Transport Of Water Droplets Research Articles

Engineered Switchable‐Wettability Surfaces for Multi‐Path Directional Transportation of Droplets and Subaqueous Bubbles (Adv. Mater. 9/2023)

Microfluidics In article number 2208645, Dongdong Xie, Faheng Zang, Guifu Ding, and co-workers propose a 2D dumbbell-patterned functional surface with switchable wettability to achieve multipath parallel directional transportation of both water droplets and subaqueous bubbles. This programmable, easy-to-fabricate, highly efficient, and cost-effective fluidic system with strong robustness against structural defects is ready for versatile fluid transportation supporting both liquid- and gas-phase matter.

Oil-Grafted Track-Assisted Directional Transport of Water Droplets and Submerged Air Bubbles on Solid Surfaces

Wettability-tailored tracks are emerging as an efficient approach to collecting and transporting underwater air bubbles as well as water from the mist. However, tailoring the surface wettability by modifying the surface structural features via physiochemical methods to create superhydrophilic-superhydrophobic contrast tracks suffers from long-term durability issues, while the emerging liquid-infused slippery surface has inherent design engineering limitations and issues from infused oil depletion. Herein, we demonstrate that by selective silicone oil grafting onto the glass substrate, it is possible to create a wettability contrast of ∼ 43°. Further, we illustrate the application of such tracks for underwater air bubble capturing and transportation in an aqueous medium with surface tension ranging from 72 to 43.5 mN/m. In addition, the potential of these nonadhesive and adhesive tracks for water collection from the mist is shown and the critical effect of the track dimension and intertrack spacing on the water harvesting rate is investigated in detail. The study illustrates that the nonadhesive nature of the oil-grafted region enables the easy transport of underwater air bubbles as well as water from the flow medium and thus offers an easy and facile approach to creating substrates for underwater air bubble collection and water harvesting.

Design and construction of a Laplace and wettability gradient field for efficient water collection.

A hydrophilic-hydrophobic hybrid surface with a vein-like pattern was prepared on a copper substrate using 3D printing technology and laser scanning technology. Under the action of the Laplace pressure gradient and wettability gradient, the superhydrophilic (SHL) vein-like pattern on the superhydrophobic (SHB) surface aided the directional transport of water droplets. The presented scheme combined with the wettability and surface pattern could achieve a water-collection efficiency of 4258.59 mg cm-2 h-1.

Adaptive IR- and Water-Gating Textiles Based on Shape Memory Fibers.

Thermoregulation is an essential function of the human body for adapting to the surrounding temperature. Stimuli-responsive smart textiles can provide effective protection of the human skin temperature from a continuously changing environment. Herein, we develop a smart textile based on shape memory polymer (SMP) fibers for adaptive regulation of IR and water transmission on human skin. An SMP textile is fabricated with hierarchical micro/nanoporous structures to enhance thermal insulation performance, and silver nanowires are coated on one side to provide asymmetric IR reflectivity and hydrophilicity. The porous SMP textile shows great tunability of thermal insulation and asymmetric wettability by deformation and recovery of the shape and structure in response to stimuli. The degree of thermal insulation is controlled by 65.7% of the original value, and the surface temperature of the SMP textile on a hot plate is successfully controlled in the IR images due to adaptive IR reflectivity. Additionally, the directional transportation of water droplets can be switched on/off according to the shape of the SMP textiles, which can be employed for sweat removal from the human skin. This IR- and water-gating smart textile can provide a feasible strategy for protecting the human skin from external environmental changes.

CuO nanomesh hierarchical structure for directional water droplet transport and efficient fog collection

Collecting water from fog is a promising approach to solve the shortage of water resources by micro-nano scale hierarchical structure material surface. Inspired by the structures of cactus and pine needles, CuO micro-nano scale structures attached to the surface of Cu foam are designed to increase the efficiency of fog collection and droplet transport. The structural evolution from CuO nanowires to CuO nanomesh and CuO nanosheets is achieved by regulating the synthesis conditions. The results show that the synthesized CuO nanomesh hierarchical structure is more favorable for fog collection, and the collection efficiency can achieve a rate of up to 34.6 mg/cm2 s. The tip structure can rapidly capture and coalesce small droplets in a foggy environment, which significantly improves the fog collection efficiency. In addition, the synthesized mesh structure and inter-neighborhood gaps have a strong ability to confine air, making the surface has small adhesion and enabling water droplets to fall quickly, which realizes the transport of the collected water efficiently and sustainably. The research work opens a new path for constructing surface structures to improve fog collection performance and has reference value and promotion for the design and development of fog collectors.

