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.
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.
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.
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.
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.
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.
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.
Due to poor chemical robustness, superhydrophobic surfaces become susceptible to failure, especially in a highly oxidative environment. To ensure the longterm efficacy of these surfaces, a more stable and environmentally friendly coating is required to replace the conventional salinization layers. Here, soot-templated surfaces with re-entrant nanostructures are precoated with poly-dimethylsiloxane (PDMS) brushes. An additional nanometer-thick lubricant layer of PDMS was then applied to increase chemical stability. The surface is superhydrophobic with a nanoscale liquid coating. Since the lubricant layer is thin, ridge formation is suppressed, which leads to low drop sliding friction and fast drop shedding. By introducing a bottom “reservoir” of a free lubricant as an oil source for self-replenishing to the upper layer, the superhydrophobic surface becomes more stable and heals spontaneously in response to alkali erosion and O2 plasma exposure. This design also leads to a higher icing delay time and faster removal of impacting cooled water drops than for uncoated surfaces, preventing icing at low temperatures.
The unique properties to combine the dual merits of both liquids and metals together make the gallium-based liquid metal (LM) droplets a class of unconventional substitute which possess great potential for a group of newly emerging areas, such as stretchable electronics, soft devices, micro sensors and actuators. In addition, LM droplets are undoubtedly an intriguing target worth of pursuing in fundamental hydrodynamic investigations due to their extremely high surface tension nature compared to classical nonmetallic fluids. Since the discovery of the diverse transformation phenomena and self-fueled droplet mollusks of LM that can move automatically in solution via single electricity or even without any external energy supply, tremendous attentions were attracted to this special fluidic object of LM droplets. Over the past decade, there has been a proliferation of explorations on LM droplet dynamics, while the involved contents are heterogeneous due to the interfacial physical/chemical activity of the LM and the diversity of the kinetic behaviors. To better understand and manipulate the droplet behavior and to promote further development of the LMs, this review is dedicated to summarize the latest progress and presents an overview on basic findings related to LM macrodroplet dynamics. Firstly, the extended definition of LM droplets and the corresponding fabrication methods are given. Then, typical works on LM droplet dynamics are systematically interpreted based on their different behavior categories. Finally, the perspectives, main obstacles and challenges restricting the development of LM droplet dynamics are pointed out.