Bouncing-to-merging transitions of droplet impact on heated liquid film*

Droplet impact on a liquid film is ubiquitous in numerous natural

and industrial processes. In many of these applications, the impacting

droplet and the impacted film contain liquids at different temperatures.

In this talk, we will discuss the findings from our study on droplet (radius,

R) impact on a heated liquid film of varied thickness (H). Using n-

tetradecane as our test liquid, we conducted a series of experiments where

the temperature difference (ΔT ) between the liquid film and the droplet

was varied from 0 to 60°C. The primary focus of this work is to identify

changes in the transition between bouncing to merging outcomes as the

film temperature increases. We will first compare the regime diagrams

where bouncing and merging outcomes were mapped in a Weber number

and normalized film thickness (H^* = H/R) space for various ΔT . The

regime maps show that, for deep pool (H^* >> 1) and shallow pool (H^∗ ≈

1) limits, ΔT exhibits varied degrees of influence on the bouncing and

merging transitions. Large ΔT , however, renders merging easier across

all H^* and beyond a critical temperature difference (ΔT ≥ 60°C for n-

tetradecane) the bouncing regime essentially disappears. Subsequently, to

explain the observed dynamics, we will discuss the role of evaporation and

vapor layer adjacent to the heated liquid film on the interfacial merging.