Investigation on the Anisotropic Wetting Properties of Water Droplets on Bio-Inspired Groove Structures Fabricated by 3D Printing and Surface Modifications.

The self-driving structure to orientate the water movement has attracted considerable attention. Inspired by the wedgelike structures of biological materials in nature, such as spider silks and cactus spines, anisotropic spreading can be realized by combining Laplace pressure gradient and hydrophilic surface. In this study, a series of groove patterns were fabricated by a combination of 3D printing and surface modification. PLA pattern was modified by the atmospheric pressure plasma, followed by grafting with hydrolyzed APTES. This work reports the anisotropic transport of water droplets on a series of designed dart-shaped groove patterns with specific angles in the main arrow and tail regions. This structure can induce capillary force to regulate droplets from the main cone to two wedgelike, whereas the droplets are hindered toward the opposite side is oat the vicinity of the groove's tail. By means of the experiment, the mechanism of water transport in this pattern was revealed. This study can contribute a potential approach to manipulate and apply anisotropic wetting in many fields.

Force-triggered rapid microstructure growth on hydrogel surface for on-demand functions

Living organisms share the ability to grow various microstructures on their surface to achieve functions. Here we present a force stamp method to grow microstructures on the surface of hydrogels based on a force-triggered polymerisation mechanism of double-network hydrogels. This method allows fast spatial modulation of the morphology and chemistry of the hydrogel surface within seconds for on-demand functions. We demonstrate the oriented growth of cells and directional transportation of water droplets on the engineered hydrogel surfaces. This force-triggered method to chemically engineer the hydrogel surfaces provides a new tool in addition to the conventional methods using light or heat, and will promote the wide application of hydrogels in various fields.

Light-Driven Liquid Conveyors: Manipulating Liquid Mobility and Transporting Solids on Demand.

The intelligent transport of materials at interfaces is essential for a wide range of processes, including chemical microreactions, bioanalysis, and microfabrication. Both passive and active methods have been used to transport droplets, among which light-based techniques have attracted much attention because they are noncontact, safe, reversible, and controllable. However, conventional light-driven systems also involve challenges related to low transport ability and instability. Because of these shortcomings, technologies that can transport and manipulate droplets and microsolids on the same surface have yet to be realized. The present work demonstrates a light-driven system referred to as a liquid conveyor that enables the transport of both water droplets and microsolids. After the incorporation of an azobenzene-based molecular motor capable of undergoing photoisomerization into the surface liquid layer of this system, an isomerization gradient was induced by exposure to ultraviolet light emitting diodes that induced flow in this layer. Various parameters were optimized, including the concentration of the molecular motor compound, the light intensity, the viscosity of the liquid layer, and the droplet volume. This process eventually achieved the horizontal transport of droplets in any direction at varied rates. As a consequence of the limited heat buildup, the lack of droplet deformation, and extremely small contact angle hysteresis in this system, microsolids on droplets were also transported. This liquid conveyor is a promising platform for high-throughput omni-liquid/solid manipulation in the fields of biotechnology, chemistry, and mechanical engineering.

Programmable Droplet Microfluidics Based on Machine Learning and Acoustic Manipulation.

Typical microfluidic devices are application-specific and have to be carefully designed to implement the necessary functionalities for the targeted application. Programmable microfluidic chips try to overcome this by offering reconfigurable functionalities, allowing the same chip to be used in multiple different applications. In this work, we demonstrate a programmable microfluidic chip for the two-dimensional manipulation of droplets, based on ultrasonic bulk acoustic waves and a closed-loop machine-learning-based control algorithm. The algorithm has no prior knowledge of the acoustic fields but learns to control the droplets on the fly. The manipulation is based on switching the frequency of a single ultrasonic transducer. Using this method, we demonstrate 2D transportation and merging of water droplets in oil and oil droplets in water, and we performed the chemistry that underlies the basis of a colorimetric glucose assay. We show that we can manipulate drops with volumes ranging from ∼200 pL up to ∼30 nL with our setup. We also demonstrate that our method is robust, by changing the system parameters and showing that the machine learning algorithm can still complete the manipulation tasks. In short, our method uses ultrasonics to flexibly manipulate droplets, enabling programmable droplet microfluidic devices.

