Portable Triboelectric Electrostatic Tweezer for External Manipulation of Droplets within a Closed Femtosecond Laser-Treated Superhydrophobic System.

Controllable droplet manipulation has diverse applications; however, limited methods exist for externally manipulating droplets in confined spaces. Herein, we propose a portable triboelectric electrostatic tweezer (TET) by integrating electrostatic forces with a superhydrophobic surface that can even manipulate droplets in an enclosed space. Electrostatic induction causes the droplet to be subjected to an electrostatic force in an electrostatic field so that the droplet can be moved freely with the TET on a superhydrophobic platform. Characterized by its high precision, flexibility, and robust binding strength, TET can manipulate droplets under various conditions and achieve a wide range of representative fluid applications such as droplet microreactors, precise self-cleaning, cargo transportation, the targeted delivery of chemicals, liquid sorting, soft droplet robotics, and cell labeling. Specifically, TET demonstrated the ability to manipulate internal droplets from the outside of a closed system, such as performing cell labeling experiments within a sealed Petri dish without opening the culture system.

Active Droplet Transport Induced by Moving Meniscus on a Slippery Magnetic Responsive Micropillar Array.

Intelligent droplet manipulation plays a crucial role in both scientific research and industrial technology. Inspired by nature, meniscus driving is an ingenious way to spontaneously transport droplets. However, the shortages of short-range transport and droplet coalescence limit its application. Here, an active droplet manipulation strategy based on the slippery magnetic responsive micropillar array (SMRMA) is reported. With the aid of a magnetic field, the micropillar array bends and induces the infusing oil to form a moving meniscus, which can attract nearby droplets and transport them for a long range. Significantly, clustered droplets on SMRMA can be isolated by micropillars, avoiding droplet coalescence. Moreover, through adjusting the arrangement of the micropillars of SMRMA, multi-functional droplet manipulation such as unidirectional droplet transport, multi-droplet transport, droplet mixing, and droplet screening can be achieved. This work provides a promising approach for intelligent droplet manipulation and unfolds broad application prospects in microfluidics, microchemical reaction, biomedical engineering, and other fields.

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.

Smart Control for Water Droplets on Temperature and Force Dual-Responsive Slippery Surfaces.

Responsive slippery lubricant-infused porous surfaces (SLIPSs), featuring excellent liquid repelling/sliding capabilities in response to external stimuli, have attracted great attention in smart droplet manipulations. However, most of the reported responsive SLIPSs function under a single stimulus. Here, we report a kind of smart slippery surface capable of on-demand control between sliding and pinning for water droplets via alternately freezing/thawing the stretchable polydimethylsiloxane sheet in different strains. Diverse parameters are quantified to investigate the critical sliding volume of the droplet, including lubricant infusion amount, laser-scanning power, and pillar spacing. By virtue of the cooperation of temperature and force fields acting on the SLIPS, we demonstrate the intriguing applications including controllable chemical reaction and on-demand electrical circuit control. We envision that this dual-responsive surface should provide more possibilities in smart control of microscale droplets, especially in active vaccine-involved biochemical microreactions where a lower temperature is highly favored.

High Performance Bubble Manipulation on Ferrofluid-Infused Laser-Ablated Microstructured Surfaces.

Manipulation of gas bubbles in an aqueous ambient environment is fundamental to both academic research and industrial settings. Present bubble manipulation strategies mainly rely on buoyancy or Laplace gradient forces arising from the sophisticated terrain of substrates. However, these strategies suffer from limited manipulation flexibility such as slow horizontal motion and unidirectional transport. In this paper, a high performance manipulation strategy for gas bubbles is proposed by utilizing ferrofluid-infused laser-ablated microstructured surfaces (FLAMS). A typical gas bubble (<2 µL) can be accelerated at >150 mm/s2 and reach an ultrafast velocity over 25 mm/s on horizontal FLAMS. In addition, diverse powerful manipulation capabilities are demonstrated including antibuoyancy motion, "freestyle writing", bubble programmable coalescence, three-dimensional (3-D) controllable motion and high towing capacity of steering macroscopic object (>500 own mass) on the air-water interface. This strategy shows terrain compatibility, programmable design, and fast response, which will find potential applications in water treatment, electrochemistry, and so on.

Hybrid femtosecond laser fabrication of a size-tunable microtrap chip with a high-trapping retention rate.

