Functional nanoporous graphene superlattice.

Two-dimensional (2D) superlattices, formed by stacking sublattices of 2D materials, have emerged as a powerful platform for tailoring and enhancing material properties beyond their intrinsic characteristics. However, conventional synthesis methods are limited to pristine 2D material sublattices, posing a significant practical challenge when it comes to stacking chemically modified sublattices. Here we report a chemical synthesis method that overcomes this challenge by creating a unique 2D graphene superlattice, stacking graphene sublattices with monodisperse, nanometer-sized, square-shaped pores and strategically doped elements at the pore edges. The resulting graphene superlattice exhibits remarkable correlations between quantum phases at both the electron and phonon levels, leading to diverse functionalities, such as electromagnetic shielding, energy harvesting, optoelectronics, and thermoelectrics. Overall, our findings not only provide chemical design principles for synthesizing and understanding functional 2D superlattices but also expand their enhanced functionality and extensive application potential compared to their pristine counterparts.

Emergent Collective Motion of Self-Propelled Condensate Droplets.

Recently, there is much interest in droplet condensation on soft or liquid or liquidlike substrates. Droplets can deform soft and liquid interfaces resulting in a wealth of phenomena not observed on hard, solid surfaces (e.g., increased nucleation, interdroplet attraction). Here, we describe a unique collective motion of condensate water droplets that emerges spontaneously when a solid substrate is covered with a thin oil film. Droplets move first in a serpentine, self-avoiding fashion before transitioning to circular motions. We show that this self-propulsion (with speeds in the 0.1-1 mm s^{-1} range) is fueled by the interfacial energy release upon merging with newly condensed but much smaller droplets. The resultant collective motion spans multiple length scales from submillimeter to several centimeters, with potentially important heat-transfer and water-harvesting applications.

Visualization and Experimental Characterization of Wrapping Layer Using Planar Laser-Induced Fluorescence.

Droplets on nanotextured oil-impregnated surfaces have high mobility due to record-low contact angle hysteresis (∼1-3°), attributed to the absence of solid-liquid contact. Past studies have utilized the ultralow droplet adhesion on these surfaces to improve condensation, reduce hydrodynamic drag, and inhibit biofouling. Despite their promising utility, oil-impregnated surfaces are not fully embraced by industry because of the concern for lubricant depletion, the source of which has not been adequately studied. Here, we use planar laser-induced fluorescence (PLIF) to not only visualize the oil layer encapsulating the droplet (aka wrapping layer) but also measure its thickness since the wrapping layer contributes to lubricant depletion. Our PLIF visualization and experiments show that (a) due to the imbalance of interfacial forces at the three-phase contact line, silicone oil forms a wrapping layer on the outer surface of water droplets, (b) the thickness of the wrapping layer is nonuniform both in space and time, and (c) the time-average thickness of the wrapping layer is ∼50 ± 10 nm, a result that compares favorably with our scaling analysis (∼50 nm), which balances the curvature-induced capillary force with the intermolecular van der Waals forces. Our experiments show that, unlike silicone oil, mineral oil does not form a wrapping layer, an observation that can be exploited to mitigate oil depletion of nanotextured oil-impregnated surfaces. Besides advancing our mechanistic understanding of the wrapping oil layer dynamics, the insights gained from this work can be used to quantify the lubricant depletion rate by pendant droplets in dropwise condensation and water harvesting.

Droplet attraction and coalescence mechanism on textured oil-impregnated surfaces.

Droplets residing on textured oil-impregnated surfaces form a wetting ridge due to the imbalance of interfacial forces at the contact line, leading to a wealth of phenomena not seen on traditional lotus-leaf-inspired non-wetting surfaces. Here, we show that the wetting ridge leads to long-range attraction between millimeter-sized droplets, which coalesce in three distinct stages: droplet attraction, lubricant draining, and droplet merging. Our experiments and model show that the magnitude of the velocity and acceleration at which droplets approach each other horizontally is the same as the vertical oil rise velocity and acceleration in the wetting ridge. Moreover, the droplet coalescence mechanism can be modeled using the classical mass-spring system. The insights gained from this work will inform future fundamental studies on remote droplet interaction on textured oil-impregnated surfaces for optimizing water harvesting and condensation heat transfer.

Staggered circular nanoporous graphene converts electromagnetic waves into electricity.

