Harnessing abundant kinetic water energy in diverse forms of river flows, ocean waves, tidal currents, raindrops, and others, is highly attractive to ease the energy crisis and satisfy the demands of scattered sensor network nodes in the Internet of things. Among them, raindrops, widely and ubiquitously distributed in nature and ambient living life, have been extensively explored and regarded as significant renewable energy carriers. Extensive efforts have been made to investigate dropletbased electricity nanogenerators in fundamental mechanism, performance, and applications for achieving sustainable energy demands of the rapidly developing society over the past decade. In this review, we introduce the remarkable progress in this field and discuss the fundamental mechanisms of droplet energy harvesting technology for achieving high-power generation. More significantly, a systematic review of droplet energy harvesting in different two-phase interfaces, including liquid-solid, liquid-liquid, and liquid-gas interfaces, is provided. Finally, this survey reveals that droplet-based electricity generators present vast potential in the power supply. At the same time, several development challenges and prospective solutions are discussed to spur future technological advancements.

Controlling acoustic streaming inside a droplet has excellent potential for enabling fluid and particle operations such as mixing, separation, and aggregation in various applications. Most concepts for generating surface acoustic waves are based on the placement of an interdigitated transducer at the side of a droplet, thus externally acting on the droplet. In this case, the flow structure inside the droplet is controlled by the relatively large scale of the interdigitated transducer compared to the droplet, thus limiting the local control of the flow. One possibility to overcome this drawback is to reduce the size of the actuator such that a highly focused ultrasound transducer can induce localized acoustic streaming in space. Here, we introduce a micro-spiral interdigitated transducer smaller than a droplet size that can generate micro-size vortices inside the droplet. This step enables a new way of controlling the flow inside the droplet, facilitating mixing, separation, aggregation, and patterning of particles. We study the characteristics of the acoustic streaming and the potential application of the flow for the separation and patterning of particles. The simplicity of the concept provides in-droplet particle manipulation toolsets for many applications such as biosensing, microbiology, and point-of-care devices.

Acoustic levitation has developed into a popular but elegant tool for the study of drops as well as soft matter due to its exceptional levitation capabilities to a variety of liquid samples. The acoustically levitated drops offer opportunities for the investigation of a wide range of fundamental issues related to liquid drops. In this review, the unique physics/chemical processes involved in acoustically levitated drops are dealt with. We first introduce the dynamics of the acoustically levitated drops, including drop oscillation, coalescence, and the associated capillary phenomena. The bubble formation and stability are also discussed. Depending on the inhibition of solid surfaces and the nonlinear effects of ultrasound, the self-assembly of colloidal particles at the air-liquid interface as well as granular matter in air is reviewed. In particular, the exploration of biological drops by using acoustic levitation is also highlighted. In the end, the concept of acoustic-levitation-fluidics and possible potential topics are proposed.

Precise control of the evaporation of multiple droplets on patterned surfaces is crucial in many technological applications, such as anti-icing, coating, and high-throughput assays. Yet, the complex evaporation process of multiple droplets on well-defined patterned surfaces is still poorly understood. Herein, we develop a digital twin system for real-time monitoring of key processes on a droplet microarray (DMA), which is essential for parallelization and automation of the operations for cell culture. Specifically, we investigate the evaporation of multiple nanoliter droplets under different conditions via experiments and numerical simulations. We demonstrate that the evaporation rate is not only affected by the environmental humidity and temperature but is also strongly linked to the droplet distribution on the patterned surfaces, being significantly reduced when the droplets are densely distributed. Furthermore, we propose a theoretical method to aid in the experimental detection of volumes and pH variation of evaporating droplets on patterned substrates. This versatile and practical strategy allows us to achieve active maneuvering of the collective evaporation of droplets on a DMA, which provides essential implications for a wide range of applications including cell culture, heat management, microreactors, biochips, and so on.

Liquid-liquid phase separation in a biotic cell system organizes complicated biochemical reactions and functions by forming membraneless compartments that allow a substrate to move across the phase boundary. On the other hand, liquid–liquid phase separation in an abiotic system gives rise to an emulsion and/or multiple droplets that hardly undergo chemical reactions. We have developed a method for the formation of phase-separated multiple droplet in a ternary mixture with a 3D-printed microchannel and demonstrated the occurrence of the iron(III) thiocyanate ligand exchange reaction in the multiple droplet. The reaction proceeded differently in the outer- and the inner-droplet phases, giving a different iron(III) complex that was identified on the basis of its color change. Surprisingly, the color change was dynamic, enabling visualization of the interphase mass transfer. At the same time, the color change dynamics synchronized with the multiple-droplet movement.

Liquid biopsy, a noninvasive technique to obtain tumor information from body fluids, is an emerging technology for cancer diagnosis, prognosis, and monitoring, providing crucial support for the realization of precision medicine. The main biomarkers of liquid biopsy include circulating tumor cells, circulating tumor DNA, microRNA, and circulating tumor exosomes. Traditional liquid biopsy detection methods include flow cytometry, immunoassay, polymerase chain reaction (PCR)-based methods, and next-generation sequencing (NGS)-based methods, which are time-consuming, labor-intensive, and cannot reflect cell heterogeneity. Droplet-based microfluidics with high throughput, low contamination, high sensitivity, and single-cell/singlemolecule/ single-exosome analysis capabilities have shown great potential in the field of liquid biopsy. This review aims to summarize the recent development in droplet-based microfluidics in liquid biopsy for cancer diagnosis.

Harvesting renewable water energy in various formats such as raindrops, waves, and evaporation is one of the key strategies for achieving global carbon neutrality. The recent decade has witnessed rapid advancement of the droplet-based electricity generator (DEG) with a continuous leap in the instantaneous output power density from 50W/m² to several kW/m². Despite this, further pushing the upper limit of the output performance of DEG is still constrained by low surface charge density and long precharging time. Here, we report a DEG incorporating the Kelvin water dropper (K-DEG) that can generate an ultrahigh instantaneous power density of 10⁵W/m² upon one droplet impinging. In this design, the Kelvin water dropper continuously replenishes the high density of surface charges on DEG, while DEG fully releases these surface charges into electric output. K-DEG with such a high output can directly light five 6-W commercial lamps and even charge a cellphone by using falling droplets.

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.

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.

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.

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.

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.

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.

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.