Droplet最新文献

Inside Back Cover: The cover image is based on the Research Article Hybrid electrodes effective for both electrowetting- and dielectrowetting-driven digital microfluidics by Geng and Cho.

The cover image illustrates hybrid electrodes for digital (droplet-based) microfluidics that can drive aqueous (conductive) as well as dielectric (non-conductive) liquid droplets on a single platform. The electrodes have capability to generate electrowetting as well as dielectrowetting driving forces. (DOI: 10.1002/dro2.58)

Special wettability fibrous materials have received a lot of attention because of their good connectivity, mechanical flexibility, large specific surface area, and ease of shape manipulation. Their outstanding performance paves the way toward efficient oil/water separation in a variety of environments and for separation requirements. This paper discusses the distinct advantages, challenges, and future research directions of various substrates for fibrous materials, such as nonwoven natural biomass fibers, fabrics, electrospinning fibers, metallic fibers, and inorganic nonmetallic fibers. The special wettability fibrous filter materials and fibrous adsorbents are summarized based on the different separation methods. The principles of preparation of various special wettabilities are introduced, and the unique advantages of fibrous adsorbents are emphasized. The preparation strategy of fibrous filter materials is discussed, as well as some representative work and research progress. The benefits, drawbacks, and research directions in terms of various materials are examined. This article emphasizes the pollution resistance of fibrous filter materials and the elasticity of fibrous adsorbents. Finally, the prospects in terms of the problems, challenges, and future development of special wettability fibrous materials used in oil/water separation are discussed.

The use of double emulsions (DEs), which represent colloidal structures composed of droplets nested within droplets, can provide for unparallel droplet manipulation in droplet-based microfluidic technology due to their unique core–shell structures. The controlled release of cores in DEs is of particular interest. However, this process remains poorly explored. In this work, the thermocapillary flow induced by a temperature gradient is used as a driving force to control the core release and the impacts of different linear temperature gradients, core diameters, shell diameter, and core/shell diameter ratios on the thermocapillary flow and core release characteristics of DE droplets consisting of a water-in-n-hexadecane-in-water system within a cylindrical microchannel are investigated. Most of the core and shell diameter conditions considered result in a double-core release process, where the inner droplet volume is partially ejected before the remaining core is rewrapped by the outer droplet, and the remaining inner droplet volume is ejected later during a second core release event. However, relatively small core diameters of 50 and 75 μm produce conditions where the full inner droplet volume is ejected during a single-core release process. In addition, we provide empirical relationships for accurately determining the time at which core release initially occurs under given DE parameters as well as for precisely determining whether the applied conditions will lead to single- or double-core release processes. Therefore, the results of this study provide insights enabling the development of accurate inner droplet release technologies under thermocapillary migration.

Programmable droplet manipulation based on external stimulation is in high demand in various modern technologies. Despite notable progress, current manipulation strategies still suffer from a common drawback such as single control means of modulating the external stimulation input, which leads to huge challenges in sophisticated and large scale-up droplet handling. Herein, a unique pattern-reconfiguration-driven droplet manipulation method is developed on conductive/nonconductive pattern surfaces under charge deposition. Contactless charge deposition induces the “edge barrier” phenomenon at the boundaries of conductive/nonconductive patterns, analogous to an invisible and tunable wall guiding droplet behaviors. The edge barrier effect can be flexibly tuned by the nonconductive surface pattern. Thus, with charge deposition, surfaces are endowed with protean control functionality. The design of conductive/nonconductive patterns can effectively enable multifunction droplet manipulations, including track-guided sliding, sorting, merging, and mixing. Moreover, dynamical pattern reconfiguration drives programmable fluidics with sophisticated and large scale-up droplet handling capabilities in a low-cost and simple approach.