Harvesting largely ignored and wasted electromagnetic (EM) energy released by electronic devices and converting it into direct current (DC) electricity is an attractive strategy not only to reduce EM pollution but also address the ever-increasing energy crisis. Here we report the synthesis of nanoparticle-templated graphene with monodisperse and staggered circular nanopores enabling an EM-heat-DC conversion pathway. We experimentally and theoretically demonstrate that this staggered nanoporous structure alters graphene's electronic and phononic properties by synergistically manipulating its intralayer nanostructures and interlayer interactions. The staggered circular nanoporous graphene exhibits an anomalous combination of properties, which lead to an efficient absorption and conversion of EM waves into heat and in turn an output of DC electricity through the thermoelectric effect. Overall, our results advance the fundamental understanding of the structure-property relationships of ordered nanoporous graphene, providing an effective strategy to reduce EM pollution and generate electric energy.

Synergistic Benefits of Micro/Nanostructured Oil-Impregnated Surfaces in Reducing Fouling while Enhancing Heat Transfer.

Liquid-liquid heat exchangers that operate in marine environments are susceptible to biofouling, which decreases the overall heat exchange between hot and cold liquids by increasing the conduction resistance. Recently, micro/nanostructured oil-impregnated surfaces have been shown to significantly reduce biofouling. However, their potential as a heat exchanger material has not been studied. Neither is it obvious since the oil used for impregnation increases the wall thickness and the associated conduction resistance. Here, by conducting extensive field and laboratory studies supported by theoretical modeling of heat transfer in oil-infused heat exchanger tubes, we report the synergistic benefits of micro/nanostructured oil-impregnated surfaces for reducing biofouling while maintaining good heat transfer. These benefits justify the use of lubricant-infused surfaces as heat exchanger materials, in particular in marine environments.

Unique ice dendrite morphology on state-of-the-art oil-impregnated surfaces.

Due to its multifaceted impact in various applications, icing and ice dendrite growth has been the focus of numerous studies in the past. Dendrites on wetting (hydrophilic) and nonwetting (hydrophobic) surfaces are sharp, pointy, branching, and hairy. Here, we show a unique dendrite morphology on state-of-the-art micro/nanostructured oil-impregnated surfaces, which are commonly referred to as slippery liquid-infused porous surfaces or liquid-infused surfaces. Unlike the dendrites on traditional textured hydrophilic and hydrophobic surfaces, the dendrites on oil-impregnated surfaces are thick and lumpy without pattern. Our experiments show that the unique ice dendrite morphology on lubricant-infused surfaces is due to oil wicking into the porous dendritic network because of the capillary pressure imbalance between the surface texture and the dendrites. We characterized the shape complexity of the ice dendrites using fractal analysis. Experiments show that ice dendrites on textured oil-impregnated surfaces have lower fractal dimensions than those on traditional lotus leaf-inspired air-filled porous structures. Furthermore, we developed a regime map that can be used as a design guideline for micro/nanostructured oil-impregnated surfaces by capturing the complex effects of oil chemistry, oil viscosity, and wetting ridge volume on dendrite growth and morphology. The insights gained from this work inform strategies to reduce lubricant depletion, a major bottleneck for the transition of micro/nanostructured oil-impregnated surfaces from bench-top laboratory prototypes to industrial use. This work will assist the development of next-generation depletion-resistant lubricant-infused ice-repellent surfaces.

Wettability-based ultrasensitive detection of amphiphiles through directed concentration at disordered regions in self-assembled monolayers.

Various forms of ecological monitoring and disease diagnosis rely upon the detection of amphiphiles, including lipids, lipopolysaccharides, and lipoproteins, at ultralow concentrations in small droplets. Although assays based on droplets' wettability provide promising options in some cases, their reliance on the measurements of surface and bulk properties of whole droplets (e.g., contact angles, surface tensions) makes it difficult to monitor trace amounts of these amphiphiles within small-volume samples. Here, we report a design principle in which self-assembled monolayer-functionalized microstructured surfaces coated with silicone oil create locally disordered regions within a droplet's contact lines to effectively concentrate amphiphiles within the areas that dominate the droplet static friction. Remarkably, such surfaces enable the ultrasensitive, naked-eye detection of amphiphiles through changes in the droplets' sliding angles, even when the concentration is four to five orders of magnitude below their critical micelle concentration. We develop a thermodynamic model to explain the partitioning of amphiphiles at the contact line by their cooperative association within the disordered, loosely packed regions of the self-assembled monolayer. Based on this local analyte concentrating effect, we showcase laboratory-on-a-chip surfaces with positionally dependent pinning forces capable of both detecting industrially and biologically relevant amphiphiles (e.g., bacterial endotoxins), as well as sorting aqueous droplets into discrete groups based on their amphiphile concentrations. Furthermore, we demonstrate that the sliding behavior of amphiphile-laden aqueous droplets provides insight into the amphiphile's effective length, thereby allowing these surfaces to discriminate between analytes with highly disparate molecular sizes.

