Advanced Anti-Icing Strategies and Technologies by Macrostructured Photothermal Storage Superhydrophobic Surfaces.

Water is the source of life and civilization, but water icing causes catastrophic damage to human life and diverse industrial processes. Currently, superhydrophobic surfaces (inspired by the lotus effect) aided anti-icing attracts intensive attention due to their energy-free property. Here, recent advances in anti-icing by design and functionalization of superhydrophobic surfaces are reviewed. The mechanisms and advantages of conventional, macrostructured, and photothermal superhydrophobic surfaces are introduced in turn. Conventional superhydrophobic surfaces, as well as macrostructured ones, easily lose the icephobic property under extreme conditions, while photothermal superhydrophobic surfaces strongly rely on solar illumination. To address the above issues, a potentially smart strategy is found by developing macrostructured photothermal storage superhydrophobic (MPSS) surfaces, which integrate the functions of macrostructured superhydrophobic materials, photothermal materials, and phase change materials (PCMs), and are expected to achieve all-day anti-icing in various fields. Finally, the latest achievements in developing MPSS surfaces, showcasing their immense potential, are highlighted. Besides, the perspectives on the future development of MPSS surfaces are provided and the problems that need to be solved in their practical applications are proposed.

Interfacial ice sprouting during salty water droplet freezing.

Icing of seawater droplets is capable of causing catastrophic damage to vessels, buildings, and human life, yet it also holds great potential for enhancing applications such as droplet-based freeze desalination and anti-icing of sea sprays. While large-scale sea ice growth has been investigated for decades, the icing features of small salty droplets remain poorly understood. Here, we demonstrate that salty droplet icing is governed by salt rejection-accompanied ice crystal growth, resulting in freezing dynamics different from pure water. Aided by the observation of brine films emerging on top of frozen salty droplets, we propose a universal definition of freezing duration to quantify the icing rate of droplets having varying salt concentrations. Furthermore, we show that the morphology of frozen salty droplets is governed by ice crystals that sprout from the bottom of the brine film. These crystals grow until they pierce the free interface, which we term ice sprouting. We reveal that ice sprouting is controlled by condensation at the brine film free interface, a mechanism validated through molecular dynamics simulations. Our findings shed light on the distinct physics that govern salty droplet icing, knowledge that is essential for the development of related technologies.

Understanding and Utilizing Droplet Impact on Superhydrophobic Surfaces: Phenomena, Mechanisms, Regulations, Applications, and Beyond.

Droplet impact is a ubiquitous liquid behavior that closely tied to human life and production, making indispensable impacts on the big world. Nature-inspired superhydrophobic surfaces provide a powerful platform for regulating droplet impact dynamics. The collision between classic phenomena of droplet impact and the advanced manufacture of superhydrophobic surfaces is lighting up the future. Accurately understanding, predicting, and tailoring droplet dynamic behaviors on superhydrophobic surfaces are progressive steps to integrate the droplet impact into versatile applications and further improve the efficiency. In this review, the progress on phenomena, mechanisms, regulations, and applications of droplet impact on superhydrophobic surfaces, bridging the gap between droplet impact, superhydrophobic surfaces, and engineering applications are comprehensively summarized. It is highlighted that droplet contact and rebound are two focal points, and their fundamentals and dynamic regulations on elaborately designed superhydrophobic surfaces are discussed in detail. For the first time, diverse applications are classified into four categories according to the requirements for droplet contact and rebound. The remaining challenges are also pointed out and future directions to trigger subsequent research on droplet impact from both scientific and applied perspectives are outlined. The review is expected to provide a general framework for understanding and utilizing droplet impact.

Efficient Photothermal Anti-/Deicing Enabled by 3D Cu2-x S Encapsulated Phase Change Materials Mixed Superhydrophobic Coatings.

Photothermal superhydrophobic surfaces are one of the most promising anti-/deicing materials, yet they are limited by the low energy density and intermittent nature of solar energy. Here, a coupling solution based on microencapsulated phase change materials (MPCMs) that integrates photothermal effect and phase change thermal storage is proposed. Dual-shell octahedral MPCMs with Cu2 O as the first layer and 3D Cu2-x S as the second layer for the first time is designed. By morphology and phase manipulation of the Cu2-x S shell, the local surface plasmonic heating modulation of MPCMs is realized, and the MPCM reveals full-spectrum high absorption with a photothermal conversion efficiency up to 96.1%. The phase change temperature and enthalpy remain in good consistency after 200 cycles. Multifunctional photothermal phase-change superhydrophobic composite coatings are fabricated by combining the hydrolyzed and polycondensation products of octadecyl trichlorosilane and the dual-shell MPCM. The multifunctional coatings exhibit excellent anti-/deicing performance under low temperature and high humidity conditions. This work not only provides a new approach for the design of high-performance MPCMs but also opens up an avenue for the anti-icing application of photothermal phase-change superhydrophobic composite coatings.

