Abstract Water‐in‐oil (W/O) droplets are widely used as microcompartments for high‐throughput biological assays, yet the continuous oil phase renders them incompatible with standard aqueous analytical instruments such as flow cytometers. To address this limitation, we present a robust microfluidic strategy for generating monodisperse water‐in‐oil‐in‐water (W/O/W) polymer capsules stabilized by food‐grade amphiphile tetraglycerol polyricinoleate (TGPR). Using a polydimethylsiloxane microfluidic device that we previously developed to generate monodisperse giant lipid vesicles, we achieved reliable capsule formation via interfacial transfer. Optimization of the channel height and operating pressures facilitated spontaneous W/O droplet splitting during the transfer process, significantly increasing the generation frequency to approximately 160 Hz while maintaining high monodispersity. The resulting TGPR capsules exhibited superior long‐term stability and thermal resistance compared to lipid vesicles. We demonstrated the utility of these capsules as robust bioreactors through successful in‐capsule PCR amplification and the cultivation of Escherichia coli . These findings establish TGPR‐stabilized capsules as a scalable, aqueous‐compatible alternative to conventional W/O droplets for synthetic biology and high‐throughput screening applications.
Abstract Hydrogel particles are attracting increasing interest as tunable microscale platforms that combine precise architectures with diverse chemical, biological, and physical functionalities. Among fabrication strategies, droplet microfluidics has emerged as a powerful technology for producing monodisperse hydrogel particles with controllable size, composition, and internal structure via precise regulation of microscale fluid dynamics. This review first outlines the fundamental principles of droplet microfluidics, including chip fabrication, channel design, droplet generation dynamics, energy input modes, and fluidic compositions, which collectively underpin reliable particle production. We then highlight compatible hydrogel precursor systems, gelation methods, and functionalization strategies for engineering tailored hydrogel particles. The versatility of such particles is further emphasized, supporting applications in cell culture, 3D bioprinting, drug delivery, and diagnostics. Finally, we provide an outlook on future directions, including system parallelization, process automation, and artificial intelligence integration, anticipated to expand the scalability, functionality, and multidisciplinary impact of droplet‐microfluidic hydrogel platforms.
Abstract Electrocatalytic hydrogen evolution reaction (HER) is crucial for green hydrogen production and the transition toward low‐carbon energy systems. However, the issues related to electrochemical gas bubbles, particularly at high current densities, have become a critical bottleneck for HER performance, resulting in active site isolation, increased ohmic resistance, and large concentration overpotential. Addressing these bubble‐related limitations is therefore essential for advancing HER efficiency. This review aims to provide a comprehensive understanding of bubble manipulation strategies for enhancing HER by (1) exploring the fundamental principles governing bubble dynamics at electrode interfaces, (2) presenting the strategies to mitigate bubble‐related issues at electrode interfaces, that is, passive strategies and active strategies, and (3) offering our insights into the challenges and opportunities for bubble dynamics in HER. By consolidating these projects, this review aims to advance the rational design of bubble management strategies and inspire innovative approaches for efficient hydrogen production.
Abstract Photothermal bubble holds promising for microfluidics, microfabrication, drug delivery, chemical engineering, and soft robotics, arising from its inherent features and advantages, such as easy formation, remote operation, rapid response, tunable flexibility, and biocompatibility. This review systematically examines advances in photothermal bubble generation and manipulation over the past decade, focusing on nucleation mechanisms, growth dynamics, and motion control. Three principal modes of bubble motion are analyzed: horizontal locomotion driven by laser translation; spontaneous bouncing/oscillatory motion under stationary laser irradiation; and three‐dimensional actuation via optimized heating configurations. We highlight representative applications demonstrating the versatility of photothermal‐driven bubble manipulation, and discuss the related limitations associated with photothermal bubble actuation in various application scenarios. Furthermore, we present outlook for key challenges and future directions for broader implementation. This review offers fundamental insights into the underlying dynamics and control mechanisms of photothermal bubbles, facilitating the transition of this technology from laboratory research to practical applications.
