Illinois researchers discover new mechanism to suppress frost spreading

A research team led by Professor Nenad Miljkovic in The Grainger College of Engineering at the University of Illinois Urbana-Champaign has published a breakthrough study in Nature Physics. The work reports the first experimental discovery of a previously unknown frost propagation mechanism—a “suspended ice bridge”—offering new pathways for anti-frosting surface design.

Frost formation plays a critical role in many engineering systems, including air-source heat pumps, refrigeration systems and aerospace applications. At the microscopic level, frost mainly spreads through the formation of “ice bridges” that connect neighboring supercooled liquid droplets, enabling freezing to propagate rapidly across a surface. For decades, these ice bridges were widely assumed to grow along the solid surface.

This assumption, largely based on conventional top-view imaging, has shaped existing theoretical models and anti-frosting strategies. However, the Illinois team’s study reveals that this long-held view is incomplete.

Using high-resolution optical microscopy combined with focal plane shift imaging (FPSI), the researchers demonstrated that ice bridges can grow in two distinct spatial modes.

On hydrophilic (water loving) surfaces, ice bridges form along the substrate, consistent with conventional understanding.

By contrast, on superhydrophobic (water repellent) surfaces, ice bridges are found to grow suspended above the surface, bridging droplets through the air rather than along the solid interface (Figure 1). This suspended or “out-of-plane” growth mode represents a fundamentally different pathway for freezing propagation and has been overlooked in previous studies due to limitations in experimental observation.

“Our study further establishes that surface wettability is the key parameter controlling the transition between the two modes,” said first author Dr. Siyan Yang, a postdoctoral researcher in Miljkovic’s lab.  

By systematically analyzing surfaces with varying contact angles, the researchers identified a critical threshold for water droplet apparent contact angle of approximately 105 degrees beyond which suspended ice bridges become dominant. This finding reveals that wettability not only influences droplet distribution and spacing, as previously known, but also fundamentally determines the three-dimensional growth pathway of ice bridges.

Miljkovic, Yang and colleagues demonstrated that the spatial mode of ice bridges is governed by the geometry of droplets and the corresponding vapor diffusion pathways. On superhydrophobic surfaces, droplet geometry shifts the shortest vapor transport path away from the substrate, leading to suspended bridge formation.

Crucially, these suspended ice bridges exhibit significantly slower growth compared to surface-attached bridges. This is attributed to their reduced thermal coupling with the cold substrate, which decreases the vapor pressure gradient driving ice growth.

As a result, frost propagation on superhydrophobic surfaces is dramatically suppressed. Experiments demonstrate a reduction in frost spreading speed by over 80 percent, highlighting the effectiveness of this newly identified mechanism.

To evaluate the practical relevance of the findings, the team extended their study to commercial finned-tube heat exchangers. The results show that surfaces promoting suspended ice bridges can significantly delay frost formation, slow frost propagation, and prolong efficient heat transfer operation. This work establishes a clear link between microscopic ice bridge behavior and macroscopic system performance, providing a new framework for anti-frosting design in energy systems.

The discovery of suspended ice bridges challenges the conventional two-dimensional perspective of frost propagation and introduces a new three-dimensional understanding of freezing dynamics.

“We believe our findings will mean more opportunities for designing advanced surfaces that control frost spreading and interfacial heat transfer,” said Miljkovic. “I expect this will influence future research in phase change phenomena, interfacial transport, and energy-efficient thermal management technologies.”

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“Growth and control of suspended ice bridges during sessile droplet freezing,” Nature Physics.

Authors: Siyan Yang^#, Fuqiang Chu^#, Vishwanath Ganesan, Parsa Faghihi, Dalia Ghaddar, Wenbo Zhang, Jiazheng Liu, Jung Bin Yang, Anxu Huang, Kalyan Boyina, Kaushik Chettiar, Sujan Dewanjee, Shayan Aflatounian, Rahat Khan, Paul V. Braun, Jie Feng, Dimos Poulikakos, Nenad Miljkovic^*
Corresponding Author: Nenad Miljkovic 

Nenad Miljkovic is a Founder Professor of mechanical science and engineering, with additional affiliations in the Department of Electrical and Computer Engineering and the Materials Research Laboratory. He also serves as the Director of the Air Conditioning and Refrigeration Center. He is an editorial board member of several international journals, including Associate Editor of the International Journal of Heat and Mass Transfer, and has chaired multiple international academic conferences. He is a recipient of the NSF CAREER Award and the ASME Bergles-Rohsenow Young Investigator Award and has been elected a Fellow of the American Society of Mechanical Engineers. His research focuses on enhancing the efficiency and reliability of energy and thermal management systems through the integration of micro- and nanoscale surface design with fundamental heat transfer mechanisms. His work has demonstrated significant application potential in areas such as air conditioning and refrigeration, energy systems, electronics cooling and water harvesting. He leads the Energy Transport Research Lab at Illinois.

Dr. Siyan Yang is a postdoctoral researcher under the supervision of Nenad Miljkovic. Her research focuses on the mechanisms of frost formation and propagation under complex environments, as well as the design of anti- and de-icing interfaces. Over the past five years, she has published more than 30 prestigious papers, including 10 first-author papers in leading journals such as Nature Physics, Proceedings of the National Academy of Sciences of the United States of America, Advanced Materials, and Cell Reports Physical Science. She has also contributed to a scholarly monograph and holds multiple invention patents.