Light-driven Locomotion of Underwater Bubbles on Ultrarobust Paraffin-impregnated Laser-ablated Fe3O4-doped Slippery Surfaces

A Review of Droplet/Bubble Transportation on Bionic Superwetting Surface

The controllable droplets/bubble transportation has a wide range of applications in the fields of biomedical, chemistry, energy, and material applications, and has aroused great attention for its significant scientific and technology importance. The main challenges derived from the liquid/solid or gas/solid contact strength and actuating energy input. Artificial superwetting surfaces inspired by nature creatures have triggered technology revolution in many fields relevant to droplet operation, and the applied actuating force improve the controllability to preferential direction. In this review, we highlights recent advancements in droplets/bubble transportation on the superwetting surfaces driven by passive or active stimulation methods inspired by bionic function interfaces. The three main superwetting surfaces including superhydrophobic surface, slippery liquid-infused porous surface, hybrid surface, various stimuli methods including gravity/buoyance, chemical/morphology gradient, heat, magnetism, electricity, light, adhesion force, and prosperous applications including micro-reaction, biochemical analysis, fog collection/antifog, energy transfer, bubble/liquid micro-robot, self-cleaning, light/circle switch have been systematically summarized. Finally, the challenges and future perspectives of research innovations and practical applications are discussed.

Interfacial fluid manipulation with bioinspired strategies: special wettability and asymmetric structures

The inspirations from nature always enlighten us to develop advanced science and technology. To survive in complicated and harsh environments, plants and animals have evolved remarkable capabilities to control fluid transfer via sophisticated designs such as wettability contrast, oriented micro-/nano-structures, and geometry gradients. Based on the bioinspired structures, the on-surface fluid manipulation exhibits spontaneous, continuous, smart, and integrated performances, which can promote the applications in the fields of heat transfer, microfluidics, heterogeneous catalysis, water harvesting, etc. Although fluid manipulating interfaces (FMIs) have provided plenty of ideas to optimize the current systems, a comprehensive review of history, classification, fabrication, and integration focusing on their interfacial chemistry and asymmetric structure is highly required. In this review, we systematically introduce development and highlight the state-of-the-art progress of bioinspired FMIs. Firstly, the biological prototype and development timeline are presented, and the underlying mechanism of on-surface fluid control on versatile structures is analyzed. Secondly, the definition and classification of FMIs as well as the strategy for controlling fluid/interface interaction are discussed. Thirdly, emergent applications of FMIs in practical scenarios including fog/vapor collection, fluid diodes, interfacial catalysis, etc. are presented. Furthermore, the challenges and prospects of interfacial liquid manipulation are concluded. We envision that this review should provide guidance for the incorporation of FMIs into suitable situations, which enlightens interdisciplinary research and practical applications in the fields of interface chemistry, materials design, bionic science, fluid dynamics, etc.

Locomotion behavior of air bubbles on solid surfaces

Air bubbles are a common occurrence in both natural and industrial settings and are a significant topic in the fields of physics, chemistry, engineering, and medicine. The physical phenomena of the contact between bubbles and submerged solid surfaces, as well as the locomotion behavior of bubbles, are worth exploring. Bubbles are generated in an unbounded liquid environment and rise due to unbalanced external forces. Bubbles of different diameters follow different ascending paths, after which they approach, touch, collide, bounce, and finally adsorb to the solid surface, forming a stable three-phase contact line (TPCL). The bubbles are in an unstable state due to the unbalanced external forces on the solid surface and the effects generated by the two-phase contact surface, resulting in different locomotion behaviors on the solid surface. Studying the formation, transport, aggregation, and rupture behaviors of bubbles on solid surfaces can enable the controllable operation of bubbles. This, in turn, can effectively reduce the loss of mechanical apparatus in agro-industrial production activities and improve corresponding production efficiency. Recent research has shown that the degree of bubble wetting on a solid surface is a crucial factor in the locomotion behavior of bubbles on that surface. This has led to significant progress in the study of bubble wetting, which has in turn greatly advanced our understanding of bubble behavior. Based on this, exploring the manipulation process of the directional motion of bubbles is a promising research direction. The locomotion behavior of bubbles on solid surfaces can be controlled by changing external conditions, leading to the integration of bubble behavior in various scientific and technological fields. Studying the dynamics of bubbles in liquids with infinite boundaries is worthwhile. Additionally, the manipulation process and mode of these bubbles is a popular research direction.

