Directional Droplet Transport Research Articles

Advanced Fog Harvesting Method by Coupling Plasma and Micro/Nano Materials.

Harvesting fog is a potential and effective way to alleviate the crisis of water resource shortage. A highly efficient and economical fog harvesting method has always been a global and common goal. Here, a promising fog harvesting method by coupling plasma and micro/nano materials is proposed, which can achieve 93% fog collection efficiency with consuming power of only 0.76 W/0.04 m2. The basic method is to utilize nanoparticles to decorate both the discharge electrode and the collecting electrode of the micro/nano electrostatic fog collector. For the discharge electrode, the nanoparticles can achieve an order of magnitude higher electric field strength and a 28.6% decrease in the operating voltage (14 kV decreases to 10 kV). For the collecting electrode, a novel composite structure of hydrophobic/hydrophilic (HB/HL) is proposed. The core advantage is the directional droplet transport at the junction of HB and HL caused by surface tension can adjust the accumulated droplets on the two sides, which avoids the droplet residue and mesh blockage in the general structure. This technology provides an innovative approach for the collection of microdroplets and a new design idea for the fog collector to deal with the water crisis.

Electric-Field-Induced Selective Directed Transport of Diverse Droplets.

Droplet directional transport is one of the central topics in microfluidics and lab-on-a-chip applications. Selective transport of diverse droplets, particularly in another liquid phase environment with controlled directions, is still challenging. In this work, we propose an electric-field gradient-driven droplet directional transport platform facilitated by a robust lubricant surface. On the platform, we clearly demonstrated a liquid-inherent critical frequency-dominated selective transport of diverse droplets and a driving mechanism transition from electrowetting to liquid dielectrophoresis. Enlightened by the Kelvin-Helmholtz theory, we first realize the directional droplet transport in another liquid phase whenever a permittivity difference exists. Co-transport of multiple droplets and various combinations of droplet types, as well as multifunctional droplet transport modes, are realized based on the presented powerful electric-field gradient-driven platform, overcoming the limitations of the surrounding environment, liquid conductivity, and intrinsic solid-liquid wetting property existing in traditional droplet transport strategies. This work may inspire new applications in liquid separation, multiphase microfluidic manipulation, chemical reagent selection, and so on.

Directional droplet transport behavior on gradient wettability wedge track with extreme wettability contrast

To enhance transport capacity of open-surfaces, a gradient wettability wedge track laid on superhydrophobic background (wedge track No. 2) was fabricated by using masking tape and alkaline-assisted oxidation. The effects of gradient wettability and extreme wettability contrast are emphasized and investigated. The maximum instant velocity of a 10 μL droplet on track No. 2 is ∼ 883.53 mm/s with final transport displacement x = 46.79 mm, which was 64.15 % faster and 11.67 % farther than that on a superhydrophilic wedge track (wedge track No. 1) respectively. Oscillation happens at the wide end of wedge tracks when kinetic energy remains. Oscillation time on wedge track No. 2 is longer, which means gradient wettability can provide an extra driving force. This novel wedge track highlights the necessity of both gradient wettability and extreme wettability contrast and will have broad prospects in directional transport on open-surfaces, biological research, energy conversion and microfluidic mixers.

Molecularly Capped Omniphobic Polydimethylsiloxane Brushes with Ultra-Fast Contact Line Dynamics.

Droplet friction is common and significant in any field where liquids interact with solid surfaces. This study explores the molecular capping of surface-tethered, liquid-like polydimethylsiloxane (PDMS) brushes and its substantial effect on droplet friction and liquid repellency. By exchanging polymer chain terminal silanol groups for methyls using a single-step vapor phase reaction, the contact line relaxation time is decreased by three orders of magnitude-from seconds to milliseconds. This leads to a substantial reduction in the static and kinetic friction of both high- and low-surface tension fluids. Vertical droplet oscillatory imaging confirms the ultra-fast contact line dynamics of capped PDMS brushes, which is corroborated by live contact angle monitoring during fluid flow. This study proposes that truly omniphobic surfaces should not only have very small contact angle hysteresis, but their contact line relaxation time should be significantly shorter than the timescale of their useful application, i.e., a Deborah number less than unity. Capped PDMS brushes that meet these criteria demonstrate complete suppression of the coffee ring effect, excellent anti-fouling behavior, directional droplet transport, increased water harvesting performance, and transparency retention following the evaporation of non-Newtonian fluids.

