Unidirectional Liquid Research Articles

Bioinspired Capillary Transistors.

Inspired by the unidirectional liquid spreading on Nepenthes peristome, Araucaria leaf, butterfly wings, etc., various microfluidic devices have been developed for water collection, irrigation, physical/chemical reaction, and oil-water separation. Despite extensive progress, most natural and artificial structures fail to enhance the Laplace pressure difference or capillary force, thus suffering from a low unidirectional capillary height (<30mm). In this work, asymmetric re-entrant structures with long overhangs and connected forward/lateral microchannels are fabricated by 3D printing, resulting in a significantly increased unidirectional capillary height of 102.3mm for water, which approximately corresponds to the theoretical limit. The overhangs can partially overlap the forward microchannels of the front structures without direct contact, thus enhancing the Laplace pressure difference and capillary force simultaneously. Based on asymmetric and symmetric re-entrant structures, capillary transistors are proposed and realized to programmably adjust the capillary direction, height, and width, which are envisioned to function as switches/valves and amplifiers/attenuators for highly efficient liquid patterning, desalination, and biochemical microreaction in 3D space.

Novel conductive Janus membranes with water diode effect assisted by an electrostatic field for efficient mass transfer and oil-water separation

Membrane separation technology is effective for treating oily wastewater but is limited by the lack of membranes that are both highly permeable and fouling-resistant. Janus membranes, with asymmetric wettability and electrostatic repulsion of charged oil droplets, offer a potential strategy for oil-water separation. This study developed novel electrically enhanced conductive Janus membranes using metal-modified organic membranes to achieve high-efficiency treatment. Conductive metal Janus membranes were fabricated using a straightforward two-step method involving simple electroless nickel plating and surface peeling. The resulting nickel-coated surface of the membrane demonstrated extreme hydrophilicity, underwater oleophobicity, and excellent conductivity (approximately 4.7 Ω). Fascinatingly, the developed membranes exhibited a “water diode” effect, facilitating unidirectional liquid transport without external pressure. Gravity-driven oil-water separation achieved a flux rate of 4874 L m−2 h−1 and over 99 % oil removal efficiency, surpassing most existing studies. During electro-filtration of oil-water emulsions, the conductive Janus membrane as a cathode generated electrostatic repulsion, preventing emulsified oil droplet adhesion. Applying an external voltage increased the flux by approximately 13 times and oil removal efficiency to 99.6 %. The membrane's separation capability remained consistent across six cycle tests, demonstrating robust stability. As all, this work first synthesized conductive “water diode” Janus membranes with high oil-water separation efficiency, offering fresh perspectives on the design and fabrication of Janus membranes as well as on the processes for treating oily wastewater.

Mechanical stretching triggered smart Janus fabrics for dynamic personal moisture/heat management

Janus fabrics, exhibiting unidirectional liquid transportation capability to foster a comfortable microclimate for our human body, have become a new research hotspot. However, integrating intelligent, adaptable functionalities into Janus fabrics to suit fluctuating environmental conditions and personal thermal perceptions remains a challenge. Herein, we proposed a smart pore-actuated Janus fabric with tunable functions based on mechanical stretching. Specifically, when the human feels hot and sweaty, they can manually stretch the spandex fabric to “open” its pores (from ∼ 41 μm to ∼ 138 μm) according to the actual situation. Meanwhile, the stretching can significantly decrease the positive breakthrough pressure of the fabric from 9.5 to 0.8 cm H2O, which can facilitate one-way sweat transportation and rapid heat dissipation (0.3 ℃ lower than that of the original fabric). Moreover, timely stopping stretching can “close” the fabric’s pores, avoiding excessive evaporation-induced chill. For comparison, in cold or rainy weather, the pores of the unstretched fabric are set to be “closed” (diameter: ∼41 μm) to prevent excessive heat loss (2.4 ℃ higher than that covered by the spandex fabric) and safeguard against external rainwater intrusion. This innovative design can maximize human comfort in diverse environments, and the underlying concept holds promise for the development of new-generation smart fabrics.

