An energy-free strategy to elevate anti-icing performance of superhydrophobic materials through interfacial airflow manipulation.

Superhydrophobic surfaces demonstrate excellent anti-icing performance under static conditions. However, they show a marked decrease in icing time under real flight conditions. Here we develop an anti-icing strategy using ubiquitous wind field to improve the anti-icing efficiency of superhydrophobic surfaces during flight. We find that the icing mass on hierarchical superhydrophobic surface with a microstructure angle of 30° is at least 40% lower than that on the conventional superhydrophobic plate, which is attributed to the combined effects of microdroplet flow upwelling induced by interfacial airflow and microdroplet ejection driven by superhydrophobic characteristic. Meanwhile, the disordered arrangement of water molecules induced by the specific 30° angle also raises the energy barriers required for nucleation, resulting in an inhibition of the nucleation process. This strategy of microdroplet movement manipulation induced by interfacial airflow is expected to break through the anti-icing limitation of conventional superhydrophobic materials in service conditions and can further reduce the risk of icing on the aircraft surface.

The roles of undercooling degree and materials surface configuration in the growth mechanism of ice layer caused by micro-droplets.

Effect mechanisms of the undercooling degree and the surface configuration on the ice growth characteristics were revealed under micro-droplets icing conditions. Preferential ice crystals appear firstly on the surfaces due to the randomness of icing, and obtain growth advantages to form protruding structures. Protruding structures block the incoming droplets from contacting the substrates, causing voids around the structures. The undercooling degree mainly affects the density and the growth rate of preferential ice crystals. With the increase of undercooling degree, the preferential ice crystals have higher density and growth rate, resulting in stronger growth advantage and higher porosity. The surface configuration affects the growth mode, and the ice layer grows with uniform mode, spreading mode and structure-induced mode on the aluminum, smooth Polytetrafluoroethylene (PTFE) and rough PTFE surface respectively, causing the needle-like, ridge-like and cluster-like ice crystals. The rough structures effectively improve the porosity of the ice layer, which is beneficial for optimizing the icephobic property of the materials. This paper provides important theoretical guidance for the design of subsequent icephobic materials.

Spontaneous Transport Mechanics of Water Droplets under a Synergistic Action of Designed Pattern and Non-Wetting Gradient.

The controllable spontaneous transport of water droplets on solid surfaces has a broad application background in daily life. Herein, a patterned surface with two different non-wetting characteristics was developed to control the droplet transport behavior. Consequently, the patterned surface exhibited great water-repellant properties in the superhydrophobic region, and the water contact angle reached 160° ± 0.2°. Meanwhile, the water contact angle on the wedge-shaped hydrophilic region dropped to 22° after UV irradiation treatment. On this basis, the maximum transport distance of water droplets could be observed on the sample surface with a small wedge angle of 5° (10.62 mm), and the maximum average transport velocity of droplets was obtained on the sample surface with a large wedge angle of 10° (218.01 mm/s). In terms of spontaneous droplet transport on an inclined surface (4°), both the 8 µL droplet and 50 µL droplet could move upward against gravity, which showed that the sample surface possessed an obvious driving force for droplet transport. Surface non-wetting gradient and the wedge-shaped pattern provided unbalanced surface tension to produce the driving forces in the process of droplet transport, and the Laplace pressure as well is produced inside the water droplet during this process. This work provides a new strategy to develop a patterned superhydrophobic surface for droplet transport.

Role of Molecular Chains Arrangement and Surface Energy State in the Low Ice Adhesion on Poly(tetrafluoroethylene).

The relation between polymer molecular chains arrangement and ice adhesion was studied at the molecular scale, and the energy states of water molecules on the poly(tetrafluoroethylene) surface were analyzed to explain the energy essence of ice adhesion. The ice adhesion on crystalline poly(tetrafluoroethylene) displayed a clear anisotropy phenomenon. Further research proved that the energy states of water molecules along the vertical direction of the molecular chains fluctuated regularly, and the water molecules in gaps between molecular chains were in the energy troughs, leading to the formation of energy traps. Water molecules needed more energy from outside to escape the energy traps, causing additional resistance to the ice movement and obvious increase of ice adhesion. Therefore, ice adhesion was closely related to the distribution of energy traps in the direction of ice removing, which mainly depended on the possibility of molecular chains perpendicularly arranged in the direction of ice removing.