Stimuli-Responsive Liquid-Crystal-Infused Porous Surfaces for Manipulation of Underwater Gas Bubble Transport and Adhesion.

Biomimetic artificial surfaces that enable the manipulation of gas bubble mobility have been explored in a wide range of applications in nanomaterial synthesis, surface defouling, biomedical diagnostics, and therapeutics. Although many superhydrophobic surfaces and isotropic-lubricant-infused porous surfaces have been developed to manipulate gas bubbles, the simultaneous control over the adhesion and transport of gas bubbles underwater remains a challenge. Thermotropic liquid crystals (LCs), a class of structured fluids, provide an opportunity to tune the behavior of gas bubbles through LC mesophase transitions using a variety of external stimuli. Using this central idea, the design and synthesis of LC-infused porous surfaces (LCIPS) is reported and the effects of the LC mesophase on the transport and adhesion of gas bubbles on LCIPS immersed in water elucidated. LCIPS are demonstrated to be a promising class of surfaces with an unprecedented level of responsiveness and functionality, which enables the design of cyanobacteria-inspired object movement, smart catalysts, and bubble gating devices to sense and sort volatile organic compounds and control oxygen levels in biomimetic cell cultures.

Biomass-Derived Carbon Heterostructures Enable Environmentally Adaptive Wideband Electromagnetic Wave Absorbers.

Although advances in wireless technologies such as miniature and wearable electronics have improved the quality of our lives, the ubiquitous use of electronics comes at the expense of increased exposure to electromagnetic (EM) radiation. Up to date, extensive efforts have been made to develop high-performance EM absorbers based on synthetic materials. However, the design of an EM absorber with both exceptional EM dissipation ability and good environmental adaptability remains a substantial challenge. Here, we report the design of a class of carbon heterostructures via hierarchical assembly of graphitized lignocellulose derived from bamboo. Specifically, the assemblies of nanofibers and nanosheets behave as a nanometer-sized antenna, which results in an enhancement of the conductive loss. In addition, we show that the composition of cellulose and lignin in the precursor significantly influences the shape of the assembly and the formation of covalent bonds, which affect the dielectric response-ability and the surface hydrophobicity (the apparent contact angle of water can reach 135°). Finally, we demonstrate that the obtained carbon heterostructure maintains its wideband EM absorption with an effective absorption frequency ranging from 12.5 to 16.7 GHz under conditions that simulate the real-world environment, including exposure to rainwater with slightly acidic/alkaline pH values. Overall, the advances reported in this work provide new design principles for the synthesis of high-performance EM absorbers that can find practical applications in real-world environments.

Liquid crystal-based open surface microfluidics manipulate liquid mobility and chemical composition on demand.

The ability to control both the mobility and chemical compositions of microliter-scale aqueous droplets is an essential prerequisite for next-generation open surface microfluidics. Independently manipulating the chemical compositions of aqueous droplets without altering their mobility, however, remains challenging. In this work, we address this challenge by designing a class of open surface microfluidic platforms based on thermotropic liquid crystals (LCs). We demonstrate, both experimentally and theoretically, that the unique positional and orientational order of LC molecules intrinsically decouple cargo release functionality from droplet mobility via selective phase transitions. Furthermore, we build sodium sulfide­loaded LC surfaces that can efficiently precipitate heavy metal ions in sliding water droplets to final concentration less than 1 part per million for more than 500 cycles without causing droplets to become pinned. Overall, our results reveal that LC surfaces offer unique possibilities for the design of novel open surface fluidic systems with orthogonal functionalities.

Enhanced condensation heat transfer using porous silica inverse opal coatings on copper tubes.

