How Frost Forms and Grows on Lubricated Micro- and Nanostructured Surfaces

Role of Molecular Water Layer State on Freezing Front Propagation Rate and Mode Studied with Thermal Imaging

In this work, we study the relationship between the molecular water layer (MWL) and frost freezing onset and propagation. The progression of frost has been reported to be governed by various localized icing phenomena, including interdroplet ice bridging, dry zones, and frost halos. Reports studying the state of water on surfaces have revealed the presence of a thin nanometer water layer on a range of surfaces. Regardless of further investigations that show environmental humidity, temperature, and surface energy to affect the thickness of the MWL on surfaces, the influence of the MWL on frost nucleation and propagation has not yet been previously addressed in the literature. To study the effect of the MWL on surface freezing events, a range of surface-functionalized glass substrates were prepared. In situ monitoring of freezing events with thermal imaging allowed studying the effect of surface chemistry and environmental relative humidity (RH) on the thickness and continuity of the MWL. We argue that the observed icing nucleation and propagation kinetics are directly related to the presence and continuity of the MWL, which can be manipulated by controlling the environmental humidity and surface chemistry.

Superhydrophobic Antifrosting 7075 Aluminum Alloy Surface with Stable Cassie-Baxter State Fabricated through Direct Laser Interference Lithography and Hydrothermal Treatment

Frost formation and accumulation can have catastrophic effects on a wide range of industrial activities. Hence, a dual-scale surface with a stable Cassie-Baxter state is developed to mitigate the frosting problem by utilizing direct laser interference lithography assisted with hydrothermal treatment. The high Laplace pressure tolerance under the evaporation stimulus and prolonged Cassie-Baxter state maintenance under the condensation stimulus demonstrate the stable Cassie-Baxter state. The dual-scale surface exhibits a lengthy frost-delaying time of up to 5277 s at -7 °C due to the stable Cassie-Baxter state. The self-removal of frost is achieved by promoting the mobility of frost melts driven by the released interfacial energy. In addition, the dense flocculent frost layer is observed on the single-scale micro surface, whereas the sparse pearl-shaped frost layer with many voids is obtained on the dual-scale surface. This work will aid in understanding the frosting process on various-scale superhydrophobic surfaces and in the design of antifrosting surfaces.

Biofilm formation of Pseudomonas aeruginosa in spaceflight is minimized on lubricant impregnated surfaces

The undesirable, yet inevitable, presence of bacterial biofilms in spacecraft poses a risk to the proper functioning of systems and to astronauts' health. To mitigate the risks that arise from them, it is important to understand biofilms' behavior in microgravity. As part of the Space Biofilms project, biofilms of Pseudomonas aeruginosa were grown in spaceflight over material surfaces. Stainless Steel 316 (SS316) and passivated SS316 were tested for their relevance as spaceflight hardware components, while a lubricant impregnated surface (LIS) was tested as potential biofilm control strategy. The morphology and gene expression of biofilms were characterized. Biofilms in microgravity are less robust than on Earth. LIS strongly inhibits biofilm formation compared to SS. Furthermore, this effect is even greater in spaceflight than on Earth, making LIS a promising option for spacecraft use. Transcriptomic profiles for the different conditions are presented, and potential mechanisms of biofilm reduction on LIS are discussed.

Curvature Adjustable Liquid Transport on Anisotropic Microstructured Elastic Film

Directional liquid transport is expected via adjusting chemical components, surface morphology, and external stimuli and is critical for practical applications. Although many studies have been conducted, there are still challenges to achieving real-time transformation of liquid transport direction on the material surface. Herein, we demonstrate a strategy to achieve curvature responsive anisotropic wetting on the elastic film with V-shaped prism microarray (VPM) microstructure, which can be used to control the direction of liquid transport. The results reveal that the curvature change of an elastic film can adjust the arrangement of V-shaped prisms on the elastic film. Correspondingly, the liquid wetting trend will change and even the moving direction reverses with varying arrangements of the V-shaped prisms on the elastic film. Meanwhile, surface hydrophobicity of the VPM elastic film also affects the liquid wetting trend and even shows the opposite transport direction of the liquid, which is up to the water wetting state on the VPM elastic film. Based on these results, the VPM elastic film can serve as a valve to control the liquid transport direction and is promising in the application of liquid directional harvest, chemical reaction, microfluidic, etc.

Creating lifting force in liquids via thermal gradients

In this paper, we explore a concept and present the first experimental evidence to show that it is possible to form a stable liquid film and create lifting force at the interface via thermal gradient to minimize interfacial rubbing of surfaces and the associated wear. The approach is based on manipulating the flow behavior via thermocapillary, which describes how a liquid can be made to flow from warm to cold regions purely by inducing a thermal gradient. We show that liquid bridges between two parallel plates can be manipulated and stabilized under a combined effect of the thermocapillary flow and the Couette flow, which describes the motion of a viscous fluid between two parallel plates in a relative sliding motion. The equilibrium stage is confirmed under different experimental conditions of a thermal gradient, interfacial gap, liquid viscosity, and liquid bridge volume. A strategy is proposed to control liquid motion and create lifting force between two plates. A theoretical model is also presented to illustrate the principle of the equilibrium stage. Creating lifting forces at the interface offers a new thermo-hydrodynamic tool for manipulating liquids behavior. This approach has the potential for controlling liquid motion in mechanical components and nature.

