Pyramid-Shaped Superhydrophobic Surfaces for Underwater Drag Reduction.

Superhydrophobic surfaces hold immense potential in underwater drag reduction. However, as the Reynolds number increases, the drag reduction rate decreases, and it may even lead to a drag increase. The reason lies in the collapse of the air mattress. To address this issue, this paper develops a pyramid-shaped robust superhydrophobic surface with wedged microgrooves, which exhibits a high gas fraction when immersed underwater and good ability to achieve complete spreading and recovery of the air mattress through air replenishment in the case of collapse of the air mattress. Pressure drop tests in a water tunnel confirm that with continuous air injection, the drag reduction reaches 64.8% in laminar flow conditions, substantially greater than 38.4% in the case without air injection, and can achieve 50.8% drag reduction in turbulent flow. This result highlights the potential applications of superhydrophobic surfaces with air mattress recovery for drag reduction.

Water Impalement Resistance and Drag Reduction of the Superhydrophobic Surface with Hydrophilic Strips.

Superhydrophobic surfaces (SHS) offer versatile applications by trapping an air layer within microstructures, while water jet impact can destabilize this air layer and deactivate the functions of the SHS. The current work presents for the first time that introducing parallel hydrophilic strips to SHS (SHS-s) can simultaneously improve both water impalement resistance and drag reduction (DR). Compared with SHS, SHS-s demonstrates a 125% increase in the enduring time against the impact of water jet with velocity of 11.9 m/s and a 97% improvement in DR at a Reynolds number of 1.4 × 104. The key mechanism lies in the enhanced stability of the air layer due to air confinement by the adjacent three-phase contact lines. These lines not only impede air drainage through the surface microstructures during water jet impact, entrapping the air layer to resist water impalement, but also prevent air floating up due to buoyancy in Taylor-Couette flow, ensuring an even spread of the air layer all over the rotor, boosting DR. Moreover, failure modes of SHS under water jet impact are revealed to be related to air layer decay and surface structure destruction. This mass-producible structured surface holds the potential for widespread use in DR for hulls, autonomous underwater vehicles, and submarines.

Multiple configuration transitions of soft actuators under single external stimulus.

Soft actuators have a wide range of applications in medical instruments, soft robotics, 3D electronics, and deployable structures, where configuration transitions are crucial for their function realization. However, most soft actuators can only morph from the initial configuration directly to the final configuration under a single external stimulus. Herein, we report a novel soft actuator by 3D printing parallel strips with crescent cross-sections onto a thin PDMS film. Multiple configuration transitions are observed when the soft actuator swells in ethyl acetate. Four factors, i.e., the geometric asymmetry of the strips, the fabrication-induced heterogeneity of the film, the differential swelling ratios of the strips and the film, and the geometric parameters of the actuator, are demonstrated to synergistically regulate the multiple configuration transitions of the actuator. Particularly, the underlying mechanisms for the configuration transitions are systematically investigated through experiments and theoretical analysis, and verified via finite element simulation. Benefitting from the multiple configuration transitions, the grasp-release-re-grab function of the actuator is demonstrated under a single stimulus. This work contributes to fundamental understanding of the morphing behaviors and the novel design of soft actuators.

Physically Entangled Antiswelling Hydrogels with High Stiffness.

Physically cross-linked hydrogels have great potential for tissue engineering because of their excellent biocompatibility and easy fabrication. However, physical cross-linking points are typically weaker compared to chemical ones and therefore cannot form robust hydrogels with excellent water stability, which greatly hinder their further applications. In this work, a novel hydrogel with high stiffness and outstanding antiswelling performance cross-linked by hydrophobic polymer chains entanglements is reported. The hydrophobic polymer polyimide (PI) is mixed with the hydrophilic polymer poly-(vinyl pyrrolidone) (PVP) to form cross-linking points between the chains. At the equilibrium swelling state, tensile modulus of the hydrogel can be up to 22.57 MPa (higher than most existing hydrogels) and the equilibrium water swelling ratio (ESR) can be as low as 125.0%. By decreasing the PI mass ratio, tensile modulus and ESR of the hydrogel can be tuned in a wide range from 22.57 to 0.005 MPa and 125.0% to 765.6%, respectively. Using PVP/PI solutions as inks, uniform structures and multi-material structures are fabricated having mechanical properties close to cartilage through a direct ink writing 3D printing platform. This current work demonstrates that entangled PVP/PI hydrogels have excellent tailoring capabilities and are promising candidates for tissue engineering applications.