Phase-change condensation is commonplace in nature and industry. Since the 1930s, it is well understood that vapor condenses in filmwise mode on clean metallic surfaces whereas it condenses by forming discrete droplets on surfaces coated with a promoter material. In both filmwise and dropwise modes, the condensate is removed when gravity overcomes pinning forces. In this work, we show rapid condensate transport through cracks that formed due to material shrinkage when a copper tube is coated with silica inverse opal structures. Importantly, the high hydraulic conductivity of the cracks promote axial condensate transport that is beneficial for condensation heat transfer. In our experiments, the cracks improved the heat transfer coefficient from ≈ 12 kW/m2 K for laminar filmwise condensation on smooth clean copper tubes to ≈ 80 kW/m2 K for inverse opal coated copper tubes; nearly a sevenfold increase from filmwise condensation and identical enhancement with state-of-the-art dropwise condensation. Furthermore, our results show that impregnating the porous structure with oil further improves the heat transfer coefficient by an additional 30% to ≈ 103 kW/m2 K. Importantly, compared to the fast-degrading dropwise condensation, the inverse opal coated copper tubes maintained high heat transfer rates when the experiments were repeated > 20 times; each experiment lasting 3-4 h. In addition to the new coating approach, the insights gained from this work present a strategy to minimize oil depletion during condensation from lubricated surfaces.

Why Are Water Droplets Highly Mobile on Nanostructured Oil-Impregnated Surfaces?

Porous lubricated surfaces (aka slippery liquid-infused porous surfaces, SLIPS) have been demonstrated to repel various liquids. The origin of this repellency, however, is not fully understood. By using surface-sensitive sum frequency generation vibrational spectroscopy, we characterized the water/oil interface of a water droplet residing on (a) an oil-impregnated nanostructured surface (SLIPS) and (b) the same oil layer without the underlying nanostructures. Different from water molecules in contact with bulk oil without nanostructures, droplets on SLIPS adopt a molecular orientation that is predominantly parallel to the water/oil interface, leading to weaker hydrogen bonding interactions between water droplets and the lubrication film, giving SLIPS their water repellency. Our results demonstrate that the molecular interactions between two contacting liquids can be manipulated by the implementation of nanostructured substrates. The results also offer the molecular principles for controlling nanostructure to reduce oil depletion-one of the limitations and major concerns of SLIPS.

Depletion of Lubricant from Nanostructured Oil-Infused Surfaces by Pendant Condensate Droplets.

Due to recent advances in nanofabrication, phase-change condensation heat transfer has seen a renaissance. Compared to conventional heat transfer surfaces, nanostructured surfaces impregnated with chemically matched lubrication films (hereinafter referred to as "nanostructured lubricated surfaces") have been demonstrated to improve vapor-side phase-change condensation heat transfer by facilitating droplet nucleation, growth, and departure. While the presence of nanoscale roughness improves performance longevity by stabilizing the lubrication film via capillary forces, such enhancement is short-lived due to the eventual loss of lubrication oil by the departing droplets. The objective of this study is to characterize oil depletion caused by pendant droplets during condensation. For our study, we nanostructured, chemically functionalized, and lubricated horizontal copper tubes that are widely used in shell-and-tube heat exchangers in power plants and process industries. Using high-speed fluorescence imaging and thermogravimetric analysis, we show that shedding droplets exert a shear force on the oil in the wetting ridge at the water-oil interface. The viscous shear draws the lubrication film from the nanostructured surface onto the upper portion of the droplet and forms a ring-like oil skirt. Through detailed theoretical analysis, we show that the thickness of this oil skirt scales with the classical Landau-Levich-Derjaguin (LLD) theory for dip-coating. Our results reveal that droplets falling from horizontal tubes break unequally and leave behind small satellite droplets that retain the bulk of the oil in the wetting ridge. This observation is in stark contrast with the earlier description of droplets shedding from tilted flat plates where the entire oil-filled wetting ridge is demonstrated to leave the surface upon droplet departure. By selecting lubrication oils of varying viscosity and spreading coefficient, we provide evidence that the contribution of the wrapping layer to the rate of oil depletion is insignificant. Furthermore, we show that due to the nanoscale features on the tubes, nearly half of the lubrication film remains on the surface after 10 h of continuous steam condensation at ambient pressure, 23 °C, and 60% relative humidity, a 2-3-fold improvement over previous results.The insights gained from this work will provide guidelines for the rational design of long-lasting nanostructured lubricated surfaces for phase-change condensation.

Dynamic Evolution of the Evaporating Liquid-Vapor Interface in Micropillar Arrays.

