Directional liquid transport has broad engineering applications, and recent efforts have demonstrated the feasibility of rapid and long-distance directional transport using external driving forces, such as light, electricity, magnetism, and mechanical force. However, the externally driven mechanism limits its application in small and intelligent areas. The manufacture of the passive directional flow surfaces remains a significant challenge. In this account, we present a passive microfluidic chip with microwedge grooved channels for improved fluid directional manipulation. Benefiting from the cooperative effect of the asymmetric resistance and capillary driving force, the microwedged channel possesses the reliable functions for long-distance, anti-gravity, and continuous directional fluid transport in a microfluidic chip. To improve these properties, we optimized the structure parameters of the microwedges, resulting in a unidirectional value of up to 7.48. Furthermore, we explored the functions and applications of these microwedge arrays in intelligent and microfluidic systems. By combining different microwedge arrays, we programed that the microfluidic chips were programed with different functions, such as microfluidic logic, liquid gating, and anti-gravity capabilities, with multi-path transportation of an overall length of 537.6 mm. This research provides important guidance on functional surfaces for the design of intelligent integrated microfluidic chips and plays a significant role in promoting practical industrial applications.

https://pubs.aip.org/aip/pof/article-pdf/doi/10.1063/5.0151005/17971845/062005_1_5.0151005.pdf

Directional fluid spreading on microfluidic chip structured with microwedge array

Using extensive molecular dynamics simulation of a coarse-grained model, we demonstrate the possibility of sustained unidirectional motion (durotaxis) of droplets without external energy supply when placed on a polymer brush substrate with stiffness gradient in a certain direction. The governing key parameters for the specific substrate design studied, which determine the durotaxis efficiency, are found to be the grafting density of the brush and the droplet adhesion to the brush surface, whereas the strength of the stiffness gradient, the viscosity of the droplet, or the length of the polymer chains of the brush have only a minor effect on the process. It is shown that this durotaxial motion is driven by the steady increase of the interfacial energy between droplet and brush as the droplet moves from softer to stiffer parts of the substrate whereby the mean driving force gradually declines with decreasing roughness of the brush surface. We anticipate that our findings indicate further possibilities in the area of nanoscale motion without external energy supply.

Unidirectional Droplet Propulsion onto Gradient Brushes without External Energy Supply

Solid‐liquid triboelectric nanogenerators (SL‐TENGs) are a new technology that combines contact electrification (CE) and electrostatic induction to collect clean energy stored in natural water. Considering their unique advantages of high energy density, wide selection of materials and being suitable for large‐scale promotion, they have attracted more and more attention in recent years, and numerous studies have shown their great potential in various applications. Many critical applications of SL‐TENGs inevitably involve sustained and stable high electrical output. To achieve stable output performance and long cycle life in these applications, the adaptability of SL‐TENGs to material selection, structural design, and working environment is necessary. Therefore, the construction of SL‐TENGs matching different applications has become a critical research direction in TENGs. This review provides a historical summary of the development of SL‐TENGs in the past few years and analyzes the key factors affecting their electrical output performance. The exciting achievements of different constructions of SL‐TENGs for practical applications is also demonstrated such as energy harvesting, self‐powered sensing, and self‐powered cathodic protection. Finally, the development prospects of SL‐TENGs and the significant challenges for their further development is discussed.

Robust Solid‐Liquid Triboelectric Nanogenerators: Mechanisms, Strategies and Applications

Double loops power management circuit of pulsed triboelectric nanogenerator with enhanced efficiency at low operating voltage and its application in self-powered flue gas monitoring system

Multibehavioral droplet manipulation in a precise and programmed manner is crucial for stoichiometry, biological virus detection, and intelligent lab-on-a-chip. Apart from fundamental navigation, merging, splitting, and dispensing of the droplets are required for being combined in a microfluidic chip as well. Yet, existing active manipulations including strategies from light to magnetism are arduous to use to split liquids on superwetting surfaces without mass loss and contamination, because of the high cohesion and Coanda effect. Here, we demonstrate a charge shielding mechanism (CSM) for platforms to integrate with a series of functions. In response to attachment of shielding layers from the bottom, the instantaneous and repeatable change of local potential on our platform achieves the desired loss-free manipulation of droplets, with a wide-ranging surface tension from 25.7 mN m-1 to 87.6 mN m-1, functioning as a noncontact air knife to cleave, guide, rotate, and collect reactive monomers on demand. With further refinement of the surface circuit, the droplets, just as the electron, can be programmed to be transported directionally at extremely high speeds of 100 mm s-1. This new generation of microfluidics is expected to be applied in the field of bioanalysis, chemical synthesis, and diagnostic kit.

Noncontact Charge Shielding Knife for Liquid Microfluidics.

Manipulating liquid is of great significance in fields from life sciences to industrial applications. Owing to its advantages in manipulating liquids with high precision and flexibility, electrowetting on dielectric (EWOD) has been widely used in various applications. Despite this, its efficient operation generally needs electrode arrays and sophisticated circuit control. Here, we develop a largely unexplored triboelectric wetting (TEW) phenomenon that can directly exploit the triboelectric charges to achieve the programmed and precise water droplet control. This key feature lies in the rational design of a chemical molecular layer that can generate and store triboelectric charges through agile triboelectrification. The TEW eliminates the requirement of the electric circuit design and additional source input and allows for manipulating liquids of various compositions, volumes, and arrays on various substrates in a controllable manner. This previously unexplored wetting mechanism and control strategy will find diverse applications ranging from controllable chemical reactions to surface defogging.

Triboelectric wetting for continuous droplet transport

Developing robust technologies to harvest energy from ambient environments is critical to applications in self‐powered electronics. As a promising candidate, droplet‐based electricity generators with transistor‐inspired architecture, which consist of electrodes and dielectric material, have manifested superior capacity to gain sustainable energy from water. Despite extensive achievement, these devices are significantly limited by insufficient flexibility and unstable performance owing to the presence of inevitable mechanical abrasion, chemical corrosion, and interfacial detachment of electrode and dielectric material. Here, a facile approach is reported to construct a monolithic integrated droplet‐based electricity generator (MI‐DEG) device that enables high flexibility and robust electrical performance. The key to such flexibility and robustness of the MI‐DEG device lies in the meticulous utilization of a thermal fusion strategy that makes specially‐designed polymer electrodes and dielectric material seamlessly integrated together. It is demonstrated that MI‐DEG exhibits stable electrical output with negligible attenuation under long‐term mechanical deformation and abrasion, as well as retains over 60% of initial electrical output under harsh corrosive environments, as opposed to the complete failure of conventional counterpart. This study provides a hopeful pathway for the rational design of flexible and robust electricity generator, making it a step closer in the practical applications.

Lili Wang

Papers

Triboelectric wetting for continuous droplet transport

Monolithic Integrated Flexible Yet Robust Droplet‐Based Electricity Generator