Graphene Oxide Aerogel Foam Constructed All-Solid Electrolyte Membranes for Lithium Batteries.

With the development of electric vehicles and products, lithium metal batteries with solid-state electrolytes have shown a broad application prospect. However, the uneven deposition of lithium, low ion conductivity, narrow electrochemical window, and high interfacial impedance limit the safety and performance of the solid-state batteries. Herein, we develop a non-ceramic solid electrolyte based on the graphene oxide aerogel frame filling with polyethylene oxide (GSPE). The resulting uniform and resilient framework structure form a continuous Li-ion adsorption zone, which ensures uniform ion-current distribution at the interface while obtaining the relatively high ionic conductivity, effectively preventing the uneven deposition of lithium, and thus greatly improving the battery stability. Comprehensive electrochemical analysis showed that GSPE achieved an ionic conductivity of 4.12 × 10-4 S cm-1 at 50 °C. The assembled LiFePO4(LFP) |GSPE| Li full battery can stably cycle for more than 100 cycles at 0.1 C, and the lithium symmetrical battery can continuously be plating-peeling for more than 600 h at 0.1 mA cm-2. The method of using the carbon aerogel structure to achieve the uniform deposition of lithium ions has explored a new possible research direction for all-solid-state batteries.

Origami Spring-Inspired Shape Morphing for Flexible Robotics.

Flexible robotics are capable of achieving various functionalities by shape morphing, benefiting from their compliant bodies and reconfigurable structures. In this study, we construct and study a class of origami springs generalized from the known interleaved origami spring, as promising candidates for shape morphing in flexible robotics. These springs are found to exhibit nonlinear stretch-twist coupling and linear/nonlinear mechanical response in the compression/tension region, analyzed by the demonstrated continuum mechanics models, experiments, and finite element simulations. To improve the mechanical performance such as the damage resistance, we establish an origami rigidization method by adding additional creases to the spring system. Guided by the theoretical framework, we experimentally realize three types of flexible robotics-origami spring ejectors, crawlers, and transformers. These robots show the desired functionality and outstanding mechanical performance. The proposed concept of origami-aided design is expected to pave the way to facilitate the diverse shape morphing of flexible robotics.

Self-Propelled Detachment upon Coalescence of Surface Bubbles.

The removal of microbubbles from substrates is crucial for the efficiency of many catalytic and electrochemical gas evolution reactions in liquids. The current work investigates the coalescence and detachment of bubbles generated from catalytic decomposition of hydrogen peroxide. Self-propelled detachment, induced by the coalescence of two bubbles, is observed at sizes much smaller than those determined by buoyancy. Upon coalescence, the released surface energy is partly dissipated by the bubble oscillations, working against viscous drag. The remaining energy is converted to the kinetic energy of the out-of-plane jumping motion of the merged bubble. The critical ratio of the parent bubble sizes for the jumping to occur is theoretically derived from an energy balance argument and found to be in agreement with the experimental results. The present results provide both physical insight for the bubble interactions and practical strategies for applications in chemical engineering and renewable energy technologies like electrolysis.

Intelligent Shape-Morphing Micromachines.

Intelligent machines are capable of switching shape configurations to adapt to changes in dynamic environments and thus have offered the potentials in many applications such as precision medicine, lab on a chip, and bioengineering. Even though the developments of smart materials and advanced micro/nanomanufacturing are flouring, how to achieve intelligent shape-morphing machines at micro/nanoscales is still significantly challenging due to the lack of design methods and strategies especially for small-scale shape transformations. This review is aimed at summarizing the principles and methods for the construction of intelligent shape-morphing micromachines by introducing the dimensions, modes, realization methods, and applications of shape-morphing micromachines. Meanwhile, this review highlights the advantages and challenges in shape transformations by comparing micromachines with the macroscale counterparts and presents the future outlines for the next generation of intelligent shape-morphing micromachines.

Soft Actuators Based on Liquid-Vapor Phase Change Composites.

Liquid-vapor phase change materials (PCMs), capable of significant volume change, are emerging as attractive actuating components in forming advanced soft composites for robotic applications. However, the novel and functional design of these PCM composites is significantly limited due to the lacking of the fundamental understanding of the mechanical properties, which further inhibits the broad applications of PCM based materials in the engineering structures requiring large deformation and high loading capacity. In this study we fabricate PCM-elastomer composites exhibiting large deformation and high output stress. Thermomechanical properties of these composites are experimentally and theoretically investigated, demonstrating enhanced deformation and loading capacity due to the induced vapor pressure. By controlling the distribution and content of the PCM inclusions, structures with tunable deformability under a relatively small strain in comparison with traditional soft materials are fabricated. Accompanying with the asymmetrical friction and deformation, complex locomotion and adaptable grabbing function are achieved with excellent performance.