Spontaneous Charging of Drops on Lubricant-Infused Surfaces

When a drop of a polar liquid slides over a hydrophobic surface, it acquires a charge. As a result, the surface charges oppositely. For applications such as the generation of electric energy, lubricant-infused surfaces (LIS) may be important because they show a low friction for drops. However, slide electrification on LIS has not been studied yet. Here, slide electrification on lubricant-infused surfaces was studied by measuring the charge generated by series of water drops sliding down inclined surfaces. As LIS, we used PDMS-coated glass with micrometer-thick silicone oil films on top. For PDMS-coated glass without lubricant, the charge for the first drop is highest. Then it decreases and saturates at a steady state charge per drop. With lubricant, the drop charge starts from 0, then it increases and reaches a maximum charge per drop. Afterward, it decreases again before reaching its steady-state value. This dependency is not a unique phenomenon for lubricant-infused PDMS; it also occurs on lubricant-infused micropillar surfaces. We attribute this dependency of charge on drop numbers to a change in surface conductivity and depletion of lubricant. These findings are helpful for understanding the charge process and optimizing solid-liquid nanogenerator devices in applications.

Controllable droplet sliding on smart shape memory slippery surface

Recently, slippery surfaces with controllable droplet sliding have aroused much attention in both fundamental research and realistic application. However, for almost all existing surfaces, constant stimuli such as thermal, light, magnetic fields, etc., are indispensable. Herein, by constructing pit structures on shape memory polymer and further infusing oil with low surface tension, we report a shape memory slippery surface that can overcome the above imperfection. Based on the shape memory performance, the surface can memorize diverse pit size as the surface is stretched or recovered. With the variation of pit structure, the sliding performances for both water and organic liquid droplets can be reversibly adjusted between the rolling and pinning states. This work, based on the shape memory effect, reports smart droplet sliding control through regulating surface microstructure, which not only provides a strategy for droplet sliding control, but also offers some fresh ideas for designing novel intelligent slippery surface.

Dynamics of Frost Propagation on Breath Figures

We investigate the process of condensation frosting on flat surfaces using thermal imaging microscopy. This method is particularly well-suited to characterize the frosting of polydisperse assemblies of dew droplets, also called breath figures, that transform into ice droplets by the propagation of frost fronts. The front propagation speed is found to be a nonmonotonous function of the characteristic droplet size of the breath figure. In our experimental conditions, the propagation speed is maximum around 70 μm s^-1 for a characteristic droplet radius of around 300 μm. We mainly show that the frost propagation speed is governed by the competition between two characteristic time scales. The first one is the freezing time of individual droplets, and the other one is the formation time of interdroplet ice bridges that grow from frozen to liquid droplets. In addition, the experiments reveal that the mean ice bridge speed is constant regardless of the characteristic radius of the liquid droplets in the breath figure. A theoretical mean-field analysis without any adjustable parameters recovers all of the features of the front propagation observed in experiments.

Cryofouling avoidance in the Antarctic scallop Adamussium colbecki

The presence of supercooled water in polar regions causes anchor ice to grow on submerged objects, generating costly problems for engineered materials and life-endangering risks for benthic communities. The factors driving underwater ice accretion are poorly understood, and passive prevention mechanisms remain unknown. Here we report that the Antarctic scallop Adamussium colbecki appears to remain ice-free in shallow Antarctic marine environments where underwater ice growth is prevalent. In contrast, scallops colonized by bush sponges in the same microhabitat grow ice and are removed from the population. Characterization of the Antarctic scallop shells revealed a hierarchical micro-ridge structure with sub-micron nano-ridges which promotes directed icing. This concentrates the formation of ice on the growth rings while leaving the regions in between free of ice, and appears to reduce ice-to-shell adhesion when compared to temperate species that do not possess highly ordered surface structures. The ability to control the formation of ice may enable passive underwater anti-icing protection, with the removal of ice possibly facilitated by ocean currents or scallop movements. We term this behavior cryofouling avoidance. We posit that the evolution of natural anti-icing structures is a key trait for the survival of Antarctic scallops in anchor ice zones.

Coral-like silicone nanofilament coatings with extremely low ice adhesion

Passive icephobic surfaces can provide a cost and energy efficient solution to many icing problems that are currently handled with expensive active strategies. Water-repellent surface treatments are promising candidates for this goal, but commonly studied systems, such as superhydrophobic surfaces and Slippery Liquid Infused Porous Surfaces (SLIPS), still face challenges in the stability and durability of their properties in icing environments. In this work, environmental icing conditions are simulated using an Icing Wind Tunnel, and ice adhesion is evaluated with a Centrifugal Adhesion Test. We show that superhydrophobic coral-like Silicone Nanofilament (SNF) coatings exhibit extremely low ice adhesion, to the point of spontaneous ice detachment, and good durability against successive icing cycles. Moreover, SNFs-based SLIPS show stably low ice adhesion for the whole duration of the icing test. Stability of surface properties in a cold environment is further investigated with water wettability at sub-zero surface temperature, highlighting the effect of surface chemistry on superhydrophobicity under icing conditions.

Frost spreading and pattern formation on microstructured surfaces

Frost is found in nature as a symphony of nucleation and heat and mass transport, cascading from angstroms to several meters. Here, we use laser-induced fluorescence microscopy to investigate the pattern formation of frost growth in experiments which tune the mesoscopic length scale by using microstructured pillar arrays as a frost condenser surface. By controlling the degree of surface supercooling and the amount of condensate, different modes of frost patterning are uncovered, ranging from complete surface coverage to fractal-looking and limited-coverage structures of spiky appearance.