Leidenfrost droplet jet engine by bubble ejection.

HYPOTHESIS: Despite the flourishing studies of Leidenfrost droplet motion in its boiling regime, the droplet motion across different boiling regimes has rarely been focused on, where bubbles are generated at the solid-liquid interface. These bubbles are probable to dramatically alter the dynamics of Leidenfrost droplets, creating some intriguing phenomena of droplet motion. EXPERIMENTS: Hydrophilic, hydrophobic, and superhydrophobic substrates with a temperature gradient are designed, and Leidenfrost droplets with diverse fluid types, volumes, and velocities travel from the hot end to the cold end of the substrate. The behaviors of droplet motion across different boiling regimes are recorded and depicted in a phase diagram. FINDINGS: A special phenomenon of Leidenfrost droplets that resembles a jet engine is witnessed on a hydrophilic substrate with a temperature gradient: the droplet traveling across boiling regimes repulsing itself backward. The mechanism of repulsive motion is the reverse thrust from fierce bubble ejection when droplets meet nucleate boiling regime, which cannot take place on hydrophobic and superhydrophobic substrates. We further demonstrate that conflicting droplet motions can occur in similar conditions, and a model is developed to predict the occurring criteria of this phenomenon for droplets in diverse working conditions, which agrees well with the experimental data.

Superhydrophobic Strategy for Nature-Inspired Rotating Microfliers: Enhancing Spreading, Reducing Contact Time, and Weakening Impact Force of Raindrops.

Wind-dispersal of seeds is a remarkable strategy in nature, enlightening the construction of microfliers for environmental monitoring. However, the flight of these microfliers is greatly affected by climatic conditions, especially in rainy days, they suffer serious raindrop impact. Here, a hierarchical superhydrophobic surface is fabricated and a novel strategy is demonstrated that the superhydrophobic coating can enhance spreading while reduce contact time and impact force of raindrops, all of which are beneficial for the rotating microfliers. When the surface rotating speed exceeds a critical value, the effect of centrifugal force becomes considerable so that the droplet spreading is enhanced. The rotating superhydrophobic surface can rotate an impacting droplet by the tangential drag force from the air boundary layer, and the rotation of the droplet generates a negative pressure zone inside it, reducing the contact time by more than 30%. The impact force by the droplet on the rotating superhydrophobic surface also has a remarkable reduction of 53% compared to that on unprocessed hydrophilic surfaces, which helps maintain the flight stability of the microfliers. This work pioneers in revealing the droplet impact effect on rotating microflier surfaces and demonstrates the effectiveness of protecting microfliers with superhydrophobic coatings, which shall guide the manufacture and flight of microfliers in rainy conditions.

Microfluidic Fabrication of a PDMS Microlens for Imaging Tunability.

A microfluidic system was created to fabricate polydimethylsiloxane (PDMS) microspheres, whose shape, surface smoothness, and size were controlled. Resulting from their excellent optical properties and elasticity prepared by the apparatus, each PDMS microsphere could act as a microlens and separate imaging unit. The focal length of the microlens was simply tuned by the forces posed on the beads. For the microlens array (MLA) application, it was constructed simply through the assembly of the monodisperse PDMS beads.

How Superhydrophobic Grooves Drive Single-Droplet Jumping.

Rapid shedding of microdroplets enhances the performance of self-cleaning, anti-icing, water-harvesting, and condensation heat-transfer surfaces. Coalescence-induced droplet jumping represents one of the most efficient microdroplet shedding approaches and is fundamentally limited by weak fluid-substrate dynamics, resulting in a departure velocity smaller than 0.3u, where u is the capillary-inertia-scaled droplet velocity. Laplace pressure-driven single-droplet jumping from rationally designed superhydrophobic grooves has been shown to break conventional capillary-inertia energy transfer paradigms by squeezing and launching single droplets independent of coalescence. However, this interesting droplet shedding mechanism remains poorly understood. Here, we investigate single-droplet jumping from superhydrophobic grooves by examining its dependence upon surface and droplet configurations. Using a volume of fluid (VOF) simulation framework benchmarked with optical visualizations, we verify the Laplace pressure contrast established within the groove-confined droplet that governs single-droplet jumping. An optimal departure velocity of 1.13u is achieved, well beyond what is currently available using condensation on homogeneous or hierarchical superhydrophobic structures. We further develop a jumping/non-jumping regime map in terms of surface wettability and initial droplet volume and demonstrate directional jumping under asymmetric confinement. Our work reveals key fluid-structure interactions required for the tuning of droplet jumping dynamics and guides the design of interfaces and materials for enhanced microdroplet shedding for a plethora of applications.