Superrepellency of underwater hierarchical structures on Salvinia leaf.

Biomimetic superhydrophobic surfaces display many excellent underwater functionalities, which attribute to the slippery air mattress trapped in the structures on the surface. However, the air mattress is easy to collapse due to various disturbances, leading to the fully wetted Wenzel state, while the water filling the microstructures is difficult to be repelled to completely recover the air mattress even on superhydrophobic surfaces like lotus leaves. Beyond superhydrophobicity, here we find that the floating fern, Salvinia molesta, has the superrepellent capability to efficiently replace the water in the microstructures with air and robustly recover the continuous air mattress. The hierarchical structures on the leaf surface are demonstrated to be crucial to the recovery. The interconnected wedge-shaped grooves between epidermal cells are key to the spontaneous spreading of air over the entire leaf governed by a gas wicking effect to form a thin air film, which provides a base for the later growth of the air mattress in thickness synchronously along the hairy structures. Inspired by nature, biomimetic artificial Salvinia surfaces are fabricated using 3D printing technology, which successfully achieves a complete recovery of a continuous air mattress to exactly imitate the superrepellent capability of Salvinia leaves. This finding will benefit the design principles of water-repellent materials and expand their underwater applications, especially in extreme environments.

Gravitational Effect in Evaporating Binary Microdroplets.

The flow in an evaporating glycerol-water binary submillimeter droplet with a Bond number Bo≪1 is studied both experimentally and numerically. First, we measure the flow fields near the substrate by microparticle image velocimetry for both sessile and pendant droplets during the evaporation process, which surprisingly show opposite radial flow directions-inward and outward, respectively. This observation clearly reveals that in spite of the small droplet size, gravitational effects play a crucial role in controlling the flow fields in the evaporating droplets. We theoretically analyze that this gravity-driven effect is triggered by the lower volatility of glycerol which leads to a preferential evaporation of water then the local concentration difference of the two components leads to a density gradient that drives the convective flow. We show that the Archimedes number Ar is the nondimensional control parameter for the occurrence of the gravitational effects. We confirm our hypothesis by experimentally comparing two evaporating microdroplet systems, namely, a glycerol-water droplet and a 1,2-propanediol-water droplet. We obtain different Ar, larger or smaller than a unit by varying a series of droplet heights, which corresponds to cases with or without gravitational effects, respectively. Finally, we simulate the process numerically, finding good agreement with the experimental results and again confirming our interpretation.

Multimaterial Microfluidic 3D Printing of Textured Composites with Liquid Inclusions.

3D printing with a high degree of spatial and compositional precision could open new avenues to the design and fabrication of functional composites. By combining the direct ink writing and microfluidics, a multimaterial 3D printing system for fabricating textured composites with liquid inclusions of programmable spatial distribution and compositions is reported here. Phase diagrams for the rational selection of desired printing parameters are determined through a combination of simple theoretical analysis and experimental studies. 1D, 2D, and 3D structures programmed with desired inclusion patterns and compositions are fabricated. Moreover, the versatility of this 3D printing framework in fabricating layered composite beams of tunable thermal property and self-healing materials is demonstrated. The proposed multimaterial microfluidic 3D printing framework could be broadly applicable for structural composites and soft robotic devices.

Evaporation-Triggered Segregation of Sessile Binary Droplets.

Droplet evaporation of multicomponent droplets is essential for various physiochemical applications, e.g., in inkjet printing, spray cooling, and microfabrication. In this work, we observe and study the phase segregation of an evaporating sessile binary droplet, consisting of a miscible mixture of water and a surfactantlike liquid (1,2-hexanediol). The phase segregation (i.e., demixing) leads to a reduced water evaporation rate of the droplet, and eventually the evaporation process ceases due to shielding of the water by the nonvolatile 1,2-hexanediol. Visualizations of the flow field by particle image velocimetry and numerical simulations reveal that the timescale of water evaporation at the droplet rim is faster than that of the Marangoni flow, which originates from the surface tension difference between water and 1,2-hexanediol, eventually leading to segregation.

A Modular Microfluidic Device via Multimaterial 3D Printing for Emulsion Generation.

