Velocity-Switched Droplet Rebound Direction on Anisotropic Superhydrophobic Surfaces.

Droplet well-controlled directional motion being an essential function has attracted much interest in academic and industrial applications, such as self-cleaning, micro-/nano-electro-mechanical systems, drug delivery, and heat-transferring. Conventional understanding has it that a droplet impacted on an anisotropic surface tends to bounce along the microstructural direction, which is mainly dictated by surface properties rather than initial conditions. In contrast to previous findings, it demonstrates that the direction of a droplet's rebound on an anisotropic surface can be switched by designing the initial impacting velocity. With an increase in impacting height from 2 to 10 cm, the droplet successively shows a backward, vertical, and forward motion on anisotropic surfaces. Theoretical demonstrations establish that the transition of droplet bouncing on the anisotropic surface is related to its dynamic wettability during impacting process. Characterized by the liquid-solid interaction, it is demonstrated that the contact state at small and large impacting heights induces an opposite resultant force in microstructures. Furthermore, energy balance analysis reveals that the energy conversion efficiency of backward motion is almost three times as that of traditional bouncing. This work, including experiments, theoretical models, and energy balance analysis provides insight view in droplet motions on the anisotropic surfaces and opens a new way for the droplet transport.

Sunflower-Inspired Superhydrophobic Surface with Composite Structured Microcone Array for Anisotropy Liquid/Ice Manipulation.

Precisely controlling the directional motion trajectories of droplets on anisotropic 3D functional surfaces has great application potential in self-cleaning, drug delivery, and droplet power generation, but it also faces huge challenges. Herein, inspired by the microcone structure in the heart of sunflowers, a nanoneedle-modified microcone array surface (NMAS) is reported. The surface is created using a combination of nanosecond laser direct engraving and electroforming and is subsequently fluorinated. Through programmable control of the laser spot, the geometric parameters and inclination angle of the microcone can be quickly and finely adjusted, thereby achieving precise control of the droplet bouncing trajectory. The results show that droplets can achieve programmable multiple bouncing behaviors on patterned functional surfaces, including gravity-defying hopping and directional water transport. It is worth noting that this functional surface has delayed freezing and anti-freezing effects. Furthermore, this functional surface has a wide range of potential applications, including surface self-cleaning, droplet capture, and droplet-based chemical microreactions, especially in the field of anti-icing operations. This opens up a new way for the directional transport of droplets on biomimetic functional surfaces.

Investigation into the optoelectrowetting droplet transport mechanism.

To explore the optoelectronic wetting droplet transport mechanism, a transient numerical model of optoelectrowetting (OEW) under the coupling of flow and electric fields is established. The study investigates the impact of externally applied voltage, dielectric constant of the dielectric layer, and interfacial tension between the two phases on the dynamic behavior of droplets during transport. The proposed model employs an improved Young's equation to calculate the instantaneous voltage and contact angle of the droplet on the dielectric layer. Results indicate that, under the influence of OEW, significant variations in the interface contact angle of droplets occur in bright and dark regions, inducing droplet movement. Moreover, the dynamic behavior of droplet transport is closely associated with various parameters, including externally applied voltage, dielectric layer material, and interfacial tension between the two phases, all of which impact the contact angle and, consequently, the transport process. By summarizing the influence patterns of the three key parameters studied, the optimization of droplet transport performance is achieved. The study employs two-dimensional simulation models to emulate the droplet motion under the influence of the electric field, investigating the OEW droplet transport mechanism. The continuous movement of droplets involves three stages: initial wetting, continuous transport, and reaching a steady position. The findings contribute theoretical support for the efficient design of digital microfluidic devices for OEW droplet movement and the selection of key parameters for droplet manipulation.

Cactus-Inspired Photonic Crystal Chip for Attomolar Fluorescence Multi-analysis.