Peristome‐mimetic Surfaces Fabricated by Nanosecond Laser for Uni‐directional Liquid Spreading

Unidirectional liquid transport, which operates without external energy input, has garnered remarkable interests in the global energy sector. A crucial breakthrough in this field involves understanding how liquids spread on the peristome surface of Nepenthes alata. This study, inspired by the microstructural traits of the Nepenthes peristome, has brought about the development of a biomimetic surface on metal substrates using nanosecond laser processing on inclined bases. This process replicates the natural peristome’s microstructure, which allows for controlled liquid spread on the surface. The research also examines how wettability influences unidirectional liquid spread. Additionally, it involves adjusting the microstructure parameters on the biomimetic surface, which leads to variations in edge curvature and microcavity wedge angles. More importantly, this study comprehensively investigates the relationship between these structural changes and the phenomenon of unidirectional liquid transport. The findings suggest that altering edge curvature and micro‐cavity wedge angles can manage the directional spread of liquids. The optimized surface, with ideal curvature and wedge angles, achieves liquid movement speeds up to 12.03mm/s. This represents a notable improvement, 2.2 and 2.7 times faster than the original structure, respectively. These findings provides valuable insight into the design of energy‐independent unidirectional liquid spreading surfaces on metal substrates.This article is protected by copyright. All rights reserved.

Sweat permeable and ultrahigh strength 3D PVDF piezoelectric nanoyarn fabric strain sensor

Commercial wearable piezoelectric sensors possess excellent anti-interference stability due to their electronic packaging. However, this packaging renders them barely breathable and compromises human comfort. To address this issue, we develop a PVDF piezoelectric nanoyarns with an ultrahigh strength of 313.3 MPa, weaving them with different yarns to form three-dimensional piezoelectric fabric (3DPF) sensor using the advanced 3D textile technology. The tensile strength (46.0 MPa) of 3DPF exhibits the highest among the reported flexible piezoelectric sensors. The 3DPF features anti-gravity unidirectional liquid transport that allows sweat to move from the inner layer near to the skin to the outer layer in 4 s, resulting in a comfortable and dry environment for the user. It should be noted that sweating does not weaken the piezoelectric properties of 3DPF, but rather enhances. Additionally, the durability and comfortability of 3DPF are similar to those of the commercial cotton T-shirts. This work provides a strategy for developing comfortable flexible wearable electronic devices.

Wedge-shaped lyophilic pattern on superlyophobic surface for unidirectional liquid guidance and lubrication enhancement

Effective maintenance of lubricants within the frictional interface is of great importance for the long-life operation of mechanical components. This paper proposes a wedge-shaped lyophilic groove with secondary structures on superlyophobic surfaces to achieve unidirectional guidance of liquid droplets. The experimental studies demonstrate its rapid and long-distance unidirectional self-driven spreading on water and oil droplets. Then, wedge-shaped lyophilic grooves were designed on non-contact area of friction pair to guide lubricant. Relatively lower and more stable friction coefficients were achieved. Further analysis reveals that wedge-shaped grooves restrict the drainage of liquid lubricants. The results of bearing experiments show that designing lyophilic wedge-shaped grooves in the non-contact area of the rolling bearing can effectively reduce the temperature rise of the outer ring.