Efficient fabrication of tilt micro/nanopillars on polypropylene surface with robust superhydrophobicity for directional water droplet rebound

SummaryThe directional rebound and transport of water droplets plays an important role in microfluidic devices, anti-fogging, and water harvesting. Herein, an extrusion compression molding and directional stretch demolding method was used to prepare a polypropylene (PP) surface with tilt micro/nanopillars with a contact angle of 157 ± 3°. The rolling angle is the highest (9 ± 4°) when the direction of rotation is opposite the tilt direction of the micro/nanopillars, showing excellent water repellency and anisotropy of the surface. Compared with the position of the first collision of the water droplet, the position of the second collision shifted ∼1.5 mm along the tilt direction of the micro/nanopillars, driven by the surface tension component during the collision. The directional rebound behavior is controlled by the droplet energy and the tilt angle. The micro/nanopillars demonstrate excellent self-cleaning property and mechanical durability, which shows the possibility of their practical engineering applications.

Liquid Confine‐Induced Gradient‐Janus Wires for Droplet Self‐Propelling Performances in High Efficiency

AbstractConstructing surface wetting gradients usually involves complex physical or chemical methods. Here, a novel Gradient‐Janus wire (GJW) can be designed based on the theory of newly Liquid Confined Modification (LCM). In LCM theory, reaction difference will be generated by the confinement of reaction solution, which constructs wettability discrepancy on the same curve surface. Thus a unique continuous Gradient‐Janus wetting region can be constructed in a long range on a 1D wire. It is demonstrated that GJW can propel water droplets to transport the distance of ≈73 mm in 0.9 s, and liquid bridge to 85 mm (the longest among this kind of study) in 0.79 s with peak velocity high up to 237 mm s−1 (over 20 times faster than droplet transport on surface of Sarracenia trichome). The mechanism is attributed to cooperation between imbalanced Laplace pressure and surface tension force to generate the driving force act on droplet, liquid bridge, or column transport, respectively. LCM directs the large‐scale facile fabrication of GJWs within 20 s. A large‐scale GJW array can achieve the high‐efficient transport of water droplet in a wide volume range (few µL to 1 mL), making it potential in fogwater harvesting.

ZnO/glass thin film surface acoustic waves for efficient digital acoustofluidics and active surface cleaning

Transparent microfluidic devices based on ZnO thin film/glass surface acoustic waves (SAWs) were explored for active surface cleaning based on its acoustofluidic performance. Acoustic waves generated from ZnO films on glass substrate were investigated and their acoustofluidic performance including transportation, jetting and nebulization were evaluated. Ash particles and starch solutions were used as model contaminants on the surface of the ZnO/glass SAW devices, and the mass loading of the contaminants on the device's surface was monitored using the SAW device with a high sensitivity of 280.0 ± 9.0 Hz/(μg/mm2). Active surface cleaning of the contaminants was demonstrated based on the transportation of water droplets, and optimized SAW powers were identified which caused strong interactions between water droplet and contaminants, thus effectively cleaning the surfaces. Studies of surface heating effects induced by SAWs showed that the cleaning efficiency was also influenced by the substrate temperature induced by SAW agitations.

Magneto Twister: Magneto Deformation of the Water-Air Interface by a Superhydrophobic Magnetic Nanoparticle Layer.