Control of the directional bounce of droplets impacting solid surfaces is crucial for many agricultural and industrial applications. However, for the universal impact process of raindrops on plant leaves, little is known about how the highly coupled and complicated fluid–structure interaction controls the postimpact motion of droplets and endows the leaves with tenacious vitality. Here, we report a leaf-like superhydrophobic cantilever to flexibly bounce droplets with well-defined directionality and controllability. Through theoretical modeling and three-dimensional fluid–solid coupling simulations, we find that the flexible cantilever significantly relieves the impacting forces of raindrops to reduce droplet fragmentation and enhance water repellency. The results further uncover the scaling relations of the droplet bouncing direction with respect to Weber number and cantilever stiffness. By this technique, the seemed disorganized postimpact movements of droplets are programmable and predictable, achieving the goal of where to point and where to hit automatically. This work advances the understanding of natural droplet impact phenomena, opens a new avenue for delicately controlling liquid motion in space with soft materials, and inspires a plethora of applications like soft robots to transport materials and energies, monitor plant growth as well as predict pathogen transmission in plants.

Planar bilayer lipid membranes (BLMs) are widely used as models for cell membranes in various applications, including drug discovery and biosensors. However, the nanometer-thick bilayer structure, assembled through hydrophobic interactions of amphiphilic lipid molecules, makes such BLM systems mechanically and electrically unstable. In this study, we developed a device to reform BLMs using a microair bubble. The device consists of a double well divided by a separator with a microaperture, where a BLM was formed by infusing a lipid-dispersed solvent and an aqueous droplet into each well in series. When the BLM ruptured, a microair bubble was injected from the bottom of the well to split the merged aqueous droplet at the microaperture, which resulted in the reformation of two lipid monolayers on the split droplets. By bringing the two droplets into contact, a new BLM was formed. An angled step design was introduced in the BLM device to guide the bubble and ensure the splitting of the merged droplet. We also elucidated the optimal bubble inflow rate for the reproducible BLM reformation. Using a 4-channel parallel device, we demonstrated the individual and repeatable reformation of BLMs. Our approach will aid the development of automated and arrayed BLM systems.

Droplet impact and breakup on meshes are relevant to a number of applications involving filters, textiles, and other spatially inhomogeneous media encountering gas-dispersed liquids. This study presents high-resolution simulation results of mm-size droplets striking wettability-patterned meshes with the goal of (a) replicating prior physical experiments, (b) identifying sensitivities to the initial conditions and wettability of the mesh wires, and (c) studying the fluid-field dynamics when droplets strike such meshes. The insights from the present model may help to advance understanding of droplet atomization on meshes, which depends on a number of parameters that are nontrivial to control in an experimental setting. The analysis is carried out by benchmarking the numerical methods used in a commercial software package for orthogonal droplet impact on a flat smooth surface, followed by a convergence analysis, and finally, simulation of specific experiments and case studies involving wettability-patterned mesh targets. We show that the wettability contrast between the hydrophilic and hydrophobic domains on the mesh as well as the contact angle hysteresis on each side play a critical role in determining whether liquid pinch-off occurs. The three-dimensional computational framework constructed in this work is a step toward predicting the postimpact behavior of droplets that strike woven meshes and other porous inhomogeneous media consisting of materials with different wetting properties.

The vast majority of prior studies on droplet impact have focused on collisions of liquid droplets with spatially homogeneous (i.e., uniform-wettability) surfaces. But in recent years, there has been growing interest on droplet impact on nonuniform wettability surfaces, which are more relevant in practice. This paper presents first an experimental study of axisymmetric droplet impact on wettability-patterned surfaces. The experiments feature millimeter-sized water droplets impacting centrally with $��W�e�<�100�$ on a flat surface that has a circular region of wettability $��{\theta }_1�$ (Area 1) surrounded by a region of wettability $��{\theta }_2�$ (Area 2), where $��{\theta }_1�<�{\theta }_2�$ (i.e., outer domain is less wettable than the inner one). Depending upon the droplet momentum at impact, the experiments reveal the existence of three possible regimes of axisymmetric spreading, namely (I) interior (only within Area 1) spreading, (II) contact-line entrapment at the periphery of Area 1, and (III) exterior (extending into Area 2) spreading. We present an analysis based on energetic principles for $��{\theta }_1�<�{\theta }_2�$, and further extend it for cases where