When microdroplets with quasi-spherical contact angles coalesce together on a low-adhesion substrate, the capillary-inertial expansion of the liquid bridge induces a dramatic out-of-plane jumping event due to symmetry breaking. From the onset of merging, droplet jumping initiates after a capillary-inertial time scale of tci ∼ 1–100 μs with characteristic jumping speeds of order vj ∼ 0.1m/s. This coalescenceinduced jumping-droplet effect is most commonly observed among a population of growing dew droplets on a superhydrophobic condenser, but can also occur by colliding deposited droplets together or during droplet sliding on fog harvesters. In this review, we cover the historical development of capillary-inertial jumping droplets, summarize the decade-long effort to rationalize the ultra-low energy conversion efficiency and critical droplet size of the phenomenon, and then present 15 variations on a theme of jumping. Capillary-inertial jumping droplets are not only a visceral illustration of the surprising power of surface tension at the microscale but they also have the potential to enhance phase-change heat transfer, enable selfcleaning surfaces, combat frost formation, harvest energy, and govern the rate of disease spread for wheat crops.
Frosting occurs due to the freezing of condensed water droplets on a supercooled surface. The nucleated frost propagates through interdroplet bridges and covers the entire surface, resulting from the deposition of highly supersaturated vapor surrounding tiny droplets. While inhibition of the formation of frost bridges is not possible, the propagation of frost can be delayed by effectively removing tiny droplets. Passive technologies, such as superhydrophobic surfaces (SHS) and hydrophobic slippery liquid-infused porous surfaces (SLIPS), rely on static growth and direct contact with densely distributed droplets. However, use of these approaches in delaying frost propagation involves challenges, as the interdroplet distance remains small. Here, we report a new approach of spontaneous droplet movement on hydrophilic SLIPS to delay the formation of interdroplet frost bridges. Surface tension forces generated by the hydrophilic oil meniscus of a large water droplet efficiently pull neighboring droplets with a diameter of less than 20 µm from all directions. This causes a dynamic separation between water droplets and an adjacent frozen droplet. Such a process delays the formation and propagation of interdroplet frost bridges. Consequently, there is significant delay in frosting on hydrophilic SLIPS compared to those on SHS and hydrophobic SLIPS.
Controlling the wetting and spreading of microdroplets is key to technologies such as microfluidics, ink-jet printing, and surface coating. Contact angle goniometry is commonly used to characterize surface wetting by droplets, but the technique is ill-suited for high contact angles close to 180°. Here, we attach a micrometric-sized droplet to an atomic force microscope cantilever to directly quantify droplet-solid friction on different surfaces (superhydrophobic and underwater superoleophobic) with sub-nanonewton force resolutions. We demonstrate the versatility of our approach by performing friction measurements using different liquids (water and oil droplets) and under different ambient environments (in air and underwater). Finally, we show that underwater superoleophobic surfaces can be qualitatively different from superhydrophobic surfaces: droplet-solid friction is highly sensitive to droplet speeds for the former but not for the latter surface.
Electrowetting on dielectric (EWOD) allows rapid movement of liquid droplets on a smooth surface, with applications ranging from lab-on-chip devices to microactuators. The in-plane force on a droplet is a key indicator of EWOD performance. This force has been extensively modeled but few direct experimental measurements are reported. We study the EWOD force on a droplet using two setups that allow, for the first time, the simultaneous measurement of force and contact angle, while imaging the droplet shape at 6000 frames/s. For several liquids and surfaces, we observe that the force saturates at a voltage of approximately 150 V. Application of voltages of up 2 kV, that is, 10 times higher than is typical, does not significantly increase forces beyond the saturation point. However, we observe that the transient dynamics, localized at the front contact line, do not show saturation with voltage. At the higher voltages, the initial front contact line speed continues to increase, the front contact angle temporarily becomes near zero, creating a thin liquid film, and capillary waves form at the liquid-air interface. When the localized EWOD forces at the contact line exceed the capillary forces, projectile droplets form. Increasing surface tension allows for higher droplet forces, which we demonstrate with mercury.
Glaucoma, the leading cause of irreversible blindness worldwide, is closely linked to aqueous overaccumulation and elevated intraocular pressure (IOP). For refractory glaucoma, aqueous shunts with valves are commonly implanted for effective aqueous drainage control and IOP stabilization. However, existing valved glaucoma implants have the disadvantages of inconsistent valve opening/closing pressures, poor long-term repeatability due to their reliance on moving parts, and complex architectures and fabrication processes. Here, we propose a novel valving concept, the droplet Laplace valve (DLV), a three-dimensional printable moving-parts-free microvalve with customizable and consistent threshold valving pressures. The DLV uses a flow discretization unit governed by capillarity, comprising a droplet-forming nozzle, and a separated reservoir to digitize continuous flow into quantifiable droplets. Unlike the classic one-time-use Laplace valves, the DLV’s unique design allows for its reusability. The opening pressure is adjustable by varying the nozzle size, like the classic Laplace valves (following the Young–Laplace equation), while the closing pressure can be modified by tuning the separation distance and the reservoir size. Various DLVs with customizable opening pressures from 5 to 11 mmHg have been demonstrated, with opening/closing pressure differences suppressed down to <0.5 mmHg (<0.15 mmHg under the best conditions). Thanks to its moving-parts-free nature and digitized flow properties, the DLV shows a highly repeatable valving performance (<1.7%, 1000 cycles) and a predictable linear flow rate–pressure correlation (R2 > 0.99). Preliminary ex vivo validation in an enucleated porcine eye confirms the DLV’s efficiency in aqueous shunting and prompt IOP stabilization. The DLV technology holds great promise in glaucoma implants for IOP management and various microsystems for flow control.