Durable Janus membrane with on-demand mode switching fabricated by femtosecond laser

Despite their notable unidirectional water transport capabilities, Janus membranes are commonly challenged by the fragility of their chemical coatings and the clogging of open microchannels. Here, an on-demand mode-switching strategy is presented to consider the Janus functionality and mechanical durability separately and implement them by simply stretching and releasing the membrane. The stretching Janus mode facilitates unidirectional liquid flow through the hydrophilic micropores-microgrooves channels (PG channels) fabricated by femtosecond laser. The releasing protection mode is designed for the in-situ closure of the PG channels upon encountering external abrasion and impact. The protection mode imparts the Janus membrane robustness to reserve water unidirectional penetration under harsh conditions, such as 2000 cycles mechanical abrasion, 10 days exposure in air and other rigorous tests (sandpaper abrasion, finger rubbing, sand impact and tape peeling). The underlying mechanism of gridded grooves in protecting and enhancing water flow is unveiled. The Janus membrane serves as a fog collector to demonstrate its unwavering mechanical durability in harsh real-world conditions. The presented design strategy could open up new possibilities of Janus membrane in a multitude of applications ranging from multiphase separation devices to fog harvesting and wearable health-monitoring patches.

Multi-inspired Janus fabrics with asymmetric coolmax patterns for rapid sweat diffusion and effective sweat-shedding

Janus fabrics with unidirectional liquid transport property have attracted wide attention for their ability to remove excess sweat and lower skin temperature. However, most Janus fabrics face the sweat removal issue on their outer surface when the sweat absorption capacity of their hydrophilic surface reaches limit, which inevitably slows down one-way sweat transport rate and induces the fabrics to become heavy/clingy. Herein, inspired by natural Namib Desert beetles and drip tips, circular Coolmax patterns (severed as sweat extraction points near the skin) and triangular Coolmax patterns (severed as sweat removal zones on the outside of the fabric) are asymmetrically sewn on superhydrophobic cotton fabrics to enable continues one-way sweat transport and allow effective sweat diffusion/shedding on its outer surface. During the sweat transporting and evaporation process, a large amount of heat is dissipated. As a result, the created Janus fabrics can maintain human body temperature about 1.3 ℃ lower than the conventional cotton fabric. Moreover, the low adhesion (∼0.2 mN cm−2) between the wet skin and the fabric makes wearers feel more comfortable. The method can be potentially used for the large-scale commercial production of Janus fabrics and provide insights for the development of smart fabrics in the future.

Peristome-mimetic surfaces using three-dimensional microcavities for unidirectional liquid transport

Peristome-mimetic surfaces using three-dimensional microcavities for unidirectional liquid transport

Janus membrane with asymmetric wettability for efficient oil/water separation

The rapid development of society and industry as well as the frequent occurrence of oil spills cause the shortage of fresh water resources, which not only affects human safety and life, but also impedes the world-wide sustainable development. To address these challenges, novel membrane materials with unique wettability properties have gained significant attention, particularly in the field of oil/water separation. In this research, we modified the hydrophobic PET fabric to achieve superhydrophilic characteristics using impregnation method. Subsequently, we electrospun hydrophobic PVDF fibers onto the superhydrophilic fabric surface, and PVDF/Ca10(PO4)6(OH)2@PET Janus membrane with asymmetric wettability was obtained. The membrane has an excellent unidirectional liquid transport capacity, and can effectively separate heavy oil or light oil, the separation efficiency is more than 90 %. The results also show that the Janus membrane can be used under alkaline conditions and has satisfactory tensile resistance and re-use performance. This work provides a new idea for Janus membrane design and effectively improves the application potential of the Janus membrane in the field of oil/water separation.

Designing integrated Janus membrane based on a double-cross-linked polyphenol hydrophilic layer for complex emulsion separation

