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."