3D-printing (3DP) technology has been developing rapidly. However, limited studies on the contribution of 3DP technology, especially multimaterial 3DP technology, to droplet-microfluidics have been reported. In this paper, multimaterial 3D-printed devices for the pneumatic control of emulsion generation have been reported. A 3D coaxial flexible channel with other rigid structures has been designed and printed monolithically. Numerical and experimental studies have demonstrated that this flexible channel can be excited by the air pressure and then deform in a controllable way, which can provide the active control of droplet generation. Furthermore, a novel modular microfluidic device for double emulsion generation has been designed and fabricated, which consists of three modules: function module, T-junction module, and co-flow module. The function module can be replaced by (1) Single-inlet module, (2) Pneumatic Control Unit (PCU) module and (3) Dual-inlet module. Different modules can be easily assembled for different double emulsion production. By using the PCU module, double emulsions with different number of inner droplets have been successfully produced without complicated operation of flow rates of different phases. By using single and dual inlet module, various double emulsions with different number of encapsulated droplets or encapsulated droplets with different compositions have been successfully produced, respectively.

Liquid-liquid displacement in slippery liquid-infused membranes (SLIMs).

Liquid-infused membranes inspired by slippery liquid-infused porous surfaces (SLIPS) have been recently introduced to membrane technology. The gating mechanism of these membranes is expected to give rise to anti-fouling properties and multi-phase transport capabilities. However, the long-term retention of the infusion liquid has not yet been explored. To address this issue, we investigate the retention of the infusion liquid in slippery liquid-infused membranes (SLIMs) via liquid-liquid displacement porometry (LLDP) experiments combined with microscopic observations of the displacement mechanism. Our results reveal that pores will be opened corresponding to the capillary pressure, leading to preferential flow pathways for water transport. The LLDP results further suggest the presence of liquid-lined pores in SLIM. This hypothesis is analyzed theoretically using an interfacial pore flow model. We find that the displacement patterns correspond to capillary fingering in immiscible displacement in porous media. The related physics regarding two-phase flow in porous media is used to confirm the permeation mechanism appearing in SLIMs. In order to experimentally observe liquid-liquid displacement, a microfluidic chip mimicking a porous medium is designed and a highly ramified structure with trapped infusion liquid is observed. The remaining infusion liquid is retained as pools, bridges and thin films around pillar structures in the chip, which further confirms liquid-lining. Fractal dimension analysis, along with evaluation of the fluid (non-wetting phase) saturation, further confirms that the fractal patterns correspond to capillary fingering, which is consistent with an invasion percolation with trapping (IPT) model.

Growth and Detachment of Oxygen Bubbles Induced by Gold-Catalyzed Decomposition of Hydrogen Peroxide.

Whereas bubble growth out of gas-oversatured solutions has been quite well understood, including the formation and stability of surface nanobubbles, this is not the case for bubbles forming on catalytic surfaces due to catalytic reactions, though it has important implications for gas evolution reactions and self-propulsion of micro/nanomotors fueled by bubble release. In this work we have filled this gap by experimentally and theoretically examining the growth and detachment dynamics of oxygen bubbles from hydrogen peroxide decomposition catalyzed by gold. We measured the bubble radius R(t) as a function of time by confocal microscopy and find R(t) ∝ t1/2. This diffusive growth behavior demonstrates that the bubbles grow from an oxygen-oversaturated environment. For several consecutive bubbles detaching from the same position in a short period of time, a well-repeated growing behavior is obtained from which we conclude the absence of noticeable depletion effect of oxygen from previous bubbles or increasing oversaturation from the gas production. In contrast, for two bubbles far apart either in space or in time, substantial discrepancies in their growth rates are observed, which we attribute to the variation in the local gas oversaturation. The current results show that the dynamical evolution of bubbles is influenced by comprehensive effects combining chemical catalysis and physical mass transfer. Finally, we find that the size of the bubbles at the moment of detachment is determined by the balance between buoyancy and surface tension and by the detailed geometry at the bubble's contact line.

Ultimate Stable Underwater Superhydrophobic State.

Underwater metastability hinders the durable application of superhydrophobic surfaces. In this work, through thermodynamic analysis, we theoretically demonstrate the existence of an ultimate stable state on underwater superhydrophobic surfaces. Such a state is achieved by the synergy of mechanical balance and chemical diffusion equilibrium across the entrapped liquid-air interfaces. By using confocal microscopy, we in situ examine the ultimate stable states on structured hydrophobic surfaces patterned with cylindrical micropores in different pressure and flow conditions. The equilibrium morphology of the meniscus is tuned by the dissolved gas saturation degree within a critical range at a given liquid pressure. Moreover, with fresh lotus leaves, we prove that the ultimate stable state can also be realized on randomly rough superhydrophobic surfaces. The finding here paves the way for applying superhydrophobic surfaces in environments with different liquid pressure and flow conditions.