Contact Time of Droplet Impact on Inclined Ridged Superhydrophobic Surfaces.

Superhydrophobic surfaces decorated with macrostructures have attracted extensive attention due to their excellent performance of reducing the contact time of impacting droplets. In many practical applications, the surface is not perpendicular to the droplet impact direction, but the impacting dynamics in such scenarios still remain mysterious. Here, we experimentally investigate the dynamics of droplet impact on inclined ridged superhydrophobic surfaces and reveal the effect of Wen (the normal Weber number) and α (the inclination angle) on the contact time τ. As Wen increases, τ first decreases rapidly until a platform is reached; if Wen continues to increase, τ further reduces to a lower platform, indicating a three-stage variation of τ in low, middle, and high Wen regions. In the middle and high Wen regions, the contact time is reduced by 30 and 50%, respectively, and is dominated by droplet spreading/retraction in the tangential and lateral directions, respectively. A quantitative analysis demonstrates that τ in the middle and high Wen regions is independent of Wen and α, while the range of middle and high Wen regions is related to α. When α < 30°, increasing α narrows the middle Wen region and enlarges the high Wen region; when α ≥ 30°, the two Wen regions remain unchanged. In addition, droplet sliding is hindered by the friction and is affected by the droplet morphology in the high Wen region. Overall, the synergistic effect of the surface inclination and macrostructures effectively promotes the detachment of impacting droplets on superhydrophobic surfaces, which provides guidance for applications of superhydrophobic surfaces.

Enhanced Moisture Condensation on Hierarchical Structured Superhydrophobic-Hydrophilic Patterned Surfaces.

Patterned surfaces combining hydrophobic and hydrophilic properties show great promise in moisture condensation; however, a comprehensive understanding of the multiscale interfacial behavior and the further controlling method is still lacking. In this paper, we studied the moisture condensation on a hybrid superhydrophobic-hydrophilic surface with hierarchical structures from micro- to nanoscale. For the first time, we demonstrated the effects of wettability difference and microstructure size on the final condensation efficiency. By optimizing the wettability difference, sub-millimeter pattern width, and microstructure size, maximum 90% enhancement of the condensation rate was achieved as compared with the superhydrophobic surface at a subcooling of 13 K. We also demonstrated the enhanced condensation mechanism by a detailed analysis of the condensation process. Our work proposed effective and systematical methods for controlling and optimizing moisture condensation on the patterned surfaces and shed light on application integration of such promising functional surfaces.

Large-Scale Dewetting via Surfactant-Laden Droplet Impact.

The dewetting phenomenon of a liquid film in the presence of a surfactant exists in various natural, industrial, and biomedical processes but still remains mysterious in some specific scenarios. Here, we investigate the dewetting behavior of water films initiated by surfactant-laden droplet impact and show that the maximum dewetting diameter can even reach more than 50 times that of the droplet size. We identify the S-type variation of the dewetting area and demonstrate its correlation to the dynamic surface tension reduction. From a viewpoint of energy conversion, we attribute the dewetting to the released surface energy caused by the surfactant addition and establish a linear relation between the maximum dewetting and the surfactant concentration in the film, i.e., dmax2 ∝ cfilm, which agrees well with the experiments. These results may advance the physics of liquid film dewetting triggered by surfactant injection, which shall further guide practical applications.

Highly efficient desalination performance of carbon honeycomb based reverse osmosis membranes unveiled by molecular dynamics simulations.

Seawater desalination is vital to our modern civilization. Here, we report that the carbon honeycomb (CHC) has an outstanding water permeability and salt rejection in the seawater desalination, as revealed by molecular dynamics simulations. More than 92% of ions are rejected by CHC at applied pressures ranging from 50 to 250 MPa. CHC has a perfect salt rejection at pressures below 150 Mpa. On increasing the applied pressure up to 150 MPa, the salt rejection reduces only to 92%. Pressure, temperature and temperature gradient are noted to play a significant role in modulating the water flux. The water flux increases with pressure and temperature. With the introduction of a temperature gradient of 3.5 K nm-1, the seawater permeability increases by 33% as compared to room temperature. The water permeability of the CHC is greater than other carbon materials and osmosis membranes including graphene (8.7 times) and graphyne (2.1 times). It indicates the significant potential of the CHC for commercial application in water purification.

