Effects of surface subcooling on the spreading dynamics of an impact water droplet

Spontaneous spreading of a liquid droplet upon a cold solid surface is ubiquitous in nature as well as critical to many industrial technologies, while the mechanism of which still remains elusive. The role of surface subcooling in a water droplet spreading behavior upon impacting on a smooth silicon surface has been experimentally investigated. Under the subcooling condition of the substrate, in the low Weber number region, the non-dimensional maximum spreading diameter decreases with the surface subcooling due to a larger viscosity dissipation and higher surface tension. However, in the case of a high Weber number, the maximum spreading factor first descends and then increases with the increasing surface subcooling. This non-monotonic tendency is attributed to the competition between the increased maximum fingering length and the reduced maximum interior spreading diameter with an increase of the surface subcooling. A sufficiently large Weber number is the prerequisite for forming fingering patterns, and a high subcooling reinforces them due to the enhanced deceleration caused by a larger surface tension and viscosity. The time at maximum spreading barely changes with the impact velocity and slightly decreases with the surface subcooling depending on the droplet size. An improved correlation of the time at maximum spreading as a function of the maximum spreading factor, droplet size, impact velocity, and surface subcooling is proposed.