Lossless and Directional Transport of Droplets on Multi‐bioinspired Superwetting V‐shape Rails

AbstractLossless and directional droplet transport is desirable in biological processes as well as in technical applications such as targeted drug therapies, bioassays, and microfluidics. Conventional methods that use surface energy and Laplace pressure gradients to achieve spontaneous droplet transport often suffer from droplet destruction and loss. Herein, an efficient strategy is reported based on a V‐shaped underwater superoleophobic rail and a V‐shaped superhydrophobic rail that delivers lossless and directional oil and water droplet transport, respectively. The V‐shaped rail not only converts the kinetic energy of the impacting droplets into planar motion but also seriously deforms the droplet to create a 3D Laplace pressure difference that directionally moves the droplet. The superoleophobic and superhydrophobic wettability of a copper rod surface is crucial for achieving lossless water and oil droplet transport, which is attributable to low adhesive forces acting on the droplets. The V‐shaped rail can also feasibly be used in droplet sensors, microchemical reactions, droplet‐based electricity generators, and water/oil separation applications, thereby significantly expanding the applications of lossless and directional droplet transport.

Preparation of Anodic Porous Alumina with Gradient Hole Size for Directional Droplet Transport.

A surface with a wettability gradient can be used as a surface where water droplets transport in one direction. In this report, a surface with a wettability gradient was formed by Al anodization and subsequent gradient etching. Since the surface wettability of anodic porous alumina formed by the Al anodization was dependent on hole size, anodic porous alumina with gradient hole size was prepared to form a surface with a wettability gradient. The surface obtained by this method showed continuously changing wettability, and it was possible to control the range and amount of change in the contact angle of the water droplets by adjusting the hole size. In addition, water droplets were observed to transport spontaneously from an area with a high contact angle to that with a low contact angle on the surface of anodic porous alumina with gradient hole size. The obtained surface with a wettability gradient can be used in various applications, such as microfluidic devices, water collection, and oil-water separation.

Programmable droplet transport on multi-bioinspired slippery surface with tridirectionally anisotropic wettability

• Multi-bioinspired tridirectionally anisotropic slippery surfaces were prepared. • The pinning effect of step edge was critical to the tridirectional anisotropy. • Vertical vibration assisted programmable droplet transports were realized. • In-plane manipulation of droplet motion in more than two directions was realized. • Droplet-based chemical micro-reactors were designed. Directional droplet transport on functional surfaces with anisotropic wettability has shown great potential applications in various fields such as water harvesting, chemical micro-reaction, and biomedical analysis. However, the in-plane manipulation of the anisotropic droplet motion in more than two directions is still a challenge. Herein, through the fusion of inspirations from rice leaves, butterfly wings and Pitcher plants, we report a tridirectionally anisotropic slippery surface (TASS) with periodic step-like micro grooves for programmable droplet transport. TASS possesses a tridirectional droplet sliding behavior, i.e. , the ultra-slipperiness along the grooves with a sliding angle of ∼2°, and the bidirectionally anisotropic sliding perpendicular to the grooves with sliding angle difference up to ∼50°, which is caused by the pinning effect of the step edge. Under the assistance of periodic vertical vibration, groove-features and droplet-volume dependent unidirectional droplets transports are realized on horizontally placed TASS, based on which two micro-reactors are designed to control the sequence of droplets merging and subsequent chemical reactions. Additionally, by utilizing the slipperiness ( i.e. , ultra-low sliding angle for liquid droplet) along the grooves simultaneously, programmable droplet transport under vertical vibration is further demonstrated on a tilted TASS. This work will provide a new avenue for the understanding of anisotropic wettability on asymmetric slippery surface, and thus offer a great opportunity to develop advanced interface for multidirectional droplet transport, chemical micro-reactor, etc .