On-Demand Transport Bubbles Adhering to Noncontiguous Patterned Superhydrophobic Surfaces Using a Superhydrophobic Tweezer

Bubble transportation and related flotation are ubiquitous phenomena in nature and industry. Various surfaces with distinct morphologies and specific wettability properties have been engineered by organisms in nature and by humans to facilitate the targeted movement of bubbles. However, existing methods predominantly rely on continuous surfaces, limiting the ability of bubbles to deviate from their path before reaching their intended destination. Therefore, directional transportation of bubbles using noncontiguous surfaces still remains a significant challenge. Inspired by water spiders' ability to capture bubbles underwater using their hydrophobic surface for survival, we propose a novel transport strategy that utilizes patterned superhydrophobic surfaces (PSHSs) and a superhydrophobic tweezer. This strategy is implemented by switching between the hood mode and puncture mode of the moving three-phase contact lines to load and unload the bubble. To quantitatively evaluate the loss ratio of the bubble during transportation, a simple and exquisite bubble-weighing apparatus is devised. Our findings indicate that circular PSHSs demonstrate superior bubble adhesion and achieve the highest bubble transport ratio of 95.1%. In order to validate the promising application of this novel method, we employ the computer numerical control (CNC) technology to facilitate the autonomous loading and precise transportation of underwater bubbles, as well as the blending and ionization of combustible gas bubbles with air bubbles at different volume ratios.

Droplet Manipulation on Bioinspired Slippery Surfaces: From Design Principle to Biomedical Applications

Droplet manipulation with high efficiency, high flexibility, and programmability, is essential for various applications in biomedical sciences and engineering. Bioinspired liquid-infused slippery surfaces (LIS), with exceptional interfacial properties, have led to expanding research for droplet manipulation. In this review, an overview of actuation principles is presented to illustrate how materials or systems can be designed for droplet manipulation on LIS. Recent progress on new manipulation methods on LIS is also summarized and their prospective applications in anti-biofouling and pathogen control, biosensing, and the development of digital microfluidics are presented. Finally, an outlook is made on the key challenges and opportunities for droplet manipulation on LIS.

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.

Directional Transport of Underwater Bubbles on Solid Substrates: Principles and Applications

The manipulation of underwater bubbles on substrates has received extensive research interest from both the scientific community and industry, including the chemical industry, machinery, biology, medicine, and other fields. Recent advances in "smart" substrates have enabled the bubbles to be transported on demand. Herein, the progress in the directional transport of underwater bubbles on various types of substrates is summarized, including planes, wires, and cones. The transport mechanism can be classified as buoyancy-driven, Laplace-pressure-difference-driven, and external-force-driven according to the driven force of the bubble. Moreover, the wide applications of directional bubble transport are reported, ranging from gas collection, microbubble reaction, bubble detection and classification, bubble switch, and bubble microrobots. Lastly, the advantages and challenges of various directional bubble transportation methods are discussed, and the current challenges and future prospects in this field are also discussed. This Review outlines the fundamental mechanisms of underwater bubble transportation on solid substrates and helps to understand the methods of optimizing bubble transportation performances.

Oil-Grafted Track-Assisted Directional Transport of Water Droplets and Submerged Air Bubbles on Solid Surfaces

Wettability-tailored tracks are emerging as an efficient approach to collecting and transporting underwater air bubbles as well as water from the mist. However, tailoring the surface wettability by modifying the surface structural features via physiochemical methods to create superhydrophilic-superhydrophobic contrast tracks suffers from long-term durability issues, while the emerging liquid-infused slippery surface has inherent design engineering limitations and issues from infused oil depletion. Herein, we demonstrate that by selective silicone oil grafting onto the glass substrate, it is possible to create a wettability contrast of ∼ 43°. Further, we illustrate the application of such tracks for underwater air bubble capturing and transportation in an aqueous medium with surface tension ranging from 72 to 43.5 mN/m. In addition, the potential of these nonadhesive and adhesive tracks for water collection from the mist is shown and the critical effect of the track dimension and intertrack spacing on the water harvesting rate is investigated in detail. The study illustrates that the nonadhesive nature of the oil-grafted region enables the easy transport of underwater air bubbles as well as water from the flow medium and thus offers an easy and facile approach to creating substrates for underwater air bubble collection and water harvesting.