Remote manipulation of superhydrophobic surfaces provides fascinating features in water interface-related applications. A superhydrophobic magnetic nanoparticle colloid layer is able to float on the water–air interface and form a stable water–solid–air interface due to its inherent water repulsion, buoyancy, and lateral capillarity properties. Moreover, it easily bends downward under an externally applied gradient magnetic field. Thanks to that, the layer creates a stable twister-like structure with a flipped conical shape, under controlled water levels, behaving as a soft and elastic material that proportionally deforms with the applied magnetic field and then goes back to its initial state in the absence of an external force. When the tip of the twister structure touches the bottom of the water container, it provides a stable magneto movable system, which has many applications in the microfluidic field. We introduce, as a proof-of-principle, three possible implementations of this structure in real scenarios, the cargo and transport of water droplets in aqueous media, the generation of magneto controllable plugs in open surface channels, and the removal of floating microplastics from the air–water interface.

Surface wettability of vertical harps for fog collection

Surface wettability is an influential property and plays a pivotal role in fog harvesting. Two types of surfaces i.e., hydrophilic and hydrophobic are generally used for fog harvesting. A hydrophilic surface provides quick nucleation whereas a hydrophobic surface carries rapid transportation of water droplets. Animals and plants live in arid environment, control their surface wetting through their naturally grown surfaces in their biological structures. These patterns provide extremely high affinity of fog collection for these surfaces by reducing the friction and pinning force of water droplets sliding down towards storage. In this study, a comparison of hydrophilic and hydrophobic surfaces is carried out and the results are expressed in terms of wetting behavior. Hydrophilic and hydrophobic vertical harps are developed by plasma treated and silane treated monofilament polyesters respectively. Traditional raschel mesh is replaced with polyester monofilament-based prototype developed in this study. The reason to replace raschel mesh (standard fog collector element) with a harp design is to capture the maximal fog by avoiding clogging. It is revealed that fog collector element(FCE) treated with hydrophobic coating improve the collection rate, however, hydrophilic treatment shows adverse effects. In addition, we examined the contact angle hysteresis (CAH) of our prototypes to verify the wettability effect.

Development of a Coating-Less Aluminum Superhydrophobic Gradient for Spontaneous Water Droplet Motion Using One-Step Laser-Ablation.

Nature shows various approaches to create superhydrophobicity, such as the lotus leaf, where the superhydrophobic (SHPB) surface arising from its hierarchical surface consists of random microscale bumps with superimposed nanoscale hairs. Some natural systems, such as the hydrophilic silk of some spider's webs, even allow the passive transport of water droplets from one part of a surface to another by creating gradients in surface tension and Laplace pressure. We look to combine both ideas and replicate the superb water repellence of the lotus leaf and the surface tension gradient-driven motion of the spider silk to form an all-metal, coating-less surface that promotes spontaneous droplet motion. We present the design, fabrication, and investigation of such superhydrophobic gradient surfaces on aluminum, which are aimed at spontaneous water droplet movement for improved surface water management. One surface demonstrates a droplet travel distance of almost 2 mm for a 11 μL droplet volume. We also present surfaces that map the theoretical ranges of the surface tension gradient surfaces tested here.

Multi-Bioinspired Janus Copper Mesh for Improved Gravity-Irrelevant Directional Water Droplet and Flow Transport.

A conceptually novel multi-bioinspired strategy based on structures and functions derived from the Namib desert beetle and lotus leaf is proposed in this paper. The proposed scheme synergistically combines the features of alternating wettability patterns and asymmetric wettability for improved directional water transport. Consequently, a Janus copper mesh, which substantially outperforms other single-bioinspired synthetic materials, is produced. The Janus copper mesh achieves directional self-transportation of tiny water droplets and continuous water flow in a gravity-irrelevant or an anti-gravity manner without energy consumption. This depends on the asymmetric wettability and alternating hydrophobic-hydrophilic wettability patterns on the hydrophobic surface of the mesh. In particular, Janus copper shows remarkable selective directional water transport in a water-oil system, rendering it a promising candidate for practical applications.

Combination of Universal Chemical Deposition and Unique Liquid Etching for the Design of Superhydrophobic Aramid Paper with Bioinspired Multiscale Hierarchical Dendritic Structure.

