In a groundbreaking revelation that challenges long-standing assumptions in the field of frost formation, researchers at the University of Illinois Urbana-Champaign have unveiled a previously unknown mechanism by which frost propagates on surfaces. Led by Professor Nenad Miljkovic from The Grainger College of Engineering, the team’s study introduces the discovery of “suspended ice bridges,” distinct spatial modes of ice bridge formation that occur in stark contrast to the conventional understanding whereby ice bridges grow strictly along the substrate. Their findings, published in the prestigious journal Nature Physics, not only deepen scientific comprehension of frost dynamics but also herald innovative strategies for designing anti-frosting surfaces critical to a wide range of engineering applications.
The formation and propagation of frost is a critical consideration in the design and operation of many technological systems, including but not limited to air-source heat pumps, refrigeration units, and aerospace components. At the microscopic scale, frost spreads primarily through the creation of ice bridges—connective formations that link neighboring supercooled liquid droplets, effectively enabling freezing fronts to advance rapidly across surfaces. For decades, it has been widely accepted, largely based on conventional top-view imaging methods, that these ice bridges advance in two dimensions, traveling along the solid substrate. The Illinois team’s novel research radically revises this view by revealing a three-dimensional aspect to ice bridge growth.
Employing advanced high-resolution optical microscopy complemented by a sophisticated technique known as focal plane shift imaging (FPSI), the researchers were able to visualize frost formation processes in unprecedented detail. This approach enabled them to identify two distinct modes of spatial ice bridge growth that depend heavily on surface wettability. On hydrophilic, or water-attracting, surfaces, ice bridges conform to existing models and propagate along the substrate, consistent with established understanding. Conversely, on superhydrophobic surfaces, which repel water, ice bridges exhibit a unique suspended growth mode, extending above the surface and bridging droplets through the air rather than along the solid interface beneath.
This suspended, or “out-of-plane,” mode of ice bridge formation represents a fundamental departure from previously accepted frost propagation models. Its discovery has been largely overlooked until now due to methodological constraints in prior experimental observations. The significance lies not only in its novelty but also in the profound implications it holds for frost management technologies. According to first author Dr. Siyan Yang, a postdoctoral researcher under Professor Miljkovic, the surface’s wettability is the pivotal parameter that controls the transition between these two ice bridge growth modes.
Through systematic experimentation varying the apparent contact angles of water droplets on different surfaces, the research team identified a critical threshold near 105 degrees. Above this value, typical of superhydrophobic surfaces, suspended ice bridges become the dominant frost propagation route. This insight adds a crucial layer to our understanding: wettability influences not just droplet behavior and spacing but fundamentally governs the three-dimensional architecture of ice bridge growth, redirecting freezing pathways and thereby affecting frost dynamics in ways not previously appreciated.
The researchers further elucidated the mechanisms governing the spatial mode of ice bridges by examining the droplet geometries and corresponding vapor diffusion pathways intrinsic to each surface type. On superhydrophobic surfaces, the geometric configuration of droplets alters the shortest path through which vapor diffuses, shifting it away from the substrate and favoring airborne bridge formation. This anatomical shift arises because droplets adopt a more spherical shape, which minimizes the area of contact with the underlying surface and affects vapor transport dynamics, creating conditions favorable for suspended ice bridges.
One of the most striking findings was the markedly slower growth rate of suspended ice bridges compared to their substrate-attached counterparts. This pronounced deceleration stems from the diminished thermal coupling between the suspended ice bridge and the cold substrate below, which effectively reduces the vapor pressure gradients responsible for driving ice accretion. Consequently, frost propagation is substantially impeded on superhydrophobic surfaces displaying suspended ice bridge formation, representing a potent natural defense against frost accumulation.
Experimentally, the Illinois team demonstrated that frost propagation speed can be diminished by more than 80 percent on surfaces promoting the suspended ice bridge mode. This breakthrough has immediate practical relevance, as it directly translates to enhanced operational efficiencies and prolonged performance lifetimes in frost-sensitive systems. To validate this, the researchers extended their experimental framework to encompass commercial finned-tube heat exchangers. These components are ubiquitous in heating, ventilation, air conditioning (HVAC), and refrigeration systems and often suffer from efficiency losses due to frost buildup.
The results obtained from tests on these heat exchangers corroborated the laboratory findings, showcasing that surfaces engineered to support suspended ice bridges can dramatically delay the onset of frost, slow its propagation, and consequently sustain optimal heat transfer performance over extended periods. This represents a crucial advancement in linking microscopic frost structure behavior to macroscopic system-level outcomes. By providing this mechanistic understanding, the research opens the door to the rational design of surfaces that strategically manipulate ice bridge formation to curb frost accumulation and improve energy efficiency.
This discovery also challenges the conventional two-dimensional framework of frost propagation, calling for a re-examination of theoretical models from a three-dimensional perspective. Recognizing that ice bridge growth can extend above the surface plane compels scientists and engineers to reconsider frost formation dynamics and interfacial heat transfer processes in materials and devices exposed to frost conditions. The new paradigm not only reshapes fundamental phase change science but could ripple across disciplines involved in thermal management and surface science.
Professor Miljkovic underscored the transformative potential of these findings by emphasizing how the deeper understanding of ice bridge formation will catalyze innovative surface engineering efforts. These efforts aim to tailor interfacial properties to regulate frost spreading deliberately, fostering more energy-efficient thermal management and phase change systems. The possibility of controlling frost at the microscale through surface wettability and geometry adjustments marks a pivotal step toward technologically advanced, frost-resilient surfaces.
Dr. Siyan Yang’s role as principal experimenter and co-author underscores the multidisciplinary expertise fueling the breakthrough. Her extensive research in frost nucleation, propagation mechanisms, and anti-icing surface design has led to numerous influential publications in high-impact journals and multiple invention patents. The convergence of physics, materials science, and engineering in this study exemplifies the burgeoning field of interface-driven energy transport phenomena.
Together with a diverse team of collaborators, Miljkovic and Yang’s pioneering work redefines the fundamental science of frost formation, presenting suspended ice bridges as a novel, three-dimensional mechanism with profound implications for future research and practical applications. This advancement represents a seminal leap, promising not only enhanced understanding but also transformative technologies for energy and thermal management systems facing the perennial challenge of frost.
Subject of Research: Frost propagation mechanisms and surface-driven ice bridge formation during sessile droplet freezing.
Article Title: Growth and control of suspended ice bridges during sessile droplet freezing
News Publication Date: 28-May-2026
Web References:
https://www.nature.com/articles/s41567-026-03296-2
http://dx.doi.org/10.1038/s41567-026-03296-2
References:
Yang, S., Chu, F., Ganesan, V., Faghihi, P., Ghaddar, D., Zhang, W., Liu, J., Yang, J.B., Huang, A., Boyina, K., Chettiar, K., Dewanjee, S., Aflatounian, S., Khan, R., Braun, P.V., Feng, J., Poulikakos, D., Miljkovic, N. (2026). Growth and control of suspended ice bridges during sessile droplet freezing. Nature Physics.
Image Credits: The Grainger College of Engineering at the University of Illinois Urbana-Champaign
Keywords
Frost propagation, ice bridges, suspended ice bridges, superhydrophobic surfaces, hydrophilic surfaces, sessile droplet freezing, surface wettability, frost mitigation, vapor diffusion pathways, thermal management, phase change phenomena, anti-frost surfaces