Automation and efficiency requirements of environmental monitoring are the pursuit of spontaneous sampling and ultrasensitivity for current sensory systems or detection apparatuses. In this work, inspired by cactus hierarchical structures, we develop a cactus-inspired photonic crystal chip to integrate spontaneous droplet sampling and fluorescence enhancement for sensitive multi-analyte detection. A conical hydrophilic pattern on hydrophobic surfaces can give rise to unidirectional Laplace pressure, which drives droplet transport to the assigned photonic crystal site. The nanostructure of photonic crystals has bigger capillarity to drive the droplet wetting uniformly into the photonic crystal matrix while performing prominent fluorescence enhancement by their photonic bandgap. A low to attomolar (2.24 × 10-19 M) fluorescence limit of detection (LOD) sensitivity can be achieved by the synergy of spontaneous droplet sampling and fluorescence enhancement. Focused on eutrophic water problems and algae pollution monitoring, a femtomolar (1.83 × 10-15 M) LOD and identification of various microcystins in urban environmental water can be achieved. The suitable integration of the unidirectional droplet transport by Laplace pressure and fluorescence enhancement by photonic crystals can achieve the spontaneous sampling and signal enhancement for ultratrace detections and sample survey of environmental monitoring and disease diagnosis.

Hanging droplets from liquid surfaces.

Natural and man-made robotic systems use the interfacial tension between two fluids to support dense objects on liquid surfaces. Here, we show that coacervate-encased droplets of an aqueous polymer solution can be hung from the surface of a less dense aqueous polymer solution using surface tension. The forces acting on and the shapes of the hanging droplets can be controlled. Sacs with homogeneous and heterogeneous surfaces are hung from the surface and, by capillary forces, form well-ordered arrays. Locomotion and rotation can be achieved by embedding magnetic microparticles within the assemblies. Direct contact of the droplet with air enables in situ manipulation and compartmentalized cascading chemical reactions with selective transport. Applications including functional microreactors, motors, and biomimetic robots are evident.

Directional pumping of water and oil microdroplets on slippery surface.

Transporting water and oil microdroplets is important for applications ranging from water harvesting to biomedical analysis but remains a great challenge. This is due to the amplified contact angle hysteresis and insufficient driving force in the micrometer scale, especially for low-surface energy oil droplets. Coalescence of neighboring droplets, which releases vast additional surface energy, was often required, but its relatively uncontrollable nature brings uncertainties to the droplet motion, and the methodology is not applicable to single droplets. Here we introduce a strategy based on slippery surface with immobilized lubricant menisci to directionally transport microdroplets. By simply mounting hydrogel dots on slippery surface, the raised menisci remotely pump microdroplets via capillary force with high efficiency, regardless of droplet size or surface energy. By proof-of-concept experiments, we demonstrate that our method allows for highly efficient water droplet collection and highly sensitive biomedical analyte detection.

Nonspecular Reflection of Droplets.

The bouncing of droplets on super-repellent surfaces normally resembles specular reflection that obeys the law of reflection. Here, the nonspecular reflection of droplet impingement onto solid surfaces with a dimple for energy-efficient, omnidirectional droplet transport is reported. With the dimple of the radius being comparable to that of the droplet, all the symmetries in the law of reflection can be broken down so that the droplet is endowed with a translational velocity finely tunable in both its direction and magnitude simply by varying the radii of the droplet and the dimple, the impinging position, and droplet Weber number. Tailoring the initial and translational velocity of impinging droplets would steer their reflected trajectories at will, thus enabling versatile droplet manipulation including trapping, shedding, antigravity transport, targeted positioning, and on-demand coalescence of droplets.

Hierarchical Nanotexturing Enables Acoustofluidics on Slippery yet Sticky, Flexible Surfaces.

The ability to actuate liquids remains a fundamental challenge in smart microsystems, such as those for soft robotics, where devices often need to conform to either natural or three-dimensional solid shapes, in various orientations. Here, we propose a hierarchical nanotexturing of piezoelectric films as active microfluidic actuators, exploiting a unique combination of both topographical and chemical properties on flexible surfaces, while also introducing design concepts of shear hydrophobicity and tensile hydrophilicity. In doing so, we create nanostructured surfaces that are, at the same time, both slippery (low in-plane pinning) and sticky (high normal-to-plane liquid adhesion). By enabling fluid transportation on such arbitrarily shaped surfaces, we demonstrate efficient fluid motions on inclined, vertical, inverted, or even flexible geometries in three dimensions. Such surfaces can also be deformed and then reformed into their original shapes, thereby paving the way for advanced microfluidic applications.

Monitoring and external control of pH in microfluidic droplets during microbial culturing.