It is urgent and significant for the further development of superhydrophobic materials to exploit a facile, low-cost, scalable, and eco-friendly method for the manufacture of superhydrophobic materials with self-cleaning, antifouling, directional transportation, and other characteristics. Herein, an outstanding superhydrophobic material composed of a flexible microconvex aramid paper substrate, micron-scale cone-shaped copper, micro-nanoscale dendritic copper oxide, and hydrophobic copper stearate film has been successfully constructed through delicate architectural design and a convenient preparation approach. Based on the microstructure evolution and composition analysis results, it is revealed that the cone-shaped copper was etched into a dendritic copper oxide structure step by step from the top to bottom and from the outside to inside in an alkaline liquid environment. Moreover, by virtue of the compositional features and structural characteristics, the constructed superhydrophobic material showcased a high contact angle (CA), low sliding angle (SA), high porosity, low surface free energy, and adhesion work. Meanwhile, the dendritic microstructure analysis, the calculation of solid-liquid interfacial tension, and the force analysis of water droplets jointly revealed the mechanism of the bounce and merged bounce of water droplets. Finally, this superhydrophobic material has the functions of self-cleaning, antifouling, and directional transportation, especially by controlling the deformation of the material to realize the transportation of water droplets in a specified direction.

Nanopumps without Pressure Gradients: Ultrafast Transport of Water in Patterned Nanotubes.

The extreme liquid transport properties of carbon nanotubes present new opportunities for surpassing conventional technologies in water filtration and purification. We demonstrate that carbon nanotubes with wettability surface patterns act as nanopumps for the ultrafast transport of picoliter water droplets without requiring externally imposed pressure gradients. Large-scale molecular dynamics simulations evidence unprecedented speeds and accelerations on the order of 1010 g of droplet propulsion caused by interfacial energy gradients. This phenomenon is persistent for nanotubes of varying sizes, stepwise pattern configurations, and initial conditions. We present a scaling law for water transport as a function of wettability gradients through simple models for the droplet dynamic contact angle and friction coefficient. Our results show that patterned nanotubes are energy-efficient nanopumps offering a realistic path toward ultrafast water nanofiltration and precision drug delivery.

Honeycomb-Inspired Robust Hygroscopic Nanofibrous Cellular Networks.

Mimicking nature is a highly efficient and meaningful way for designing functional materials. However, constructing bioinspired nanofibrous 3D cellular networks with robust mechanical features is extremely challenging. Herein, a biomimetic, super-flexible, highly elastic, and tough nanofibrous membrane (NFM)-based water harvester is reported with a highly ordered honeycomb-inspired gradient network structure, self-assembled from electrospun spider-silk-like humped nanofibers. The resultant NFM exhibits super flexibility, high tensile strength (2.9MPa), superior elasticity, and decent toughness (3.39MJm-3 ), allowing it to be used as the framework of hygroscopic materials. The resulting hygroscopic NFM displays excellent moisture absorption performance, which can be used as an efficient water harvester with a superhigh equilibrium moisture absorption capacity of 4.60gg-1 at 95% relative humidity for 96h, fast moisture absorption and transport rates, and long-term durability, achieving directional transport and collection of tiny water droplets. This work paves the way for the design and development of multifunctional NFMs with a honeycomb-inspired gradient network structure.

Ultrafast self-propelled water droplet transport on a graphene-covered nanocone

Cacti, a common drought-tolerant plant, enable the directional transport of droplets from the tip of the spikes to the roots in order to collect moisture from the air, creating conditions for cactus survival in arid areas. Inspired by this interesting natural phenomenon, a novel design for a monolayer graphene-covered nanocone (GNC) is proposed to realize ultrafast water droplet transport from the tip to the end of the GNC. The results show that the self-driving speed of a water droplet can reach ∼80 m s−1 driven by the Young–Laplace pressure difference. The rule of energy change during the droplet self-driving process indicates that the potential energy of the droplet and the interaction energy between the droplet and the GNC undergo cooperation and competition successively, resulting in the droplet first speeding up and then slowing down to a steady moving state. A larger droplet can extend the high-speed stage, which is beneficial to long-distance self-driving of droplets in a water-harvesting process. Continuum theory in the self-driving of a droplet at a microscale is used to describe the steady moving process, in order to further understand the rule in GNC-based water transport. The findings will open a broad range of novel perspectives for spontaneous droplet transport and water harvesting by graphene-covered functional surfaces.