Abstract Electrowetting‐on‐dielectric (EWOD) enables electrical modulation of liquid contact angle and is widely used for droplet actuation; however, its reliance on bulky high‐voltage power supplies limits portability. Triboelectric nanogenerators (TENGs) provide a lightweight, mechanically driven alternative, yet existing EWOD–TENG systems still rely on electronic controllers or rapid manual actions. Here, we present a hand‐powered and mechanically programmable EWOD platform that integrates a contact‐separation TENG (CS‐TENG) with a mechanically encoded punch‐card switch array, both driven by a single hand‐crank mechanism. Hand cranking simultaneously generates high voltage for EWOD actuation and advances a punch‐card tape to sequentially trigger electrode switching according to predefined punch‐hole patterns. Unlike conventional voltage sources, the CS‐TENG delivers a constant‐charge output per cycle, leading to distinct EWOD behavior. An EWOD–TENG model with trapped charge elucidates key phenomena, including bias‐dependent asymmetric EWOD arising from dielectric charge trapping and stepwise voltage attenuation caused by capacitive EWOD loading during sequential switching. The punch‐card switch array converts physical hole patterns into time‐synchronized electrode activation, enabling programmable droplet manipulation without electronic controllers. The resulting platform executes predefined droplet operations using only a hand crank as the sole energy and control input, achieving autonomous, portable, and robust droplet control for field‐deployable microfluidic systems.
The unique one-dimensional structure and outstanding optoelectronic properties of carbon nanotubes (CNTs) have motivated extensive research into their directed assembly. Although the droplet evaporation method offers a straightforward strategy for aligning CNTs, controlling the final deposited structure remains challenging due to the inherently high aspect ratio of CNTs. In this study, we prepared four substrates with distinct wettability and friction properties through surface modification. Time-resolved contact angle analysis was employed to monitor the evolution of the three-phase contact line and the evaporation kinetics of CNT droplets on these substrates. Combined with detailed microstructural characterization, we found that on hydrophobic, highly friction surfaces (NBE-modified silica), strong friction stabilizes the contact line during droplet drying. At the same time, rapid solvent evaporation induces droplet shrinkage, leading to a pronounced circumferential flow along the droplet periphery. This flow promotes the formation of long-range aligned CNT orientation bands with a width of similar to 150 m (orientation degree of 9.2 degrees) after drying. These findings provide insights for the design and modification of substrates used in evaporation-driven oriented assembly of CNTs.
Abstract Cavitation phenomena are widespread in nature and industrial applications. Researchers have conducted in‐depth studies on the interaction of cavitation bubbles with a single boundary. However, when cavitation bubbles interact simultaneously with multiple boundaries, for example, a rigid wall and free surface, the process and mechanisms are still unclear. Here, experiments are carried out to study the dynamics of cavitation bubbles within a liquid layer. The results show that due to the combined effect of a rigid wall and free surface, the bubbles exhibit a significant downward migration in the liquid layer. Based on Kelvin impulse theory, we calculate the impulse experienced by the bubble and find a good consistency between the downward migration of the bubble and the impulse it experiences. By varying the initial position of the bubble in the liquid layer, we find that the confinement effect of the rigid wall significantly affects the bubble's collapse morphology, leading to different post‐collapse phenomena of free surface motion.
Precise control of particle deposition patterns from evaporating colloidal droplets is vital in manufacturing advanced materials and devices. Despite significant progress, the interplay between substrate mechanical anisotropy and gravity in shaping the patterns remains underexplored. Here, we report an intriguing phenomenon of orientation-dependent sub-microliter droplet evaporation and particle deposition on inclined, stretched soft substrates. We demonstrate that sequential control of substrate stretching, inclination, and rotation can effectively tailor complex deposition patterns. Stretching creates an anisotropic wetting state, which pinches the droplet into a spindle shape during late-stage evaporation. At the droplet's tapered ends, reduced contact angles enhance local capillary flows, driving particle accumulation and forming an elongated ring pattern with periodic rim height gradient. Inclination biases both surrounding airflow and particle sedimentation, resulting in asymmetric, downward-cumulated deposition patterns. Remarkably, rotating the inclined stretched substrate reorients this asymmetry, allowing precise manipulation of the deposit's densest regions and final morphology. This work deepens the fundamental understanding of anisotropic droplet dynamics and offers a simple yet powerful strategy for engineering tailored micropatterns.