BACKGROUND: Cell-based experimentation in microfluidic droplets is becoming increasingly popular among biotechnologists and microbiologists, since inherent characteristics of droplets allow high throughput at low cost and space investment. The range of applications for droplet assays is expanding from single cell analysis toward complex cell-cell incubation and interaction studies. As a result of cellular metabolism in these setups, relevant physicochemical alterations frequently occur before functional assays are conducted. However, to use droplets as truly miniaturized bioreactors, parameters like pH and oxygen availability should be controlled similar to large-scale fermentation to ensure reliable research. RESULTS: Here, we introduce a comprehensive strategy to monitor and control pH for large droplet populations during long-term incubation. We show the correlation of fluorescence intensity of 6-carboxyfluorescein and pH in single droplets and entire droplet populations. By taking advantage of inter-droplet transport of pH-mediating molecules, the average pH value of several million droplets is simultaneously adjusted in an a priori defined direction. To demonstrate the need of pH control in practice, we compared the fermentation profiles of two E. coli strains, a K12-strain and a B-strain, in unbuffered medium with 5 g/L glucose for standard 1 L bioreactors and 180 pL droplets. In both fermentation formats, the commonly used B-strain E. coli BL21 is able to consume glucose until depletion and prevent a pH drop, while the growth of the K12-strain E. coli MG1655 is soon inhibited by a low pH caused by its own high acetate production. By regulating the pH during fermentation in droplets with our suggested strategy, we were able to prevent the growth arrest of E. coli MG1655 and obtained an equally high biomass yield as with E. coli BL21. CONCLUSION: We demonstrated a comparable success of pH monitoring and regulation for fermentations in 1 L scale and 180 pL scale for two E. coli strains. This strategy has the potential to improve cell-based experiments for various microbial systems in microfluidic droplets and opens the possibility for new functional assay designs.

Excellent Fog-Droplets Collector via Integrative Janus Membrane and Conical Spine with Micro/Nanostructures.

Inspired by a cactus spine and trichomes integrated fog collection system, a strategy is presented to design a micro/nanostructured conical spine and Janus membrane integrative system (MNCS+JM). In this strategy, the surface of conical spine can be covered with rough micro and nanostructure (MNCS), so that the tiny fog-droplets can be captured, coalesced, and transported. Janus membrane (JM) with inside hydrophobic surface and outside hydrophilic surface is further used to control the water collection in process of droplet transport when the Janus membrane is vertically placed with different positions on the MNCS, thus MNCS+JM propel the droplet continuously for transport-coalescence-transport in a circle of droplet transport and collection. It is demonstrated that a higher fog collection rate can be achieved effectively, which is attributed to a cooperation effect between the Laplace pressure in difference and the released surface energy in droplet coalescence, in addition to wettability force of superhydrophobic-hydrophilic difference in the Janus membrane. This strategy of MNCS+JM offers an insight into the surface of materials to control the droplet transport for water collection in efficiency, which is significant to be extended into the realms of applications such as high-efficiency water collection systems, microfluidics devices, and others.

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.

High-speed directional transport of condensate droplets on superhydrophobic saw-tooth surfaces.

HYPOTHESIS: Most droplets on high-efficiency condensing surfaces have radii of less than 100 µm, but conventional droplet transport methods (such as wettability-gradient surfaces and structural-curvature-gradient surfaces) that rely on the unbalanced force of three-phase lines can only transport millimeter-sized droplets efficiently. Regulating high-speed directional transport of condensate droplets is still challenging. Therefore, we present a method for condensate droplet transportation, based on the reaction force of the superhydrophobic saw-tooth surfaces to the liquid bridge, the condensate droplets could be transported at high speed and over long distances. EXPERIMENTS: The superhydrophobic saw-tooth surfaces are fabricated by femtosecond laser ablation and chemical etching. Condensation experiments and luminescent particle characterization experiments on different surfaces are conducted. Aided by the theoretical analysis, we illustrate the remarkable performance of condensate droplet transportation on saw-tooth surfaces. FINDINGS: Compared with conventional methods, our method improves the transport velocity and relative transport distance by 1-2 orders of magnitude and achieves directional transport of the smallest condensate droplet of about 2 µm. Furthermore, the superhydrophobic saw-tooth surfaces enable multi-hop directional jumping of condensate droplets, leading to cross-scale increases in transport distances from microns to decimeters.

On-chip mixing of cancer cells and drug using LED enabled 2D opto-wetting droplet platforms.

