Nanoengineered surfaces
Wetting
Despite extensive studies on flat homogeneous surfaces, recent advances in micro/nanofabrication and coating technologies have enabled the emergence of nanoengineered surfaces with heterogeneous topography and wettability. Fundamental understanding of the liquid wetting behaviors on these surfaces is important for various applications including thermal management, microfluidics, lab-on-a-chip devices, and anti-icing materials.
At the AECR Lab, we are dedicated to explore the variation in surface wettability due to structural topography and chemical heterogeneity in the phase change process. These studies cannot only help to expand our knowledge boundaries of interfacial heat and mass transport, but also provide insights into other research topics in our lab, for example, directional droplet manipulation, condensation, dehumidification, anti-icing, and desalination.
Interactions between liquids and solids are ubiquitous in nature. The wettability of solid surfaces is typically characterized by the wetting angle of a liquid droplet resting on the solid surface.
Condensation
Condensation of water vapor is ubiquitous in nature and of significant importance in various applications involved with phase change heat transfer, such as power generation, indoor environmental control, water harvesting and desalination. In recent years, with the advancements of micro/nanofabrication, nanostructured superhydrophobic surfaces have attracted widespread attention due to their potential for improving the droplet mobility and, in some cases, leading to the coalescence-induced droplet jumping.
At the AECR Lab, we are theoretically and experimentally investigating enhanced condensation heat transfer for water and low-surface-fluids condensation by superhydrophobic, amphiphobic and biphilic surfaces. Our studies cover the fabrication, characterization, and wetting dynamics of superhydrophobic materials during condensation, for exploring the role of surface structure and chemistry on water nucleation, growth, and departure characteristics. Meanwhile, we are interested to create scalable and durable superhydrophobic surfaces that can be applied in industries for multiple years.
Freezing and icing
Frost and ice has long been a problem in human society, which may result in disasters, like downed power lines, damaged crops, stalled aircraft, as well as decreased performance of ships, wind turbines, and heating ventilation and air conditioning (HVAC) systems. Understanding and control of the liquid freezing on solid mediums have a crucial impact on our daily life and industrial production.
The current approach to fabricate anti-frosting surfaces focuses on developing superhydrophobic nanostructures to increase the energy barrier for ice nucleation [46], reduce both contact angle hysteresis, and the ice adhesion strength. Many studies have shown that superhydrophobic surfaces can successfully prevent icing of individual droplets being deposited on the surface or impacting the surface at some prescribed velocity. The overall phase change heat transfer on icephobic surfaces, in general, is intentionally sacrificed to suppress the nucleation of water and ice. Therefore, towards energy efficiency of heat transfer devices in frigid environments, a paradoxical issue between anti-icing and condensation enhancement has stumped scientists for years.
At the AECR Lab, we reconcile the conflict between ice inhibition and condensation enhancement by creating a biphilic structural topography. By leveraging the wetting contrast, the patterned hydrophilic structures on superhydrophobic substrate induce a droplet wetting transition, which spontaneously tunes the interfacial thermal barrier and nucleation rates of water and ice in the sequential condensation-freezing process. Owing to the varying interfacial thermal barrier at the liquid-solid interface, the biphilic topography not only reduces the thermal resistance beneath small droplets to enhance the overall heat transfer, but also simultaneously retards the heat dissipation of the few anchored large droplets to delay the ice nucleation.
Through a combined experimental and theoretical investigation, we reveal, for the first time, the correlation between the onset of droplet freezing and its characteristic radius. This fundamental study of supercooled condensation freezing offers a new insight for controlling the multiphase transitions with surface topography, and will guide rational design for more advanced anti-icing materials with high energy efficiency.