Droplet Directional Movement on the Homogeneously Structured Superhydrophobic Surface with the Gradient Non-Wettability.

The surface with the gradient non-wettability intensely appeals to researchers because of its academic significance and applications for directional droplet movement. Herein, we developed a homogeneous structure superhydrophobic surface with the gradient non-wettability by a combination strategy of chemical etching and vapor diffusion modification. As a consequence, the as-prepared surface exhibits a remarkable gradient characteristic of water repellency, and the water contact angle is mainly located within the range of 162 ± 0.5 to 149 ± 0.4°. Meanwhile, the sliding angle also exhibits a corresponding change from 3 to 11°. On this basis, the gradient characteristic of non-wettability induces the distinguishing droplet adhesion on the surface, that is, from 19 µN for the most hydrophobic end to 57 µN for the opposite one. Because of the difference of the water adhesion force, droplets on the as-prepared surface can well roll alongside a specific direction (i.e., gradient direction of non-wettability). In terms of dynamic impact droplets, they can rapidly rebound off the sample surface with the short contact time of 12.8 ms, and the finally fallen droplets mainly deviate toward weaker regions because of water repellency. To analyze this phenomenon, it is found that the asymmetric mechanic behavior is mainly caused by the unbalanced retraction force between the both ends of the impact droplet. This work provides a novel strategy to construct the homogeneous structure superhydrophobic surface with the gradient non-wettability for the applications in the droplet movement control or transport.

Selective nucleation of ice crystals depending on the inclination angle of nanostructures.

Heterogeneous nucleation is decided by many factors, and surface morphology is one of the most important elements. This paper reports the selective ice nucleation and growth process on a series of nanorods with different inclinations, which were rarely mentioned in previous research studies. It is found that the nanorods with special inclinations can cause the selective nucleation of ice crystals because of the spatial geometry matching. On this basis, we can regulate the ice crystal types (mainly including cubic ice and hexagonal ice) accordingly and even improve the freezing efficiency via controlling the inclinations of surface nanorods. In particular, cubic ice occupies the dominant role in the ice crystal on the surface of 45°-inclination nanorods, yet 90°-inclination nanorods are more beneficial for the formation of hexagonal ice. The shape of the nanorods not only controls the type of ice crystal, but also changes the freezing efficiency because different ice crystals have an unequal nucleation energy barrier. There are no apparent differences in the freezing efficiency on nanostructures with 45°, 75° and 90° inclination nanorods, and 60°-inclination nanorods are more favorable for ice nucleation. Our studies can promote the understanding on the selective nucleation of ice crystals and provide a theoretical basis for achieving the regulation of freezing efficiency.

Statistically understanding the roles of nanostructure features in interfacial ice nucleation for enhancing icing delay performance.

Freezing is a spontaneous phase transformation process, which is mainly governed by heterogeneous ice nucleation. This work aims at the discussion of the roles of nanostructure geometrical features in interfacial ice nucleation. Two kinds of superhydrophobic nanostructures with sealed layered porous and open cone features were designed and fabricated by means of wet-chemical processing methods. Both the resultant surfaces exhibited a larger extent of improvement of non-wettability, especially in the aspect of droplet movement. Comparing with the sealed layered nanoporous structures, the open nanocone structures only induced a sliding angle of 1°. During the freezing process, the solid-liquid contact type highly determined the macroscopic freezing process, and resulted in a difference of icing delay time of ∼170 s (and freezing temperature of ∼3.7 °C) between both superhydrophobic nanostructures. Also, the precooling time, the period before the moment of a droplet instantaneously becoming turbid, occupied a dominant role (∼90%) in the entire freezing time. The ice nucleation behavior was analyzed in detail according to the statistical results of 500 cycles of freezing temperatures, demonstrating that the ice nucleation probability of nanocone structures is less than that of layered nanoporous structures. This is in line with the ice nucleation temperatures of both as-prepared superhydrophobic nanostructures. As a consequence, there was a greater distinction in the ice nucleation rate, especially in the solid-liquid interface nucleation rate, by two orders of magnitude.