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.

Deposition of micro/macroscale water droplets on grooved hydrophobic surfaces

Hydrophobic surfaces containing groove structures are frequently found in nature. Understanding the contact line dynamics of water droplets on such surfaces is important for analyzing the droplet motion and utilizing it for directional droplet transport. Although the visualization technique has been significantly improved, less attention has been paid to the contact line dynamics of droplets on grooved hydrophobic surfaces. Here, we fabricated hydrophobic surfaces containing grooves using a facile laser technique and visualized the advancing and receding contact line dynamics on the surfaces through high-speed imaging. In addition, the geometry of the groove structure, the droplet volume, and the inclination angle of the surface were systematically varied, and their effects on the sizes and shapes of the residual droplets deposited on the groove structures because of the sliding droplet were studied. Minute and uniform water droplets were deposited on the grooved surfaces when the structures were perpendicular to the droplet's moving path. As the droplet volume, surface inclination angle, and solid fraction of the grooved surfaces increased, the sizes of the residual droplets deposited on the grooves increased. A simple dimensionless analysis indicated that the size of the residual droplet could be predicted using the parameters tested in this study.

Steerable directional bouncing and contact time reduction of impacting droplets on superhydrophobic stepped surfaces

HypothesisThe unbalanced capillary force provided by wettability patterns, non-uniform/asymmetric microtextures enables directional droplet transport, while macrotextures have shown potentials in reducing the contact time. Inspired by these findings, we design millimeter superhydrophobic stepped surfaces to simultaneously achieve highly steerable directional bouncing and contact time reduction of impacting droplets. ExperimentsThe stepped surfaces are fabricated by computerized numerical control, chemical oxidation, hydrophobic treatment. Systematic impact experiments are conducted under Weber number ranging from 10.5 to 20.5, two step heights (0.5 mm and 1.0 mm) and extensive impact positions. FindingsCompared with known microtextured surfaces, the stepped surfaces exhibit excellent performance with the maximum lateral movement distance about 8 times of droplet radius, controllable rebound angle ranging from ∼ 32° to ∼ 90° and up to ∼ 30 % reduction in contact time. Particularly, we divide the directional bouncing characteristics into six regimes and attribute the variation of rebound velocity by the synergistic effects of viscous dissipation, excess surface energy, excess kinetic energy. It is demonstrated that the contact time is reduced by liquid mass redistribution and asymmetry enhancement. Predictive models of contact time that incorporate the coupling effects of impact position, impact velocity and step height are also established.

Directional transport of droplets impacting on superhydrophobic opening triangular groove surfaces

Directional droplet transport is of great importance to various processes including heat transfer, water harvesting and microfluidics. Here, a facile superhydrophobic opening triangular groove surface was proposed, and the directional transport behavior of droplets impacting on the surface was observed. The results show that the essence of the directional transport on the opening triangular groove surface can be attributed to the interaction between the impinging droplet and the groove sidewall, and the directional transport distance is regulated by the contact area during the interaction. Further, by controlling the depth of triangular groove, the triangular opening angle, the impacting point position and the Weber number, the contact area between the droplet and the groove sidewall during the interaction can be changed, leading to the variation of directional transport distance. This study provides a new method for directional droplet transport on superhydrophobic surfaces and offers more options for the manipulation of droplet behavior.

