observe lower maximum drop spread (Dmax) values viz-à-viz impact on Super hydrophobic or Leidenfrost drops which display similar behavior. Furthermore, the drop is acted upon by hydrodynamic instabilities which lead to formation of fingers which give rise to an interesting petal shaped pattern. Our study aims characteristics these various morphological transitions in this regime where there is partial freezing accompanied by bouncing and sticking depending on the impact Weber number. This research thus aims to further our knowledge of drop impact on dry ice with the view of helping us better understand development of liquid repellant coatings and application where drag reductions is important.