Droplets of microliter size serve as miniaturized reaction chambers for practical lab on a chip (LoC) applications. The transportation and coalescence of droplets are indispensable for realizing microfluidic mixing. Light can be used as an effective tool for droplet manipulation. We report a novel platform for LED-based transport and mixing of cell-encapsulated microdroplets for evaluating dose response of cancer drugs. Microcontroller enabled LEDs (Light-emitting diodes) were used to actuate droplet movement on Azobenzene coated planar silicon substrates. Droplet transport was initiated by the spatial gradient in solid-liquid interfacial tension developed through LED triggered photoisomerization of Azobenzene substrate. Detailed UV-Visible characterization of Azobenzene molecule was performed for different LED light intensities and wavelengths. A complete standalone opto-wetting toolbox was developed by integrating various components such as a microcontroller, UV LED (385 nm), blue LED (465 nm), and Azobenzene coated photoresponsive substrate. 2D transport of DI water droplets (10-30µl) along simple trajectories was demonstrated using this device. Subsequently, the proposed opto-wetting platform was used for performing drug evaluation through on-chip mixing of droplets containing cancer cells (A549-Lung cancer cells) and cancer drug (paclitaxel). Separate cell viability analysis was performed using MTT assays, where the cytocompatibility of Azobenzene and UV light (385 nm) on A549 cells were studied. The dosage response of paclitaxel drug was studied using both MTT (3-(4,5-Dimethylthiazolyl-2)-2,5-diphenyltetrazolium bromide) and live-dead cell assays. The results obtained indicate the potential use of our device as a cost-effective, reliable opto-wetting microfluidic platform for drug screening experiments.

Transport and collection of water droplets interacting with bioinspired fibers.

Inspired by natural creatures, such as spider silk, scientists have designed various bionic materials with special wettability. Compared with two-dimensional and three-dimensional materials, one-dimensional materials, such as fiber, with special wettability can dynamically transport or manipulate liquid droplets, attracting widespread attention. This paper reviews the latest developments of bioinspired fibers in the directional steering and transportation of droplets. Firstly, some fundamental theories of droplet's directional motion are reviewed. The movement of liquid droplets on the fiber is usually affected by several factors/features of the artificial fibers. Then the advantages and disadvantages of four representative methods for preparing bioinspired fibers with different features are reviewed. Subsequently, this paper continues to analyze the influencing factors of bioinspired fibers for efficient water collection and droplet driving direction, and the advantages of different structural combinations. Specifically, a section was dedicated to discuss droplets detachment from fibers. Finally, we provide some improved ideas and outlooks for developing of one-dimensional bioinspired fibers.

Electrothermally Assisted Surface Charge Density Gradient Printing to Drive Droplet Transport.

Surface 2019, surface charge density (SCD) gradient printing-driven droplet transport, has attracted considerable attention as a novel and effective approach, which adopts the water droplet impacting a nonwetting surface to create a reprintable SCD gradient pathway conveniently and realizes the high-velocity and long-distance transport of droplets. In the present work, we further investigated the effects of electrothermal behavior on SCD gradient printing on hydrophobic surfaces by considering the droplet impact dynamics. After the electrothermal function was activated, the wettability of the hydrophobic surface improved in terms of the spreading factor history and the infiltration depth, which increased the probability of solid/liquid contact electrification to generate a more favorable SCD gradient. Since the hydrophobic surface was negatively charged by droplet impact, polarized droplets rolled forward along the preprinted SCD gradient pathway due to opposite charge attraction. Based on these results, we designed a SCD gradient printer with an electrothermal function for hydrophobic surfaces. Subsequently, the kinematic parameters of rolling droplets on hydrophobic surfaces were observed and quantified to evaluate the improvements resulting from the electrothermal function.

Tension gradient-driven oil/water interface rapid particle self-assembly and its application in microdroplet motion control.