Directional droplet transport on switchable ratchets by mechanowetting

Materials with a mechanical response to an external stimulus are promising for application in miniaturized cargo and fluid manipulation in microfluidic (lab-on-a-chip) systems and microsystems in general. One of the main challenges in droplet microfluidics is the precise control of the droplet motion, and existing technologies have drawbacks that can compromise the droplet contents. Here, we demonstrate how an on–off switchable ratchet topography combined with a simple actuation strategy can be exploited to accurately manipulate mm-sized droplets. Because of the mechanowetting principle, the three-phase line dynamically attaches to these deforming ratchets, affecting the droplet displacement in a controlled matter. We show that such topographies are capable of transporting droplets over a surface in a stepwise fashion. We calculate the forces generated by the surface using both a theoretical description of the three-phase line and fluid simulations, and we identify the window of applicability in terms of the droplet size relative to the sawtooth dimensions. Our results enable the design of microfluidic systems with deforming wall topographies for controlled droplet manipulation.

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.

3D-printed mesh membranes with controllable wetting state for directional droplet transportation

Mesh membranes have a wide range of applications including fog harvesting and oil-water separation. However, due to the wetting energy barrier, water droplets are usually pinned in the meshes and there are still substantial difficulties in achieving directional transport. In this paper, mesh membranes for directional droplet transport are achieved by controlling the surface wetting state with sub-millimeter structures. The effect of mesh structures on the surface wetting state is studied theoretically. And accordingly, a unique filament stack structure is presented to acquire the desired wetting state, which can be further switched on-demand by incorporating the filament swelling. The well-designed mesh structures can be 3D printed efficiently without post-processes such as surface modification. It provides a facile and effective way to develop scalable mesh membranes for directional and selective droplet transport.

Wettability Difference Induced Out-of-Plane Unidirectional Droplet Transport for Efficient Fog Harvesting.

Securing freshwater resources is a global issue for ensuring sustainable development. Fog harvesting is attracting great attention as a method to collect water without any energy input. Previous reports that were inspired by insects and plants have given insights such as the effectiveness of in-plane wettability and structural differences for droplet transport, which might enhance artificial water harvesting efficiency. However, further efforts to transfer droplets while maintaining performance are needed because droplet motion owing to these effects is limited to the in-plane direction. In this study, we report droplet transport between three-dimensional copper wire structures with nanostructured hydrophobic and superhydrophilic features. This mechanism enhanced the fog harvesting capability by more than 20% compared with the cumulative value of individual wires. In addition, the relationship between the droplet height and spacing of wires affected the performance. Our results show the importance of out-of-plane directional droplet transport from the wire surface assisted by differences in wire wettability, which minimizes limiting factors of fog harvesting including clogging and droplet shedding. Furthermore, the proposed arrangement reduces the overall system width compared with that of a two-dimensional arrangement while maintaining the amount of harvested water. These results provide a promising approach to designing large-scale and highly efficient fog harvesters.

A combined structural and wettability gradient surface for directional droplet transport and efficient fog collection

HypothesisThe droplet manipulation behavior is affected by chemical structural driving force (including the superposition of electric, magnetic, optical and thermal fields), which directly determine transportation velocity. A lot of research has focused on a single driving force that induces the directional transportation behavior, which affects its performance. ExperimentsA simple method for preparing wettability gradient conical copper needles (WGCCN) combining structural gradient and chemical gradient was formulated. The effect of droplet volume and tilt angles on droplet transport velocity was systematically studied. The process of droplet transport was revealed through theoretical model and mechanical analysis. Finally, the application of WGCCN and its array model in fog collection were explored. FindingsA continuous chemical gradient in the conical structure gradient induces the droplet directional transportation, and the transportation velocity depends on the droplet volume. In addition, under the cooperation effect of multiple driving force, the droplet can still be transported in a directional orientation even if it is tilted at a certain angle. The simple droplet manipulation behavior portends that the droplets directional transport behavior can be applied in microfluidic manipulation by cooperation of effective multiple driving force with satisfactory results.