In this Letter, we propose a new (to the best of our knowledge), promising concept of a hybrid femtosecond (fs) laser processing method composed of single-point scanning and holographic light modulation fabrication for manufacturing a tunable-size microtrap chip. The hybrid method not only ensures key microfluidic device precision but also greatly improves the fabrication speed. By using a new asymmetry-bracket-shaped microtrap design with a mechanical strain stretching method, real-time size-tunable trapping is obtained, and a 100% particle trapping retention is realized, ignoring the flow fluctuation. Finally, the microtrap array is successfully applied to trap single yeast cells and hold them for $\sim{10}\;{\rm h}$∼10h without escaping.

Reversible Tuning between Isotropic and Anisotropic Sliding by One-Direction Mechanical Stretching on Microgrooved Slippery Surfaces.

Dynamically responsive liquid-infused interfacial materials have broad technological implications in manipulating droplet motions. However, present works are mainly about reversible tuning of the isotropic slippery surface; the reversible switching between isotropic and anisotropic sliding has not been deeply explored. Here, we report a kind of liquid-infused elastic-grooved surface (LIEGS) by femtosecond laser ablation and realize reversible switching between isotropic and anisotropic sliding by one-direction mechanical stretching. Under mechanical stretching and strain release, droplet motion can be reversibly switched between the sliding and pinned states along the perpendicular direction to the grooves, whereas the droplet keeps sliding along the parallel direction to the grooves. The mechanism of reversible switching mainly contributes to the decrease of film thickness during the stretching process in which the film thickness decreases from 13 to 4 µm with the increase of the strain from 0 to 60%. Finally, we demonstrate the real-time flexible control over a droplet sliding/pinned on the strain-changing LIEGS.

Photothermal Actuation of Diverse Liquids on an Fe3O4-Doped Slippery Surface for Electric Switching and Cell Culture.

The photoinduced manipulation of liquids on a slippery lubricant-infused porous surface (SLIPS) has attracted a tremendous amount of attention because of its merits of contactless stimulation and excellent spatial and temporal control. However, tedious fabrication methods by a combination of template transfer and fluorination for a photothermal-material-doped SLIPS and the lack of deeper systematically quantitative analysis with respect to droplet hydrokinetics are greatly perplexing in both academic research and industrial applications. Here we demonstrate a kind of Fe3O4-doped SLIPS by one-step femtosecond laser cross-scanning, which can readily steer diverse liquids toward arbitrary directions with a fast velocity of up to 1.15 mm/s in the presence of a unilateral NIR stimulus. The underlying mechanism is that the wettability gradient force (Fwet-grad) induced by the temperature gradient arising from asymmetric near-infrared-irradiation (NIR) loading would be generated within 1 s to actuate a targeted droplet's sliding behavior. Through tuning the NIR irradiating sites, we can slide a targeted droplet with controllable directions and routes. On the basis of fundamental physics, we have quantitatively analyzed the relationship among Fe3O4-doped content, lubricant rheological performance, droplet wettability variations, Fwet-grad, and the sliding velocity for diverse liquid species. Accordingly, we can remotely steer liquid droplets to realize the on-off state of an electrical circuit on demand, the droplet fusion of a microfluidic reactor, and the culture/inhibition of biological cells.

A Fast Room Temperature NH3 Sensor Based on an Al/p-Si/Al Structure with Schottky Electrodes.

In this paper, an electrical-based NH3 sensor with an Al/p-Si/Al structure is reported. The p-Si substrate is microstructured by fs-laser irradiation and then etched by 30% alkaline solution. This sensor works well at room temperature with fast response/recovery for NH3 gas at 5-100 ppm concentration. However, when the sensor is annealed in N2/H2 forming gas or short-circuited for Al/Si electrodes, its sensitivity decreases drastically and almost vanishes. Further I-V and FT-IR results show that the two back-to-back Schottky diodes on the device play a key role in its sensing performance.

Femtosecond-laser-patterned origami Janus membrane toward enhanced water fog harvesting.

