Session: Rising Stars of Mechanical Engineering Celebration & Showcase

Paper Number: 149958

149958 - Dynamics of Droplet Impact on Liquid Films: Coalescence and Mixing 

Droplet impact on liquid films plays a critical role in many engineering processes. For example, the quality of ink-jet printing (both 2D and 3D) greatly depends on the impact dynamics and the subsequent mixing of the ink droplets on the thin film created by the previously deposited ink droplets. In spray cooling and thermal spray processes, liquid droplets are continuously sprayed onto substrates, often at either elevated or reduced temperatures compared to the droplets. The efficacy of the spray cooling and the quality of spray coating rely on controlled deposition, mixing, and heat transfer during droplet impaction of the wetted substrates. It's apparent that in the context of these applications, two critical processes during droplet impacts are (a) interfacial merging controlled by the micro-scale gas layer trapped between the droplet and film interface and (b) post-coalescence mixing between the droplet liquid and the previously deposited liquid layer during the curing/solidification process. 

Recognizing the significance of droplet impact on liquid surfaces in various engineering applications, our primary objective of this project is to investigate and provide critical insights that can be directly applied to improve the control and efficiency of 2D/3D ink-jet printing, spray cooling, thermal sprays, and other related processes.

Our study employs a comprehensive experimental setup to study droplet impact on liquid films. We use a droplet generator to dispense droplets onto a glass container with a square cross-section, which houses a liquid film of variable thickness. Depending on the specific measurements, we utilize several optical diagnostics. High-speed shadowgraphy imaging is used to observe the overall impact dynamics, high-speed particle image velocimetry is employed to measure the impact-induced flow field, and laser-induced fluorescence is used to quantify the mixing dynamics.

In the first two years, we explored two problems. The first problem investigates the bouncing-to-merging transitions when droplets impact a heated liquid film. This is particularly important for 3D printing and IC engines. We varied the film thickness (H), film temperature (T), and impact Weber number (We) to isolate the critical conditions for the bouncing-to-merging transitions. When data are plotted in an H-We regime map, three distinct impact regimes can be observed at room temperature, including bouncing, merging, and late-merging. With an increase in T, the bouncing regime initially remains unchanged, but later, the regime becomes smaller and eventually disappears. Furthermore, as the boiling point of the liquid increases, the critical temperature where the bouncing regime disappears will also increase. These behaviors were elucidated by assessing the changes in the interfacial gaseous layer due to increased evaporation rates. Subsequently, scaling laws were established between the critical impact velocity required for merging and the film temperature, which explains the experimentally observed trends.

In the second problem, we studied the post-coalescence mixing dynamics using a combination of shadowgraphy, particle image velocimetry (PIV), and planar laser-induced fluorescence (PLIF). Simultaneous PIV and PLIF are used to characterize the vorticity fields and scalar mixing processes. From PIV data, we show how toroidal vortices are generated after the coalescence of the droplet and the impacted pool surface translates through the pool. We also extract the temporal evolution of the vorticity as the vortex translates downwards and is stretched once it reaches the bottom of the pool. Using the PLIF images, we quantify the concentration of the droplet liquid. The mean and variance of concentration and the scalar dissipation rate will be analyzed to quantify the mixing dynamics. Additionally, shadowgraphy captures the evolution of the impact cavity formed. A few statistical quantities, including mean, variance, and joint probability distribution functions are used to quantify the mixing dynamics for a range of impact velocities and film thicknesses. Subsequently, a scaling analysis is presented to describe the underlying physics.

Presenting Author: Abhishek Saha University of California San Diego

Presenting Author Biography: Abhishek Saha is an Associate Professor in the Department of Mechanical and Aerospace Engineering at the University of California San Diego. His research group focuses on three areas, (1) dynamics of droplets in the context of propulsion, manufacturing, and disease transmission; and (2) Dynamics and instabilities in reactive flows. Before joining UCSD, Prof. Saha was a research staff at Princeton University. He completed his Ph.D. at the University of Central Florida, where he received the university-wide Outstanding Dissertation award. Prof. Saha is a recipient of the NSF Faculty Early Career Development (CAREER) Award in 2022.

Authors:

Abhishek Saha University of California San Diego