A fabric-based hydrovoltaic electricity generator with multi-component carbon black for sustainable energy output.

Water evaporation-induced electricity generators are considered a promising green energy-harvesting technology to alleviate the increasingly serious fossil energy crisis. Previous water evaporation-induced electricity generators mainly focused on single component carbon black, limiting the improvements in energy output. At present, there are relatively few studies on multi-component carbon black for improving electricity-generation performance. Herein, inspired by plant transpiration, we designed a fabric-based water evaporation-induced electricity generator (FWEG) based on multi-component carbon black, which can maintain a voltage of 0.65 V for more than 48 h. Through the synergistic effect of multi-component carbon black-enhanced oxygen-containing functional density, the FWEG can generate an enhanced output current of 61.61 µA without any additional energy input. Moreover, we show that the FWEG can be integrated readily to charge commercial capacitors or directly power LED lights and calculators for a long time. This work provides new insights for designing high-performance hydrovoltaic electricity generators for sustainable green energy harvesting.

Versatile bubble maneuvering on photopyroelectric slippery surfaces.

Contactless bubble manipulation with a high spatiotemporal resolution brings a qualitative leap forward in a variety of applications. Despite considerable advances, light-induced bubble maneuvering remains challenging in terms of robust transportation, splitting and detachment. Here, a photopyroelectric slippery surface (PESS) with a sandwich structure is constructed to achieve the versatile bubble manipulation. Due to the generated dielectric wetting and nonuniform electric field under the irradiation of near infrared (NIR) light, a bubble is subject to both the Laplace force and dielectrophoresis force, enabling a high-efficiency bubble steering. We demonstrate that the splitting, merging and detachment of underwater bubbles can be achieved with high flexibility and precision, high velocity and agile direction maneuverability. We further extend the capability of bubble control to microrobots for cargo transportation, micropart assembly and transmission of gear structures. We envision this robust bubble manipulation strategy on the PESS would provide a valuable platform for various bubble-involved processes, ranging from microfluidic devices to soft robotics.

An electrothermal platform for active droplet manipulation.

The smart control of droplet transport through surface structures and external fields provides exciting opportunities in engineering fields of phase change heat transfer, biomedical chips, and energy harvesting. Here we report the wedge-shaped slippery lubricant-infused porous surface (WS-SLIPS) as an electrothermal platform for active droplet manipulation. WS-SLIPS is fabricated by infusing a wedge-shaped superhydrophobic aluminum plate with phase-changeable paraffin. While the surface wettability of WS-SLIPS can be readily and reversibly switched by the freezing-melting cycle of paraffin, the curvature gradient of the wedge-shaped substrate automatically induces an uneven Laplace pressure inside the droplet, endowing WS-SLIPS the ability to directionally transport droplets without any extra energy input. We demonstrate that WS-SLIPS features spontaneous and controllable droplet transport capability to initiate, brake, lock, and resume the directional motion of various liquid droplets including water, saturated NaCl solution, ethanol solution, and glycerol, under the control of preset DC voltage (∼12 V). In addition, the WS-SLIPS can automatically repair surface scratches or indents when heated and retain the full liquid-manipulating capability afterward. The versatile and robust droplet manipulation platform of WS-SLIPS can be further used in practical scenarios such as laboratory-on-a-chip settings, chemical analysis and microfluidic reactors, paving a new path to develop advanced interface for multifunctional droplet transport.

Ultrasonic tweezer for multifunctional droplet manipulation.

Spatiotemporally controllable droplet manipulation is essential in diverse applications, ranging from thermal management to microfluidics and water harvesting. Despite considerable advances, droplet manipulation without surface or droplet pretreatment is still challenging in terms of response and functional adaptability. Here, a droplet ultrasonic tweezer (DUT) based on phased array is proposed for versatile droplet manipulation. The DUT can generate a twin trap ultrasonic field at the focal point for trapping and maneuvering the droplet by changing the position of the focal point, which enables a highly flexible and precise programmable control. By leveraging the acoustic radiation force resulting from the twin trap, the droplet can pass through a confined slit 2.5 times smaller than its own size, cross a slope with an inclination up to 80°, and even reciprocate in the vertical direction. These findings provide a satisfactory paradigm for robust contactless droplet manipulation in various practical settings including droplet ballistic ejection, droplet dispensing, and surface cleaning.

Recent advances in superwetting materials for separation of oil/water mixtures.

Engineering surfaces or membranes that allow an efficient oil/water separation is highly desired in a wide spectrum of applications ranging from oily wastewater discharge to offshore oil spill accidents. Recent advances in biomimetics, manufacturing, and characterization techniques have led to remarkable progress in the design of various superwetting materials with special wettability. In spite of exciting progress, formulating a strategy robust enough to guide the design and fabrication of separating surfaces remains a daunting challenge. In this review, we first present an overview of the wettability theory to elucidate how to control the surface morphology and chemistry to regulate oil/water separation. Then, parallel approaches are considered for discussing the separation mechanisms according to different oil/water mixtures, and three separation types were identified including filtration, adsorption and other separation types. Finally, perspectives on the challenges and future research directions in this research area are briefly discussed.

Steerable drops on heated concentric microgroove arrays.

Guided drop transport is of great importance in various water and thermal management technologies. Unidirectional drop transport on a hot surface has been widely developed, but a bidirectional reversal is still challenging. Here, we report a steerable transport of drop impinging on heated concentric microgroove arrays, on which the directionality of drop transport is dictated by the drop boiling modes. In the transition boiling state, the driving force originated from the Laplace pressure difference rendered by the microgrooves, which enables the drop rebounding towards the center of curvature. While in the film boiling state, a net force towards the opposite side is generated between the grooves and the penetrated liquid, that drives the drop far away from the center of curvature. Our experimental and theoretical results uncover that the lateral displacement is controlled by both the Weber number and off-center distance. These findings strengthen our fundamental understanding of drop impact dynamics at high temperatures and are essential for effective cooling of hot-spot cores and drop sieving.

High-Efficiency Directional Ejection of Coalesced Drops on a Circular Groove.

Coalescence-induced drop jumping has received significant attention in the past decade. However, its application remains challenging as a result of the low energy conversion efficiency and uncontrollable drop jumping direction. In this work, we report the high-efficiency coalescence-induced drop jumping with tunable jumping direction via rationally designed millimeter-sized circular grooves. By increasing the surface-droplet impact site area and restricting the oscillatory deformation, the energy conversion efficiency of the jumping droplet reaches 43.5%, 600% as high as the conventional superhydrophobic surfaces. The droplet jumping direction can be tuned from 90° to 60° by varying the principal curvature of the circular groove, while the energy conversion efficiency remains unchanged. We show through theoretical analysis and numerical simulations that the directional jumping mainly originates from reallocation of droplet momentum enabled by the asymmetric liquid bridge impact. Our study demonstrates a simple yet effective method for fast, efficient, and directional droplet removal, which warrants promising applications in jumping droplet condensation, water harvesting, anti-icing, and self-cleaning.