Fog harvesting is an effective way to relieve water shortages in arid regions; thus, improving the efficiency of fog harvesting is urgently needed for both academic research and practical applications. Here, we report an origami patterned Janus (O-P-Janus) membrane using laser-ablated copper foams inspired by origami handcraft and traditional Chinese architecture. Compared to the planar fully ablated Janus membrane, our O-P-Janus membrane, with selectively ablated rectangular areas, exhibits an exceptional water collection rate (WCR) of approximately 267%. The underlying physical mechanism of WCR enhancement is revealed and attributed to the enhanced fog adsorbing capacity on the upper superhydrophobic origami structures and the accelerated removal of accumulated droplets beneath the lower superhydrophilic V-shaped tips. This O-P-Janus membrane with excellent fog collection performance should open up a new avenue for both device designs and potential applications toward structuring-enhanced fog collection and microfluidic control platforms.

Bioinspired magnetism-responsive hybrid microstructures with dynamic switching toward liquid droplet rolling states.

The functionality of tunable liquid droplet adhesion is crucial for many applications such as self-cleaning surfaces and water collectors. However, it is still a challenge to achieve real-time and fast reversible switching between isotropic and anisotropic liquid droplet rolling states. Inspired by the surface topography on lotus leaves and rice leaves, herein we report a biomimetic hybrid surface with gradient magnetism-responsive micropillar/microplate arrays (GMRMA), featuring dynamic fast switching toward different droplet rolling states. The exceptional dynamic switching characteristics of GMRMA are visualized and attributed to the fast asymmetric deformation between the two different biomimetic microstructures under a magnetic field; they endow the rolling droplets with anisotropic interfacial resistance. Based on the exceptional morphology switching surface, we demonstrate the function of classification and screening of liquid droplets, and thus propose a new strategy for liquid mixing and potential microchemical reactions. It is expected that this intelligent GMRMA will be conducive to many engineering applications, such as microfluidic devices and microchemical reactors.

Sustaining Robust Cavities with Slippery Liquid-Liquid Interfaces.

The formation of a stable gas cavity on the surfaces of solid bodies is essential for many practical applications, such as drag reduction and energy savings, owing to the transformation of the originally sticky solid-liquid interface into a free-slip liquid-vapor interface by the creation of either liquid repellency or a Leidenfrost state on the surfaces. Here, it is shown that the simple infusion of a textured sphere with a smooth, slippery liquid layer can more easily create and sustain a stable gas cavity in a liquid at lower impact velocities compared to a dry solid sphere with the same contact angle. With a key parameter of curvature ratio, the early lamella dynamics during water entry of spheres and drops impact on planes are first unified. With the perspective of wetting transition, the unforeseen phenomenon of prone to cavity formation are successfully explained, which is the preferential lamella detachment from a slippery surface due to the higher viscosity of the lubricant relative to air. It is envisioned that the findings will provide an important and fundamental contribution to the quest for energy-efficient transport.

Robust Underwater Air Layer Retention and Restoration on Salvinia-Inspired Self-Grown Heterogeneous Architectures.

Salvinia's long-term underwater air layer retention ability has inspired researchers to develop artificial microstructures. However, Salvinia has an exquisite combination of a complicated hollow structure and heterogeneous chemical properties, which makes artificial reproduction beyond the capabilities of traditional fabrication techniques. In addition, under extremely low underpressure conditions, the mechanism of retention and restoration of the underwater air layer of Salvinia remains unclear. Herein, by combining the shape memory polymer "top-constrained self-branching (TCSB)" and hydrophilic SiO2 microspheres trapping, four-branch hollow microstructures with heterogeneous chemical properties are fabricated. By applying underpressure, the crucial role of hydrophilic apexes is unveiled in air layer restoration. Through the calculation of the surface energy, the underlying mechanism is well interpreted. This study holds great promise for developing Salvinia-inspired artificial structures and reveals the underlying mechanism of the robust air retention and recovery capability of Salvinia leaves in extreme environments.

Magnetism-Actuated Superhydrophobic Flexible Microclaw: From Spatial Microdroplet Maneuvering to Cross-Species Control.

The flexible maneuvering of microliter liquid droplets is significant in both fundamental science and practical applications. However, most current strategies are limited to the rigid locomotion on confined geographies platforms, which greatly hinder their practical uses. Here, we propose a magnetism-actuated superhydrophobic flexible microclaw (MSFM) with hierarchical structures for water droplet manipulation. By virtue of precise femtosecond laser patterning on magnetism-responsive poly(dimethylsiloxane) (PDMS) films doped with carbonyl iron powder, this MSFM without chemical contamination exhibits powerful spatial droplet maneuvering advantages with fast response (<100 ms) and lossless water transport (∼50 cycles) in air. We further performed quantitative analysis of diverse experimental parameters including petal number, length, width, and iron element proportion in MSFM impacting the applicable maneuvering volumes. By coupling the advantages of spatial maneuverability and fast response into this versatile platform, typical unique applications are demonstrated such as programmable coalescence of droplets, collecting debris via droplets, tiny solid manipulation in aqueous severe environments, and harmless living creature control. We envision that this versatile MSFM should provide great potential for applications in microfluidics and cross-species robotics.