Efficient separation materials with multifunctional and switchable properties are urgently needed to treat submicron-scale oily droplets. Herein, tannic acid (TA) and polyethylenimine (PEI) were blended with poly(vinylidene fluoride) (PVDF) and used to coat a polydimethylsiloxane (PDMS)-modified non-woven fabric immersed in a CuSO4/H2O2 coagulation bath. TA and PEI form a phenolamine hydrophilic network that migrates to the surface and firmly binds to the PVDF substrate. CuSO4 catalyses the H2O2 oxidation, promoting the self-polymerisation of phenol hydroxyl group for forming a carboxyl group. Furthermore, the remaining Cu2+ ions complex with TA to form a metal–phenol cross-linked network. A double-cross-linked hydrophilic layer with polyphenols as connection points was formed on the PVDF surface. The hydrophilic contact angle of the membrane reached 0°. A rough structure with PDMS micro-/nanoparticles was constructed on the PVDF underside via surface nanocasting and the hydrophobic contact angle of the membranes reached 165°. In experiments, each side of the obtained Janus membranes demonstrated surface superhydrophilic and superlipophilic properties and a high flux and excellent separation efficiency for various oil–water emulsions. The integrated Janus membranes are suitable for unidirectional liquid collection, oil–water emulsion separation, cationic pigment adsorption and dealing with complex dye–emulsion pollutants. The integrated structure formed by the hydrophilic layer and the membrane enhances the fouling resistance of the membrane and the removal rate of pollutants reached >99 %. This research presents a simple approach to prepare Janus membranes with asymmetric wettability.

Hierarchically designed 3-D printed porous nylon fabric-based personal thermoregulatory for radiative and directional wick-evaporative cooling

Energy consumption is increasing with global warming which remains a great challenge towards sustainable growth. Radiative cooling is an emergent technology towards personalized thermoregulation that radiates heat to outer space to attain self-cooling providing thermal comfort to humans while outdoors through economical way without any energy consumption. Herein a hydrophobic-hydrophilic porous nylon fabric (PNF) composite having unique pores across the composite with hierarchical silver nanoparticles decorated zinc stannate nanorods (Ag@ZTO) synthesized directly over the PNF. We designed the composite by novel and quick three-dimensional (3D) printing technique for the first time that utilizes synergistic innovative integration of radiative and wick-evaporation cooling. The high reflectance of solar irradiance (89.5%) and high emissivity of human body thermal radiation (91.8%) permitted the textile to reduce the temperature of simulated skin by 19.3 °C under direct sunlight demonstrating its superior passive cooling capability. An excellent water vapor transmission rate, water-wicking, washability, durability, tensile strength (70.8% compared to bare PNF) is demonstrated for the composite and a high one-way transport index R (1368 %) shows its unidirectional liquid transport behavior. This approach of designing fabric through tailoring the properties by cost-effective techniques offers new routes for personal thermoregulatory and moisture management towards advanced functional textiles introducing a pioneering direction to sustainable energy.

Highly Durable Janus Fabrics Based on Transfer Prints for Personal Moisture Management.

Janus fabrics with moisture management ability have great potential for improving both physiological and psychological comfort of human body. However, current methods for creating Janus fabrics are typically complex, environmentally unfriendly, and costly. More importantly, the prepared Janus fabrics have demonstrated insufficient mechanical properties and poor fastness, rendering them unsuitable for practical applications. Here, this work proposes a method for constructing Janus fabrics through thermal transfer printing of hydrophobic transfer prints onto a superhydrophilic cotton fabric, followed by creation of a conical micropore array on the fabric surface. The as-prepared Janus fabrics exhibit excellent unidirectional liquid transport capacity, capable of transporting 50µL water completely in 11.6 s in the positive direction. Attributed to the durable property of the transfer prints, the Janus fabrics are capable of withstanding over 900 friction cycles and 250 home laundry cycles, which is a great advance in this research field. Additionally, the fabrication process has no detrimental effect on the fabric's breathability, elasticity, and flexibility. Furthermore, the Janus fabric can maintain human body temperature 3.6 °C cooler than that worn with cotton fabric. The fabrication method can provide useful insights for the design and creation of durable Janus fabrics to maximize personal comfort.

Bioinspired Underwater Superoleophilic Two-Dimensional Surface with Asymmetric Oleophobic Barriers for Unidirectional and Long-Distance Oil Transport.