Axial spreading of droplet impact on ridged superhydrophobic surfaces.

HYPOTHESIS: Due to the complex hydrodynamics of droplet impact on ridged superhydrophobic surfaces, quantitative droplet spreading characteristics are unrevealed, limiting the practical applications of ridged superhydrophobic surfaces. During droplet impacting, the size ratio (the ratio of the ridge diameter to the droplet diameter) is an important factor that affects droplet spreading dynamics. EXPERIMENTS: We fabricated ridged superhydrophobic surfaces with size ratios ranging from zero to one, and conduct water droplet impact experiments on these surfaces at varied Weber numbers. Aided by the numerical simulations and theoretical analysis, we illustrate the droplet spreading dynamics and reveal the law on the maximum axial spreading coefficient. FINDS: The results show that the droplet spreading and retraction dynamics on ridged superhydrophobic surfaces are significantly asymmetric in the axial and spanwise directions. Focusing on the maximum axial spreading coefficient, we find it decreases first and then increases with increasing size ratios, indicating the existence of the critical size ratio. The maximum axial spreading coefficient can be reduced by 25-40% at the critical size ratio compared with that on flat surfaces. To predict the maximum axial spreading coefficient, two theoretical models are proposed respectively for size ratios smaller and larger than the critical size ratio.

Ultimate jumping of coalesced droplets on superhydrophobic surfaces.

HYPOTHESIS: Jumping of coalesced droplets on superhydrophobic surfaces (SHSs) is widely used for enhanced condensation, anti-icing/frosting, and self-cleaning due to its superior droplet transport capability. However, because only a tiny fraction (about 5%) of the released excess surface energy during coalescence can be transformed into jumping kinetic energy, the jumping is very weak, limiting its application. METHODS: We experimentally propose enhanced jumping methods, use machine learning to design structures that achieve ultimate jumping, and finally combine experiments and simulations to investigate the mechanism of the enhanced jumping. FINDING: We find that a more orderly flow inside the droplets through the structure is the key to improve energy transfer efficiency and that the egg tray-like structure enables the droplet to jump with an energy transfer efficiency 10.6 times higher than that of jumping on flat surfaces. This energy transfer efficiency is very close to the theoretical limit, i.e., almost all the released excess surface energy is transformed into jumping kinetic energy after overcoming viscous dissipation. The ultimate jumping enhances the application of water droplet jumping and enables other low surface energy fluid such as R22, R134a, Gasoline, and Ethanol, which cannot jump on a flat surface, to jump.

Patterning a Superhydrophobic Area on a Facile Fabricated Superhydrophilic Layer Based on an Inkjet-Printed Water-Soluble Polymer Template.

An elaborated surface with a superhydrophilic area and a superhydrophobic area was fabricated by inkjet printing a water-soluble polymer template on a superhydrophilic layer. Titanate was used to generate the superhydrophilic layer with an in situ reaction. A water-soluble polymer template was inkjet printed on the facile fabricated superhydrophilic layer. Superhydrophobic treatment was carried out on the inkjet-printed surface with perfluorinated molecules. A superhydrophilic-superhydrophobic patterned surface (SSPS) was obtained by washing out the water-soluble polymer template. Various patterns of SSPS were fabricated with the different water-soluble polymer templates. Then, adhesion and deposition of water droplets were studied on the SSPS with the different wetting abilities on the surface. Meanwhile, a microreaction with a microfluidic chip was realized on the SSPS. In this work, systematic research on fabricating an SSPS based on a facile fabricated superhydrophilic layer with an inkjet-printed water-soluble polymer template is presented. It will have great potential for patterning materials, fabricating devices, and researching interfaces, such as microdroplet self-removal, analyte enrichment, and liquid-liquid interface reaction.

A viable method to enhance the electrical conductivity of CNT bundles: direct in situ TEM evaluation.

