Engineers redefine how heat transfers on advanced surfaces

When University of Texas at Dallas researchers tested a new surface that they designed to collect and remove condensates rapidly, the results surprised them.

The mechanical engineers’ design collected more condensates, or liquid formed by condensation, than they had predicted based on a classic physics model.

The finding revealed a limitation in the existing model and inspired the researchers to develop a new theory to explain the phenomenon, which they outline in an article published online March 13 in the physical science journal Newton.

The theory is critical to the researchers’ work to develop innovative surfaces for applications such as harvesting water from air without electricity.

“This new theory can help us better design surfaces that condense water or other fluids,” said Dr. Xianming (Simon) Dai, the study’s corresponding author and associate professor of mechanical engineering in the Erik Jonsson School of Engineering and Computer Science.

Dai develops surfaces for a range of applications, including water harvesting and refrigeration. His goal is to develop a surface that collects and removes condensed droplets quickly in a continuous process.

The researchers’ new theory addresses rolling droplets, or small droplets that roll off a surface during dropwise condensation, a process in which vapor condenses on a surface and forms tiny liquid droplets. Dropwise condensation is favorable for surfaces that aim to shed condensates quickly because the drops roll off and clear the surface faster, making room for the surface to collect more condensates.

Deepak Monga PhD’24, a research scientist in Dai’s lab, noticed that a new surface he was working on contained areas that did not show visible droplets. Indeed, conventional theory does not consider condensation in those areas.

After further investigation, the researchers discovered that the areas actually had contributed to condensation, counter to the half-century-old theory’s prediction. The droplets were so early in the formation process that they are invisible. The classical theory did not consider the speed at which the new surface could collect and shed these condensates.

“It’s all about speed. The classical heat transfer theory does not account for the high speed at which our newer surface removes condensates,” said Monga, the paper’s lead author. “In the new model, we included disappearing frequency to accommodate the high-speed rolling.”

Dr. Yaqing Jin, assistant professor of mechanical engineering, led the experiments to measure and visualize the activity of the tiny water droplets by integrating a time-resolved particle image velocimetry system with a long-distance microscope, which records the flow and motions of microscopic particles inside a small droplet.

“The system has a very high resolution, which allows you to see the flow velocity and distribution of a small droplet, so we can understand how this small droplet is rolling or sliding across a surface,” Jin said.

Monga has used the new theory to design a surface, which he presented at The American Society of Mechanical Engineers’ 2024 Summer Heat Transfer Conference and earned a best presentation award.

The research was supported by a Defense Advanced Research Projects Agency Young Faculty Award, a National Science Foundation Faculty Early Career Development Program (CAREER) award, and the Department of Energy.

Other co-authors of the paper include mechanical engineering doctoral student Dylan Boylan; research associate Dhanush Bhamitipadi Suresh MS’21, PhD’24; Jyotirmoy Sarma MS’18, PhD’22; and Dr. Pengtao Wang from Oak Ridge National Laboratory.