Transport of ions and water is essential for diverse physiological activities and industrial applications. As the dimension approaches nano and even angstrom scale, ions and water exhibit anomalous behaviors that differ significantly from the bulk. One of the key reasons for these distinctive behaviors is the prominent influence of surface effects and related transport properties occurring at the interface under such (sub)nanoconfinement. Therefore, exploring nanofluidic transport at the interfaces could not only contribute to unraveling the intriguing ion and water transport behaviors but also facilitate the development of nanofluidic devices with tunable mass transport for practical applications. In this review, we focus on three crucial interfaces governing ion and water transport, namely liquid-gas interface, liquid-solid interface, and liquid-liquid interface, with emphasis on elucidating their intricate interfacial structures and critical roles for nanofluidic transport phenomena. Additionally, potential applications associated with liquid-gas, liquid-solid, and liquid-liquid interfaces are also discussed. Finally, we present a perspective on the pivotal roles of interfaces on nanofluidics, as well as challenges in this advancing field.
In the present mini-review, droplet impacting on a liquid pool, jet impingement, and binary droplet collision of nonreacting liquids are first summarized in terms of basic phenomena and the corresponding nondimensional parameters. Then, two representative hypergolic bipropellant systems, a hypergolic fuel of N,N,N’,N’-tetramethylethylenediamine (TMEDA) and an oxidizer of white fuming nitric acid (WFNA) and a monoethanolamine-based fuel (MEA-NaBH4) and a high-density hydrogen peroxide (H2O2), are discussed in detail to unveil the rich underlying physics such as liquid-phase reaction, heat transfer, phase change, and gas-phase reaction. This review focuses on quantifying and interpreting the parametric dependence of the gas-phase ignition induced by droplet collision of liquid hypergolic propellants. The advances in droplet collision of hypergolic propellants are important for modeling the real hypergolic impinging-jet (spray) combustion and for the design optimization of orbit-maneuver rocket engines.
Droplets and bubbles have a wide range of applications in industry, agriculture, and daily life, and their controllable manipulation is of significant scientific and technological importance. Versatile magnetically responsive manipulation strategies have been developed to achieve precise control over droplets and bubbles. To manipulate nonmagnetic droplets or bubbles with magnetic fields, the presence of magnetic medium is indispensable. Magnetic additives can be added to the surface or interior of droplets and bubbles, allowing for on-demand manipulation by direct magnetic actuation. Alternatively, magnetically responsive elastomer substrates can be used to actuate droplets and bubbles by controlling the deformation of microstructures on the substrates through magnetic stimulation. Another strategy is based on untethered magnetic devices, which enables free mobility, facilitating versatile manipulation of droplets and bubbles in a flexible manner. This paper reviews the advances in magnetically responsive manipulation strategies from the perspective of droplets and bubbles. An overview of the different classes of magnetic medium, along with their respective corresponding droplet/bubble manipulation methods and principles, is first introduced. Then, the applications of droplet/bubble manipulation in biomedicine, microchemistry, and other fields are presented. Finally, the remaining challenges and future opportunities related to regulating droplet/bubble behavior using magnetic fields are discussed.
Preventing the accretion of droplets on surfaces is vital and slippery liquid-infused porous surfaces (SLIPS) have promising application prospects, such as surface selfcleaning and droplet transportation. In this work, controllable self-transport of bouncing droplets on ultraslippery surfaces with wedge-shaped grooves is reported. The impact behaviors of droplets on SLIPS under various impact velocities and diameters are explored, which can be classified as hover, total bounce, partial bounce, Worthington jet, and crush. SLIPS with wedge-shaped grooves were designed to transport accreted droplets. An energy and transport model is established to explain the impact and self-transport mechanism, where the Laplace pressure and moving resistance between droplets play a key role. Finally, SLIPS with branched wedgeshaped grooves were designed for droplet self-transport and demonstrated advantages. This work provides a general reference for spontaneous motion control of sessile droplets, droplets with initial impacting velocity, or even liquid films.
Gravity-induced drainage is one of the main destabilizing mechanisms for soap bubbles and foams. Here we show that solely through acoustic levitation without introducing any chemical stabilizers, liquid drainage in the bubble film can be completely inhibited, therefore leading to a significant enhancement of bubble lifetime by more than two orders of magnitude and enabling the bubble to survive puncturing by a needle. Based on sound simulation and force analysis, it has been found that acoustic radiation force, exerted on both the inner and outer surfaces of the levitated bubble, acts in opposite directions, thus providing a squeezing effect to the bubble film. The hydrostatic pressure that induces drainage has been balanced by the acoustic radiation pressure exerted on both sides of the film, which is at the origin of the sound stabilization mechanism. This study provides new insights into the interplay between sound and soap bubbles or films, thus stimulating a wide range of fundamental research concerning bubble films and expanding their applications in bio/chemical reactors.