Capillary assisted passively pumped thermal management devices have gained importance due to their simple design and reduction in energy consumption. The performance of these devices is strongly dependent on the shape of the curved interface between the liquid and vapor phases. We developed a transient laser interferometry technique to investigate the evolution of the shape of the liquid-vapor interface in micropillar arrays during evaporation heat transfer. Controlled cylindrical micropillar arrays were fabricated on the front side of a silicon wafer, while thin-film heaters were deposited on the reverse side to emulate a heat source. The shape of the meniscus was determined using the fringe patterns resulting from interference of a monochromatic beam incident on the thin liquid layer. We studied the evolution of the shape of the meniscus on these surfaces under various operating conditions including varying the micropillar geometry and the applied heating power. By monitoring the transient behavior of the evaporating liquid-vapor interface, we accurately measured the absolute location and shape of the meniscus and calculated the contact angle and the maximum capillary pressure. We demonstrated that the receding contact angle which determines the capillary pumping limit is independent of the microstructure geometry and the rate of evaporation (i.e., the applied heating power). The results of this study provide fundamental insights into the dynamic behavior of the liquid-vapor interface in wick structures during phase-change heat transfer.

High-resolution liquid patterns via three-dimensional droplet shape control.

Understanding liquid dynamics on surfaces can provide insight into nature's design and enable fine manipulation capability in biological, manufacturing, microfluidic and thermal management applications. Of particular interest is the ability to control the shape of the droplet contact area on the surface, which is typically circular on a smooth homogeneous surface. Here, we show the ability to tailor various droplet contact area shapes ranging from squares, rectangles, hexagons, octagons, to dodecagons via the design of the structure or chemical heterogeneity on the surface. We simultaneously obtain the necessary physical insights to develop a universal model for the three-dimensional droplet shape by characterizing the droplet side and top profiles. Furthermore, arrays of droplets with controlled shapes and high spatial resolution can be achieved using this approach. This liquid-based patterning strategy promises low-cost fabrication of integrated circuits, conductive patterns and bio-microarrays for high-density information storage and miniaturized biochips and biosensors, among others.

Non-wetting droplets on hot superhydrophilic surfaces.

Controlling wettability by varying surface chemistry and roughness or by applying external stimuli is of interest for a wide range of applications including microfluidics, drag reduction, self-cleaning, water harvesting, anti-corrosion, anti-fogging, anti-icing and thermal management. It has been well known that droplets on textured hydrophilic, that is superhydrophilic, surfaces form thin films with near-zero contact angles. Here we report an unexpected behaviour where non-wetting droplets are formed by slightly heating superhydrophilic microstructured surfaces beyond the saturation temperature (>5 °C). Although such behaviour is generally not expected on superhydrophilic surfaces, an evaporation-induced pressure in the structured region prevents wetting. In particular, the increased thermal conductivity and decreased vapour permeability of the structured region allows this behaviour to be observed at such low temperatures. This phenomenon is distinct from the widely researched Leidenfrost and offers an expanded parametric space for fabricating surfaces with desired temperature-dependent wettability.

Unified model for contact angle hysteresis on heterogeneous and superhydrophobic surfaces.

Understanding the complexities associated with contact line dynamics on chemically heterogeneous and superhydrophobic surfaces is important for a wide variety of engineering problems. Despite significant efforts to capture the behavior of a droplet on these surfaces over the past few decades, modeling of the complex dynamics at the three-phase contact line is needed. In this work, we demonstrate that contact line distortion on heterogeneous and superhydrophobic surfaces is the key aspect that needs to be accounted for in the dynamic droplet models. Contact line distortions were visualized and modeled using a thermodynamic approach to develop a unified model for contact angle hysteresis on chemically heterogeneous and superhydrophobic surfaces. On a surface comprised of discrete wetting defects on an interconnected less wetting area, the advancing contact angle was determined to be independent of the defects, while the relative fraction of the distorted contact line with respect to the baseline surface was shown to govern the receding contact angle. This behavior reversed when the relative wettability of the discrete defects and interconnected area was inverted. The developed model showed good agreement with the experimental advancing and receding contact angles, both at low and high solid fractions. The thermodynamic model was further extended to demonstrate its capability to capture droplet shape evolution during liquid addition and removal in our experiments and those in literature. This study offers new insight extending the fundamental understanding of solid-liquid interactions required for design of advanced functional coatings for microfluidics, biological, manufacturing, and heat transfer applications.