In Situ Electric-Induced Switchable Transparency and Wettability on Laser-Ablated Bioinspired Paraffin-Impregnated Slippery Surfaces.

Switchable wetting and optical properties on a surface is synergistically realized by mechanical or temperature stimulus. Unfortunately, in situ controllable wettability together with programmable transparency on 2D/3D surfaces is rarely explored. Herein, Joule-heat-responsive paraffin-impregnated slippery surface (JR-PISS) is reported by the incorporation of lubricant paraffin, superhydrophobic micropillar-arrayed elastomeric membrane, and embedded transparent silver nanowire thin-film heater. Owing to its good flexibility, in situ controllable locomotion for diverse liquids on planar/curved JR-PISS is unfolded by alternately applying/discharging low electric-trigger of 6 V. Simultaneously, optical visibility can be reversibly converted between opaque and transparent modes. The switching principle is that in the presence of Joule-heat, solid paraffin would be melt and swell within 20 s to enable a slippery surface for decreasing light scattering and frictional force derived from contact angle hysteresis (FCAH ). Once Joule-heat is discharged, undulating rough surface would reconfigure by cold-shrinkage of paraffin within 8 s to render light blockage and high FCAH . Upon its portable merit, in situ thermal management, programmable visibility, as well as steering functionalized droplets by electric-activated JR-PISSs are successfully deployed. Compared with previous Nepenthes-inspired slippery surfaces, the current JR-PISS is more competent for in situ harnessing optical and wetting properties on-demand.

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

Manipulating underwater bubbles (UGBs) is realized on morphology-tailored or stimuli-responsive slippery lubricant-impregnated porous surface (SLIPS). Unfortunately, the volatile lubricants (e. g., silicone oil, ferrofluid) greatly decrease their using longevity. Designed is light-responsive paraffin-infused Fe3O4-doped slippery surface (LR-PISS) by incorporation of hybrid lubricants and superhydrophobic micropillar-arrayed elastometric membranes resulted from one-step femtosecond laser vertically scanning. Upon LR-PISS, the dynamic motion control bwteen pinning and sliding along free routes over UGB could be realized by alternately loading/discharging NIR-trigger. The underlying principle is that when the NIR was applied, UGB would be actuated to slide along the NIR trace because the irradiated domain melts for a slippery surface within 1.0 s. Once the NIR is removed, the liquefied paraffin would be reconfigured to solid phase for pinning a moving UGB within 0.5 s. Newly explored hydrokinetics imparts us with capability of steering UGBs to arrange any desirable patterns and switch light-path behaving as the light-control-light optical shutter. In comparison with previously reported SLIPS, current LR-PISS unfolds unparalleled ultrarobust antidisturbance ability even in flowing liquid ambient. More significantly, even subjected to physical damage, underwater LR-PISS is capable of in situ self-healing within 13 s under the assistance of remote NIR. The results here could inspire the design of robust bubble manipulator and further boost their applications in optofluidics and all-optical modulators.

Biomimetic Mechanoswitchable Interfaces for High-Performance Spatial Gas Bubble Maneuvering.

The on-demand manipulation of gas bubbles in aqueous ambient environments is fundamental to many fields such as microfluidics and biochemical microanalysis. However, most bubble manipulation strategies are limited to restricted locomotion on the confined surfaces without spatial convenience of transport. Herein, we report a kind of biomimetic bubble manipulator with mechanoswitchable interfaces (MSIs), featuring the advantages of parallel bubble control and spatial maneuvering flexibility. By the synergic action between Janus aluminum membrane and superaerophilic microfiber array, the gas-MSI interfacial adhesion can be reversibly switched to achieve capturing/releasing underwater bubbles. Moreover, the adhesion force of MSI can be readily tuned by diverse experimental parameters including surface roughness, fiber number, diameter, and spacing of the neighboring microfibers, which are further systematically investigated. Relying on this mobile platform, we demonstrate a series of powerful applications including bubble parallel control, bubble array regrouping, arbitrary bubble transport and even manipulating underwater solids through bubbles, which are otherwise challenging for conventional approaches. We envision that this versatile platform will bring new insights into potential applications, such as cross-species sample control and handheld gas microsyringe.