Abstract Magnetic digital microfluidics offers a promising alternative to the dominant electrowetting‐based digital microfluidics but faces significant challenges regarding system portability, sample contamination, and limited functionality. Here, we report a magneto‐silicobotic system utilizing silicone oil–based ferrofluid droplets as inert microrobots for versatile manipulation of droplets and solid particles. The system combines a cost‐effective and portable microcoil array with a millimeter‐scale permanent magnet to generate localized magnetic fields for precise actuation. It achieves precise droplet manipulation with an average tracking error of 0.15 mm, stable operation over 360 cycles, and speeds up to 52 mm/s. It can transport droplets 50 times its volume, achieve maximum transport speeds of 35 mm/s, maintain long‐term residual‐free transport, and handle diverse liquids, including organic solvents and biological samples. We further design a ferrofluid‐based pipette to perform droplet splitting and dispensing, thus completing all basic operations of digital microfluidics. Additionally, we extend the robot's capability to include the transport and automated assembly of solid particles using integrated visual feedback. We envision that this portable, additive‐free platform will have a significant impact on point‐of‐care testing, miniaturized biochemical assays, and automated lab‐on‐a‐chip systems.
Abstract Omniphobic surfaces have attracted considerable interest in recent years due to their various applications. Such surfaces conventionally require a fluorine‐based surface chemistry with a low surface energy, which is achieved through surface modification using per‐ and polyfluoroalkyl substances (PFAS). However, PFAS, as emerging contaminants, pose serious health risks to humans and wildlife. The discovery of doubly reentrant texture has paved the way for designing fluorine‐free omniphobic surfaces, but current methods for fabricating this unique texture involve complex processes and expensive equipment. In this work, we demonstrate that the simple and inexpensive digital light processing three‐dimensional printing can be used to fabricate not only doubly reentrant but also triply reentrant micropillars. Our printed micropillars with doubly reentrant features display repellency towards liquids with a wide range of surface tensions after surface modification using a hydrocarbon silane. Water and ethanol droplets remain in the Cassie–Baxter state during the entire evaporation process, indicating the robust omniphobicity of our surface. Moreover, the printed micropillars with triply reentrant features exhibit omniphobicity even when the micropillars are composed of hydrophilic resin. Our simple method for achieving fluorine‐free omniphobicity has the potential of facilitating the development of sustainable liquid‐repellent surfaces and the mitigation of PFAS contamination.
Abstract Wettability characterizations are crucial to many disciplines, including dropwise condensation heat transfer, microfluidics, and self‐cleaning materials. Droplet goniometry is a standard technique for such characterizations due to its relatively simple setup and execution. However, goniometry is severely limited when analyzing irregular droplets and multi‐droplet systems, and alternative methods often require additional equipment, exhaustive scans, or non‐physics‐based modeling, hindering their effectiveness. In response, we created the reverse catch light method, which constructs physics‐informed, three‐dimensional digital twins of droplets from a single overhead image. We locate droplet contact lines and point light reflections on a real droplet surface, then iteratively solve the Young‒Laplace equation to generate candidate droplet surfaces whose unique reflections match those observed in our image. We experimentally validated this method with goniometry across varied droplet contact angles, volumes, liquid types, and substrates. Furthermore, we highlight new capabilities that are impractical with classical goniometry, such as constructing digital twins of irregularly shaped droplets or sliding droplets for hysteresis characterization. Finally, we demonstrate the ability to obtain rich, spatiotemporal statistical data from multi‐droplet systems that would be impossible with classical goniometry. Our findings suggest that this methodology could be easily deployed for enhanced in situ diagnostics of otherwise difficult‐to‐analyze, realistic systems.