Carbon nanotubes (CNTs) exhibit outstanding electrical and mechanical properties, but these superior properties are often compromised as nanotubes are assembled into bulk structures, which limits the use of CNT assemblies. Despite much work in this field, few studies have made in situ observations of the relationship between electrical conductivity and the amount of nanowelding within pristine CNT assemblies at the microscopic scale. Here, we report in situ transmission electron microscopy observations of electrical conductivity increase of CNT bundles. High-temperature Joule heating was applied to a CNT bundle to fuse adjacent carbon nanofibers with graphitic carbon bonds, as this causes the electrical conductivity of the CNT bundle to increase three orders of magnitude. Apart from the welding process of the cross-over CNT bundles, we further observed a new case of welding process of parallel CNT bundles. Here, we not only obtain the relationship between electrical conductivity of CNT bundles and their merging processes, but also show the effect of the relationship between electrical conductivity and Joule-heating induced temperature on CNT bundles, which follows the natural logarithm law. Improving effective inter-bonding between neighboring nanotubes would help facilitate large-scale development of high-performing, bulk-carbon-based materials from nanostructures in applications such as flexible devices, energy storage, and electrocatalysis.

Directional Transportation of Impacting Droplets on Wettability-Controlled Surfaces.

Although a superhydrophobic surface could realize rapid rebounding (i.e., short contact time) of an orthogonal impacting droplet, the rebounding along the original impacting route may limit its engineering application; in contrast, the directional transportation seems to be more promising. Here, we achieve directional transportation of a droplet impacting a wettability-controlled surface. When the droplet eccentrically impacts on the boundary between the superhydrophobic part and the hydrophilic part, it undergoes spreading, retracting, departure, throwing, and breaking up stages, and finally bounces off directionally. The directional transportation distance could even reach more than ten times the droplet size, considered the adhesion length (i.e., covering length on the hydrophilic part by the droplet at the maximum spreading) is optimized. However, there is a critical adhesion length, above which the directional transportation does not occur. To be more generalized, the adhesion length is de-dimensionalized by the maximum spreading radius, and the results show that as the dimensionless adhesion length increases, the transportation distance first increases and then decreases to zero. Under the present impacting conditions, the optimal dimensionless adhesion length corresponding to the maximum transportation distance is near 0.4, and the critical dimensionless adhesion length is about 0.7. These results provide a fundamental understanding of droplet directional transportation and could be useful for related engineering applications.

Fabricating High-Resolution Metal Pattern with Inkjet Printed Water-Soluble Sacrificial Layer.

The metal pattern plays a crucial role in various optoelectronic devices. However, fabrication of high-resolution metal patterns has serious problems including complicated techniques and high cost. Herein, an inkjet printed water-soluble sacrificial layer was proposed to fabricate a high-resolution metal pattern. The water-soluble sacrificial layer was inkjet printed on a polyethylene glycol terephthalate (PET) surface, and then the printed surface was deposited with a metal layer by evaporating deposition. When the deposited surface was rinsed by water, the metal layer deposited on the water-soluble sacrificial layer could be removed. Various high-resolution metal patterns were prepared, which could be used in electroluminescent displays, strain sensors, and 3D switches. This facile method could be a promising approach for fabricating high-resolution metal patterns.

Departure Velocity of Rolling Droplet Jumping.

Droplet jumping phenomenon widely exists in the fields of self-cleaning, antifrosting, and heat transfer enhancement. Numerous studies have been reported on the static droplet jumping while the rolling droplet jumping still remains unnoticed even though it is very common in practice. Here, we used the volume of fluid (VOF) method to simulate the droplet jumping induced by coalescence of a rolling droplet and a stationary one with corresponding experiments conducted to validate the correctness of the simulation model. The departure velocity of the jumping droplet was the main concerned here. The results show that when the center velocity of the rolling droplet (V0 = ωR, where ω is the angular velocity of the rolling droplet and R is the droplet radius) is fixed, the vertical departure velocity satisfies a power law which can be expressed as Vz,depar = aRb. When the droplet radius is fixed, the vertical departure velocity first decreases and then increases if the center velocity exceeds a critical value. Interestingly, the critical center velocity is demonstrated to be approximately 0.76 times the capillary-inertial velocity, corresponding to a constant Weber number of 0.58. Different from the vertical departure velocity, the horizontal departure velocity is basically proportional to the center velocity of the rolling droplet. These results deepen the understanding of the droplet jumping physics, which shall further promote related applications in engineering fields.

Controllable fabrication of uniform ruthenium phosphide nanocrystals for the hydrogen evolution reaction.

Uniform ruthenium phosphide (Ru2P and RuP) nanocrystals grown in situ on N- and P-codoped graphene were obtained by pyrolyzing tris(triphenylphosphine) ruthenium(ii) chloride (TPP-Ru) with pyritic acid (PA). Interestingly, Ru2P and RuP can be controllably prepared by varying the molar ratio of PA to TPP-Ru. As an efficient electrocatalyst for the hydrogen evolution reaction (HER), Ru2P exhibited a better performance than RuP in alkaline medium.