Directional Droplet Transport on Functional Surfaces with Superwettabilities

AbstractDirectional droplet transport on smart surfaces with superwettabilities arouse extensive attentions from both the scientific community and industry due to the exciting applications in healthcare, environmental protection, and energy harvesting. The past decades have witnessed the progress and achievements in mechanism disclosure, device preparation, and applied investigation. Herein, various droplet transport schemes are summarized, such as electrohydrodynamic strategy, self‐propelled droplet on anisotropic surfaces, gravity driven droplet on slippery lubricant impregnated porous surfaces (SLIPS), and other engineering strategies. Accordingly, the basic mechanism of droplet manipulation, devices configuration, and key features are presented. The recent cutting‐edge applications of directional droplet transport, including digital microfluidics, electric energy harvesting, fag/water collection, and oil‐water separation are also reviewed. At the end, current challenges and future perspectives in this field are discussed.

Directional Droplet Transport Mediated by Circular Groove Arrays. Part II: Theory of Effect.

In the first part of this research, we reported the experimental study of the drop impact on the superhydrophobic circular groove arrays, which resulted in a directional droplet transport. In the second part, we further explored the influence of the Weber number (We), ridge height (H), and the deviation distance (r) between the impacting point and the center of curvature on the lateral offset distance (ΔL) of bouncing drops. The suggested theoretical analysis is in reasonable agreement with the experimental observations. We demonstrate that a Cassie-Wenzel wetting transition occurred within the microstructures of the relief under the threshold Weber number, for example, We ≅ 19-25, which switched the nature of drop bouncing. The dynamic pressure plays a decisive role in the directional droplet transport. The reported investigation may shed light on the solid-liquid interactions occurring on the patterned hierarchical surfaces and open up new opportunities for directional droplet transportation.

Gradient Quasi‐Liquid Surface Enabled Self‐Propulsion of Highly Wetting Liquids

AbstractSelf‐propulsion of highly wetting liquids is important in heat exchanger, air conditioning, and refrigeration systems. However, it is challenging to achieve such a spontaneous motion as these liquids tend to wet all the surfaces due to their ultralow surface tensions. Despite that extensive asymmetric surface structures and gradient chemical coatings are developed for directional droplet transport, they will be flooded and covered by these liquids. Here, this challenge is addressed by creating a gradient quasi‐liquid surface to achieve the self‐propulsion of droplets with surface tensions down to 10.0 mN m−1. Such a surface engineered by tethering flexible polymers with gradient grafting density shows ultralow contact angle hysteresis (<1o) to highly wetting liquids. Thus, the surface can simultaneously provide sufficient driving forces through the gradient wettability and negligible retention forces through the slippery boundary lubrication for spontaneous droplet movement. Moreover, continual self‐propulsion of tiny droplets is achieved by spraying highly wetting liquids in simulated condensation conditions and demonstrates that adding temperature gradient can further accelerate the self‐propulsion. The study provides a new paradigm to promote passive removal of highly wetting droplets, leading to potential impacts in enhancing condensation heat transfer regardless of surface orientations.

Droplet Retention on Superhydrophobic Surfaces: A Critical Review

AbstractWetting phenomena and superhydrophobic surfaces are ubiquitous in nature and have recently been explored widely in scientific and engineering applications. The understanding and control of surface superhydrophobicity are not only fundamentally intriguing but also practically important to provide unique and regulated functionalities to natural species and industrial applications. Here, the fundamentals of wetting phenomena are critically reviewed that especially apply to superhydrophobicity, putting an emphasis on the clarification of contact angles, the quantification of droplet retention force, and the role of contact line. The fundamentals of how the droplet retention is determined by the surface features are discussed and advanced analytical models for the prediction of contact angles and retentive forces are introduced. Applications are further discussed whose functionalities largely depend on the droplet retention, including directional droplet transport, anti‐icing, and water harvesting.