Evaporation-triggered microdroplet nucleation and the four life phases of an evaporating Ouzo drop.

Evaporating liquid droplets are omnipresent in nature and technology, such as in inkjet printing, coating, deposition of materials, medical diagnostics, agriculture, the food industry, cosmetics, or spills of liquids. Whereas the evaporation of pure liquids, liquids with dispersed particles, or even liquid mixtures has intensively been studied over the past two decades, the evaporation of ternary mixtures of liquids with different volatilities and mutual solubilities has not yet been explored. Here we show that the evaporation of such ternary mixtures can trigger a phase transition and the nucleation of microdroplets of one of the components of the mixture. As a model system, we pick a sessile Ouzo droplet (as known from daily life-a transparent mixture of water, ethanol, and anise oil) and reveal and theoretically explain its four life phases: In phase I, the spherical cap-shaped droplet remains transparent while the more volatile ethanol is evaporating, preferentially at the rim of the drop because of the singularity there. This leads to a local ethanol concentration reduction and correspondingly to oil droplet nucleation there. This is the beginning of phase II, in which oil microdroplets quickly nucleate in the whole drop, leading to its milky color that typifies the so-called "Ouzo effect." Once all ethanol has evaporated, the drop, which now has a characteristic nonspherical cap shape, has become clear again, with a water drop sitting on an oil ring (phase III), finalizing the phase inversion. Finally, in phase IV, all water has evaporated, leaving behind a tiny spherical cap-shaped oil drop.

Receding dynamics of contact lines and size-dependent adhesion on microstructured hydrophobic surfaces.

The microstructure size on textured surfaces of a given solid fraction exhibits an important effect on their properties. To understand the size effect on surface adhesion, we study the receding dynamics of the microscopic three-phase contact lines, the adhesive properties, and the relation between them on microstructured surfaces. Two types of surfaces are used, which are micropillar and micropore, respectively. First, the receding process of the contact line is directly observed by laser scanning confocal microscopy (LSCM), which shows distinct characteristics on the two types of surfaces. The micro contact line experiences pinnning, sliding, and rupture on micropillar-patterned surfaces while no rupture occurs on micropore-patterned surfaces. The three-dimensional morphology of the micromeniscus on the micropillared surfaces and the two-dimensional scanning of the cross-sections of the micromeniscus along the diagonal direction are imaged. Based on the images, the local contact angles around the micropillar at the receding front, and the curvatures of the micro-meniscus are obtained. Then, the adhesive force on these surfaces is measured, which surprisingly shows an increasing trend with the size of the microstructure for micropillared surfaces but no obvious size dependence for micropored surfaces. Wetting hysteresis is also measured to testify the similar trend with the size for the two types of surfaces. Further investigation shows that the monotonic increase of the adhesive force with the increasing size of micropillars is due to the growing difficulty of the detachment of the contact lines. The underlying mechanism responsible for the size dependence of the adhesive force is the enhancement of the local reduced pressure exerted on the top of the micropillar with increasing size, resulting from the concave profile of the outer micromeniscus.

Influence of fluid flow on the stability and wetting transition of submerged superhydrophobic surfaces.

Superhydrophobic surfaces have attracted great attention for drag reduction application. However, these surfaces are subject to instabilities, especially under fluid flow. In this work, we in situ examine the stability and wetting transition of underwater superhydrophobicity under laminar flow conditions by confocal microscopy. The absolute liquid pressure in the flow channel is regulated to acquire the pinned Cassie-Baxter and depinned metastable states. The subsequent dynamic evolution of the meniscus morphology in the two states under shear flow is monitored. It is revealed that fluid flow does not affect the pressure-mediated equilibrium states but accelerates the air exchange between entrapped air cavities and bulk water. A diffusion-based model with varying effective diffusion lengths is used to interpret the experimental data, which show a good agreement. The Sherwood number representing the convection-enhanced mass transfer coefficient is extracted from the data, and is found to follow a classic 1/3-power-law relation with the Reynolds number as has been discovered in channel flows with diffusive boundary conditions. The current work paves the way for designing durable superhydrophobic surfaces under flow conditions.