Flows within a droplet help clarify spray cooling

When a cooling spray is spritzed on a hot surface, its many tiny droplets heat up as the surface cools down.

But what happens above the surface is a lot more complex: The flow of the fluid within the droplets also affects the heat transfer, and these dynamics are not well understood. To bridge this gap, Saha et al. simulated the hydrodynamics within a single droplet impacting a hot surface.

“The droplet hydrodynamics affect the liquid flow above the hot surface, which in turn affects the heat transfer,” said author Rishav Saha.

Depending on how much time the droplet spends in contact with the hot surface, the hydrodynamics and heat transfer vary. The stagnant portion of the liquid — its center — heats up more than its spreading edges, meaning the surface underneath the stagnant area cools more.

“If you want to really generalize it, for example, if I’m standing on a hot plate, my foot really gets hot faster, but if I run there, the heat transfer is quite different,” Saha said. “Within the droplet, some portion of the water is stagnant, and some portion is running really fast.”

This creates a cold spot underneath the stagnant region of the droplet, where the lack of spreading speeds up the heat transfer process. Along the edges, the droplet initially spreads out then flows back, forming a raised rim.

The researchers used water droplets in their simulations but plan to expand their study to include other fluid types in addition to structured surfaces — all of which would change the flow and, in effect, the thermodynamics.

Source: “Local heat flux distribution at the solid-liquid interface during droplet impact and spreading at a hot substrate,” by Rishav Saha, Manish Kumar, Simona Tonini, Gianpietro E. Cossali, Bernhard Weigand, Physics of Fluids (2026). The article can be accessed at https://doi.org/10.1063/5.0350250 .

This paper is part of the Complex Drop Impact Collection, learn more here .