HYPOTHESIS: A binary mixture was used during injection with one water-miscible component and the other water-immiscible, which can help particles to migrate toward and then self-assemble at the interface. EXPERIMENTS: The ethanol-tetrachloromethane binary mixture was used to verify the self-assembly method, with the diameter of droplets being about 1 mm. As the ethanol diffused into the colloidal solution, the colloidal particles efficiently moved towards and self-assembled on the oil/water interface, while a colloidal particle film with high-coverage was able to rapidly form on the droplet surface even in an ultra-low concentration colloidal solution. The effects of ethanol concentration and particle concentration on self-assembly were investigated. FINDINGS: The driving force for self-assembly originated from the tension gradient generated by ethanol's concentration gradient at the particle/liquid interfaces, where the concentrations of ethanol and the colloidal solution had significant effects on self-assembly. The simulation and calculations results aligned well with experiments, providing the theoretical basis for this self-assembly method. Further, as-prepared magnetic particle-coated droplets transformed into a non-wetting soft solid, which had long lifetimes and could be precisely moved, coalesced, and transferred in various two-dimensional and three-dimensional liquid environments. Thus, wider applications are facilitated, such as droplet transfer, microreactor and other potential fields.

Enhanced Fog Harvesting through Capillary-Assisted Rapid Transport of Droplet Confined in the Given Microchannel.

A novel integrated bioinspired surface is fabricated by using an innovative capillarity-induced selective oxidation method, to achieve the combination of the fog-collecting characteristics of a variety of creatures, i.e., the micronanostructures of spider silk, the wettable patterns of desert beetle, the conical structure of cactus spine, and the hierarchical microchannel of Sarracenia trichome. The fog is captured effectively via multistructures on the cone tips, and captured droplet is collected and confined in the microchannel to realize rapid transport via the formation of wettable pattern on the surface and the introduction of wettable gradient in the microchannel. Consequently, the fog harvest efficiency reaches 2.48 g/h, increasing to nearly 320% compared to the normal surface. More interestingly, similar to Sarracenia trichome, the surface also presents two transport modes, namely, Mode I (water transport along dry microchannel) and Mode II (succeeding water slippage on the water film). In Mode II, the velocity of 34.10 mm/s is about three times faster than that on the Sarracenia trichome. Such a design of integrated bioinspired surface may present potential applications in high-efficiency water collection systems, microfluidic devices, and others.

Magneto-Responsive Shutter for On-Demand Droplet Manipulation.

Magnetically responsive structured surfaces enabling multifunctional droplet manipulation are of significant interest in both scientific and engineering research. To realize magnetic actuation, current strategies generally employ well-designed microarrays of high-aspect-ratio structure components (e.g., microcilia, micropillars, and microplates) with incorporated magnetism to allow reversible bending deformation driven by magnets. However, such magneto-responsive microarray surfaces suffer from highly restricted deformation range and poor control precision under magnetic field, restraining their droplet manipulation capability. Herein, a novel magneto-responsive shutter (MRS) design composed of arrayed microblades connected to a frame is developed for on-demand droplet manipulation. The microblades can perform two dynamical transformation operations, including reversible swing and rotation, and significantly, the transformation can be precisely controlled over a large rotation range with the highest rotation angle up to 3960°. Functionalized MRSs based on the above design, including Janus-MRS, superhydrophobic MRS (SHP-MRS) and lubricant infused slippery MRS (LIS-MRS), can realize a wide range of droplet manipulations, ranging from switchable wettability, directional droplet bounce, droplet distribution, and droplet merging, to continuous droplet transport along either straight or curved paths. MRS provides a new paradigm of using swing/rotation topographic transformation to replace conventional bending deformation for highly efficient and on-demand multimode droplet manipulation under magnetic actuation.

Biomimetic Water-Repelling Surfaces with Robustly Flexible Structures.

Biomimetic liquid-repelling surfaces have been the subject of considerable scientific research and technological application. To design such surfaces, a flexibility-based oscillation strategy has been shown to resolve the problem of liquid-surface positioning encountered by the previous, rigidity-based asymmetry strategy; however, its usage is limited by weak mechanical robustness and confined repellency enhancement. Here, we design a flexible surface comprising mesoscale heads and microscale spring sets, in analogy to the mushroomlike geometry discovered on springtail cuticles, and then realize this through three-dimensional projection microstereolithography. Such a surface exhibits strong mechanical robustness against ubiquitous normal and shear compression and even endures tribological friction. Simultaneously, the surface elevates water repellency for impacting droplets by enhancing impalement resistance and reducing contact time, partially reaching an improvement of ∼80% via structural tilting movements. This is the first demonstration of flexible interfacial structures to robustly endure tribological friction as well as to promote water repellency, approaching real-world applications of water repelling. Also, a flexibility gradient is created on the surface to directionally manipulate droplets, paving the way for 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.