Ultralow-Voltage-Driven Smart Control of Diverse Drop's Anisotropic Sliding by in Situ Switching Joule Heat on Paraffin-Infused Microgrooved Slippery Surface.

Stimuli-responsive anisotropic slippery surfaces (ASSs) have demonstrated intriguing performance in manipulating the behaviors of some liquids. However, most present methods have been limited to conductive droplets, certain specific conductive platforms, and higher manipulation temperature that greatly hinder its practical applications. Here, an electric-responsive paraffin-infused ASS (ER-PIASS) composed of paraffin, microgrooved PDMS, and flexible embedded silver nanowire heater is reported. Owing to the fast electric-response of ER-PIASS, smart control between anisotropic sliding and pinning for diverse liquids can be realized by remotely loading and discharging electric-stimuli. The underlying mechanism is that the generated Joule heat melts the solidified paraffin to slide a pinning droplet once an electric-trigger is loaded due to the formation of a slippery air/liquid/liquid/solid system. Once the voltage is discharged, the liquefied paraffin would rapidly solidify to stick to a slipping droplet because of the recovery of a frictional air/liquid/solid system. Additionally, the effect of the groove's height (h), spacing between two adjacent grooves (d), and thickness of the paraffin layer on the anisotropic degree was quantitatively studied and an optimized value of 75° is thus harvested. Through tuning the recipe of the hybrid lubricant, the responsive voltage and temperature for ER-PIASS can be dramatically decreased to ultralow figures of 2.0 V and 34.2 °C. By taking advantage of this ultralow-voltage-driven biocompatible ER-PIASS, we enable the anisotropic smart control of cell culture medium and yeast droplets for their directional coalesce, growth, and fission. We believe that such stimuli-responsive surfaces will be promising candidates for manipulating droplets' directional sliding behavior and further bloom the studies of flexible microfluidics devices.

Unidirectional Transport and Effective Collection of Underwater CO2 Bubbles Utilizing Ultrafast-Laser-Ablated Janus Foam.

Manipulating gas bubbles in aqueous ambient is of great importance for applications in water treatment, gas collection, and matter transport. Here, a kind of Janus foam is designed and fabricated by one-step ultrafast laser ablation of one side of the copper film, which is treated to be superhydrophobic. Janus foam exhibits not only the capability of unidirectional transport of underwater bubbles but also gas collection with favorable efficiency up to ∼15 mL cm-2 min-1. The underlying physical mechanism is attributed to the cooperation of the buoyancy, adhesion, and wetting gradient forces imposed on the bubbles. As a paradigm, the underwater chemical reaction between the unidirectional CO2 gas flow and the alkaline phenolphthalein solution is demonstrated via Janus foam. This facile and low-cost fabrication approach for Janus foam will find broad potential applications in effective bubble transport, carbon capture, and controllable chemical reactions under aqueous conditions.

Rapid and Wide-Range Detection of NOx Gas by N-Hyperdoped Silicon with the Assistance of a Photovoltaic Self-Powered Sensing Mode.

Wide-dynamic-range NOx sensors are vital for the environment and health purposes, but few sensors could achieve wide-range detection with ultralow and ultrahigh concentrations at the same time. In this article, the microstructured and nitrogen-hyperdoped silicon (N-Si) for NOx gas sensing is investigated systematically. Working by the change of surface conductivity, the sensor is ultrasensitive to low concentrations of NOx down to 11 ppb and shows a rapid response/recovery time of 22/33 s for 80 ppb. When the NOx concentration increases and exceeds a threshold value (10-50 ppm), an n-p conduction-type transition is observed due to the inversion of the conduction type of major carriers, which limits the dynamic range of the sensor at high concentration. However, when the sensor works in a photovoltaic self-powered mode under the asymmetric light illumination, the limitation can be successfully overcome. Therefore, with the combination of the two working principles, a wide dynamic range stretching over 6 orders of magnitude (∼0.011-4000 ppm) can be achieved.