Frost spreads more slowly on superhydrophobic surfaces via suspended ‘ice bridges’
Frost is a familiar nuisance, on car windshields, refrigerator coils, aircraft wings and heat exchangers, but the physics of how it spreads has an important subtlety that engineers may now be able to exploit. A study published earlier this year in Nature Physics and reported July 9 by Physics World reveals that frost does not always grow along the surface. On superhydrophobic materials, it leaps through the air.
The discovery, made by a team led by Siyan Yang and Nenad Miljkovic at the University of Illinois Urbana-Champaign, together with colleagues at the University of Science and Technology Beijing and ETH Zurich, overturns a long-standing assumption in frost physics: that ice bridges, the thin connectors that link freezing water droplets, always form along the substrate.
Two modes of frost growth
When water droplets freeze on a cold surface, they do not simply turn into ice in place. As a droplet freezes, the latent heat released during freezing causes neighboring supercooled droplets to begin freezing as well, connected by growing “ice bridges,” thin ice filaments that propagate from the frozen droplet to its unfrozen neighbors. The speed and geometry of this bridge propagation determine how quickly frost covers a surface.
On hydrophilic surfaces (contact angle below approximately 90°), ice bridges form as 2D “causeways” growing along the substrate. The bridge is in direct contact with the surface, allowing efficient heat transfer and relatively fast propagation.
On superhydrophobic surfaces, materials engineered to repel water with contact angles greater than 150 degrees, the team discovered a fundamentally different mechanism. Instead of growing along the surface, ice bridges form suspended above it: 3D filaments that arc through the air from one droplet to the next, touching the surface only at the droplet bases.
The critical angle
The transition between the two modes occurs at a critical apparent contact angle of approximately 105°. Above this threshold, the suspended-bridge mechanism dominates. The discovery was made possible by focal plane shift imaging (FPSI), a profilometry technique that reconstructs 3D structure from a series of 2D images at different focal depths. Standard optical microscopy compresses the vertical dimension and cannot distinguish between a bridge resting on the surface and one suspended above it.
Once the team knew what to look for, the implications became clear.
Slower propagation
The suspended bridges propagate more than 80% slower than their surface-bound counterparts. The reasons are twofold: the bridges are longer (they must arc through the air rather than follow the surface) and they exchange heat less efficiently with the substrate (the air gap acts as a thermal insulator).
The practical significance was tested directly. On meter-sized finned-tube aluminium heat exchangers, the type used in air conditioning, refrigeration and heat pump systems, applying a superhydrophobic coating nearly doubled the frost propagation time and delayed the onset of frost coverage.
What this means for anti-icing design
The finding provides a mechanistic explanation for why superhydrophobic surfaces have long been observed to suppress frost formation, even as the specific physics remained unclear. More importantly, it suggests a design principle: rather than focusing solely on delaying the initial nucleation of ice, which has been the dominant anti-icing strategy, surfaces can be engineered to control the geometry of ice-bridge growth itself, favoring the slower suspended mode.
The work was supported by the Air Conditioning and Refrigeration Center (ACRC) at UIUC and the International Institute for Carbon Neutral Energy Research (WPI-I2CNER) at Kyushu University.
Sources:
1. Yang, S., Chu, F., Ganesan, V. et al. “Growth and control of suspended ice bridges during sessile droplet freezing.” Nature Physics (2026). DOI: 10.1038/s41567-026-03296-2
2. Dumé, I. “Frost spreads across surfaces via suspended ‘ice bridges.'” Physics World, July 9, 2026. https://physicsworld.com/a/frost-spreads-across-surfaces-via-suspended-ice-bridges/