Meniscus-Induced Directional Self-Transport of Submerged Bubbles on a Slippery Oil-Infused Pillar Array with Height-Gradient

Directional manipulation of submerged bubbles is fundamental for both theoretical research and industrial production. However, most current strategies are limited to the upward motion direction, complex surface topography, and additional apparatuses. Here, we report a meniscus-induced self-transport platform, namely, a slippery oil-infused pillar array with height-gradient (SOPAH) by combining femtosecond laser drilling and replica mold technology. Owing to the unbalanced capillary force and Laplace pressure difference, bubbles on SOPAH tend to spontaneously transport along the meniscus gradient toward a higher elevation. The self-transport performances of bubbles near the pillars depend on the complex meniscus shape. Significantly, to understand the underlying transport mechanism, the 3D meniscus profile is simulated by solving the Young-Laplace equation. It is found that the concave valleys formed between the adjacent pillars can change the gradient direction of the meniscus and lead to the varied transport performances. Finally, by taking advantage of a water electrolysis system, the assembled SOPAH serving as a bubble-collecting device is successfully deployed. This work should not only bring new insights into the meniscus-induced self-transport dynamics but also benefit potential applications in the field of intelligent bubble manipulation.

Wetting Ridge-Guided Directional Water Self-Transport

Directional water self-transport plays a crucial role in diverse applications such as biosensing and water harvesting. Despite extensive progress, current strategies for directional water self-transport are restricted to a short self-driving distance, single function, and complicated fabrication methods. Here, a lubricant-infused heterogeneous superwettability surface (LIHSS) for directional water self-transport is proposed on polyimide (PI) film through femtosecond laser direct writing and lubricant infusion. By tuning the parameters of the femtosecond laser, the wettability of PI film can be transformed into superhydrophobic or superhydrophilic. After trapping water droplets on the superhydrophilic surface and depositing excess lubricant, the asymmetrical wetting ridge drives water droplets by an attractive capillary force on the LIHSS. Notably, the maximum droplet self-driving distance can approach ≈3 mm, which is nearly twice as long as the previously reported strategies for direction water self-transport. Significantly, it is demonstrated that this strategy makes it possible to achieve water self-transport, anti-gravity pumping, and chemical microreaction on a tilted LIHSS. This work provides an efficient method to fabricate a promising platform for realizing directional water self-transport.

Stimuli-Responsive Liquid-Crystal-Infused Porous Surfaces for Manipulation of Underwater Gas Bubble Transport and Adhesion

Biomimetic artificial surfaces that enable the manipulation of gas bubble mobility have been explored in a wide range of applications in nanomaterial synthesis, surface defouling, biomedical diagnostics, and therapeutics. Although many superhydrophobic surfaces and isotropic-lubricant-infused porous surfaces have been developed to manipulate gas bubbles, the simultaneous control over the adhesion and transport of gas bubbles underwater remains a challenge. Thermotropic liquid crystals (LCs), a class of structured fluids, provide an opportunity to tune the behavior of gas bubbles through LC mesophase transitions using a variety of external stimuli. Using this central idea, the design and synthesis of LC-infused porous surfaces (LCIPS) is reported and the effects of the LC mesophase on the transport and adhesion of gas bubbles on LCIPS immersed in water elucidated. LCIPS are demonstrated to be a promising class of surfaces with an unprecedented level of responsiveness and functionality, which enables the design of cyanobacteria-inspired object movement, smart catalysts, and bubble gating devices to sense and sort volatile organic compounds and control oxygen levels in biomimetic cell cultures.

Magnetic-Actuated Robot Enables High-Performance Underwater Bubble Maneuvering on Laser-Textured Biomimetic Slippery Surfaces

Controllable underwater gas bubble (UGB) transport on a surface is realized by geography-/stimuli-induced wettability gradient force (F_wet-grad). Unfortunately, the high-speed maneuvering of UGBs along free routes on planar surfaces remains challenging. Herein, a regime of magnetism-actuated robot (MAR) mounting on biomimetic laser-ablated lubricant-impregnated slippery surfaces (LA-LISS) is reported. Leveraging on LA-LISS, MAR-entrained UGBs can move along arbitrary directions through the loading of a tracing magnetic trigger. The underlying hydrodynamics is that MAR-entrained UGBs would be actuated slipping upon a giant magnetic-induced towing force (F_M//). Once the magnetism stimuli is discharged, F_M// vanishes immediately to immobilize the UGBs on LA-LISS. Thanks to the MAR's robust bubble affinity, a typical UGB (20 μL) on the optimized LA-LISS can be accelerated at 500 mm/s² and gain an ultrafast velocity of over 205 mm/s that far exceeds previously reported figures. Moreover, fundamental physics renders MAR antibuoyancy, steering locomotive UGBs on the inclined LA-LISS. Significantly, an MAR propelling UGBs to configure desirable patterns, realize on-demand coalescence, remedy the cutoff switch, as well as facilitate a programmable light-control-light optical shutter is successfully deployed. Compared with previous smart surfaces, the current multifunctional regime is more competent for harnessing UGBs featuring an unparalleled transport velocity independent of the feeble F_wet-grad.