Cold granular targets slow the bulk freezing of an impacting droplet

TL;DR

This study investigates how cold granular targets delay droplet freezing and alter impact dynamics, revealing that pore solidification and surface effects can control solidified droplet morphology in subfreezing impacts.

Contribution

It demonstrates that cold granular materials significantly slow bulk droplet freezing and modify impact behavior, providing new insights into controlling solidification in droplet impacts.

Findings

Bulk freezing delayed by an order of magnitude with increased undercooling

Solidification in pores causes impact dynamics changes

Cold grains can mimic effects on impact behavior

Abstract

When making contact with an undercooled target, a drop freezes. The colder the target is, the more rapid the freezing is supposed to be. In this research, we explore the impact of droplets on cold granular material. As the undercooling degree increases, the bulk freezing of the droplet is delayed by at least an order of magnitude. The postponement of the overall solidification is accompanied by substantial changes in dynamics, including the spreading-retraction process, satellite drop generation, and cratering in the target. The solidification of the wetted pores in the granular target primarily causes these effects. Owing to the small size of pores, solidification there is sufficiently fast to match the characteristic timescales of the impact dynamics at moderate undercooling degrees. As a result, the hydrophilic impact appears `hydrophobic', and the dimension of the solidified droplet shrinks. A monolayer of cold grains on a surface can reproduce these consequences. Our research presents a potential approach to regulate solidified morphology for subfreezing drop impacts. It additionally sheds light on the impact scenario of strong coupling between the dynamics and solidification.