Abstract Water transport across lipid bilayers is a fundamental biological process, but most model systems fail to capture the molecular complexity of biological cell membranes. As a result, the extent to which the membrane composition choreographs its permeability and how it is modulated by small molecules is poorly understood. Here, we use droplet interface bilayers (DIBs) formed using either synthetic 1,2‐dioleoyl‐sn‐glycero‐3‐phosphocholine (DOPC) or biologically sourced phosphatidylcholine (PC) and phosphatidylethanolamine (PE) lipids to examine how lipid biomimicry influences passive water permeability. We also study the effect of two “membrane‐active” molecules: cholesterol, which in our membranes should reduce permeability, and niacin, which has been reported to enhance membrane permeability. We find that biomimetic PC/PE membranes exhibit an approximately eightfold higher baseline water permeability than synthetic DOPC membranes. The presence of cholesterol reduces water permeability in both types of membranes, while niacin has an opposing effect, increasing water permeability in a concentration‐dependent manner. However, only synthetic membranes show full recovery and overshoot of baseline levels of water permeability with the addition of niacin, whereas biomimetic membranes display limited recovery. Our work shows that the use of biologically relevant phospholipid mixtures in model membranes is essential to accurately capture the behavior of the membrane with respect to water permeability.
Abstract The efficiency of industrial electrocatalytic reactions depends not only on generating gaseous products but also on their detachment from the catalyst surface. Bubble adhesion at the solid–liquid interface blocks active sites, increases mass‐transfer resistance, elevates overpotentials, and consumes extra energy, thereby slowing reactions and reducing economic viability. Conventional strategies, such as tailoring catalyst microstructures or optimizing reactor flow fields, control bubble behavior passively and lack adaptability to varying conditions. In contrast, external physical fields, including acoustic, magnetic, thermal, mechanical, and optical inputs, offer active regulation. They provide noncontact operation, rapid responsiveness, and low energy consumption. By modifying interfacial tension, inducing microflows, applying localized forces, or altering solution properties, these approaches lower the detachment barrier, enhance mass transport, and boost catalytic performance. This review summarizes advances in bubble management using external energy fields, emphasizing the underlying physicochemical coupling mechanisms. It compares the strengths and limitations of different fields and outlines future directions, including multi‐field synergy and adaptive feedback control. Together, these insights provide a framework for designing efficient strategies for interfacial bubble regulation.
Pore condensation enables vapor-liquid transitions below saturation humidity, yet the microscopic dynamics of the embryo formation and cluster growth within the pore remain unresolved. Through molecular simulations on tunable-wettability surfaces (124(degrees)-90(degrees)), we discover that wettability influences condensation kinetics: reducing contact angles 124(degrees)to 90(degrees)from to decreases the molecular displacement by approximate to 60% and the molecular potential energy by approximate to 10% for the condensed water molecules, increasing vapor capture tenfold. Pores enhance nucleation on hydrophobic surfaces (124(degrees)) by restricting molecular mobility, an effect attenuated with increasing wettability. Specifically, we identify four distinct cluster evolution modes: intra-pore (confined growth), on-plane, tumble (pore-to-pore migration), and bounce. Analysis of the above modes reveals that spatial confinement accelerates initial nucleation but suppresses later-stage coalescence. As a result, the cluster growth rate of the intra-pore mode can be initially higher than that of the flat reference but later drops below. This work establishes a microscopic mechanism for pore condensation.
Abstract Against the backdrop of carbon neutrality, the demand for sustainable energy keeps growing. Among numerous new energy technologies, the triboelectric nanogenerator (TENG) has garnered widespread attention in recent years due to its ability to harvest widely distributed tiny mechanical energy from the environment. With the advancement of TENG technology, solid‒liquid interface‐based TENGs have not only enabled the recovery of mechanical energy from droplets but have also been utilized for self‐powered sensing of liquid samples, demonstrating broad application potential. In recent years, in addition to research on triboelectric materials, the development of novel electrode structures has also been regarded as a crucial approach for enhancing the electrical output performance of solid‒liquid interface‐based TENGs. In this review, we traced the developmental trajectory of electrode structures in solid‒liquid interface‐based TENGs, and compiled the corresponding equivalent circuit diagrams and signal characteristics for devices featuring three representative electrode structures. Meanwhile, we summarized the representative applications of each electrode structure in energy conversion and self‐powered sensing and discussed the suitable application scenarios for each electrode structure. We believe that this review guides the in‐depth use of solid‒liquid interface‐based TENGs in energy harvesting and self‐powered sensing.