Unidirectional and long-distance liquid transport is critically important to a range of practical applications, e.g., water harvesting, microfluidics, and chemical reactions. Great efforts have been made on liquid manipulation; most of which, however, are limited in the air environment. It is still a great challenge to achieve unidirectional and long-distance oil transport in an aqueous environment. Herein, we have successfully fabricated an underwater superoleophilic two-dimensional surface (USTS) with asymmetric oleophobic barriers to arbitrarily manipulate oil in aqueous medium. The behavior of oil on USTS was carefully investigated, of which the unidirectional spreading capability was originated from the anisotropic spreading resistance resulted from the asymmetric oleophobic barriers. Accordingly, an underwater oil/water separation device has been developed, which can achieve continuous and efficient oil/water separation and further prevent the secondary pollution caused by oil volatilization.

4D Printing of Butterfly Scale-Inspired Structures for Wide-Angle Directional Liquid Transport.

Unidirectional liquid transport has been extensively explored for water/fog harvesting, electrochemical sensing, and desalination. However, current research mainly focuses on linear liquid transport (transport angle α= 0°), which exhibits hindered lateral liquid spreading and low unidirectional transport efficiency. Inspired by the wide-angle (0°< α< 180°) liquid transport on butterfly wings, this work successfully achieves linear (α= 0°), wide-angle, and even ultra-wide-angle (α= 180°) liquid transport by four-dimensional (4D)printing of butterfly scale-inspired re-entrant structures. These asymmetric re-entrant structures can achieve unidirectional liquid transport, and their layout can control the Laplace pressure in the forward (structure-tilting) and lateral directions to adjust the transport angle. Specifically, high transport efficiency and programmable forward/lateral transport paths are simultaneously achieved by the ultra-wide-angle transport, where liquid fills the lateral path before being transported forward. Moreover, the ultra-wide-angle transport is also validated in 3D space, which provides an innovative platform for advanced biochemical microreaction, large-area evaporation, and self-propelled oil-water separation.

Design and Modification of Janus Polyvinylidene Fluoride/Deacetylated Cellulose Acetate Nanofiber Membrane and its Multifunctionality

AbstractA novel type of Janus polyvinylidene fluoride/deacetylated cellulose acetate (PVDF/D‐CA) nanofiber membrane with multifunctional application is successfully fabricated by sequential electrospinning. The morphology, chemical composition, surface wettability, oil–water separation performance, and unidirectional liquid transmission capability have been systematically analyzed. It has asymmetric wettability property in air and in liquid systems for its bottom and top sides. It can be used as a water‐removing, or oil‐removing substance for oil–water mixture and emulsion under a harsh environment only under gravity driving force (DF) by simply switching the different sides. It possesses on‐demand water and oil collection ability, and moisture‐wicking performance due to its switchable permeation and unidirectional liquid transmission capability. Its multifunction application can be easy to control by adjusting the thickness of the top and bottom sides through controlling the electrospinning time. Its excellent mechanical property, chemical stability, durability under harsh environment and outstanding anti‐pollution ability endow the membrane with powerful cyclic stability and reusability.

Multi-pores Janus paper with unidirectional liquid transport property toward information encryption/decryption

Janus membranes with unidirectional liquid transport property have tremendous applications in fields of functional fabrics, water harvesting, smart wound dressings and so on. Paper shows distinct advantages to other materials, such as hydrophilicity, low-cost, flexible and so on. However, there is still less attention on how to construct a Janus paper with unidirectional liquid transport capacity and its functional applications. Here, a multi-pores Janus paper with asymmetric wetting properties is developed through consecutive laser microdrilling, hydrophobic modification and air-plasma treatment. As a result, the prepared Janus paper can allow the water to transport from the hydrophobic side to the hydrophilic side, while it blocks the water to flow in the reverse direction. Meanwhile, it can also achieve antigravity unidirectional water transport. In addition, the mechanism for unidirectional liquid transport is investigated. Finally, as a proof-of-concept, the Janus paper is utilized for applications of information encryption/decryption.

Self‐powered virtual olfactory generation system based on bionic fibrous membrane and electrostatic field accelerated evaporation

AbstractOlfactory plays an important role in virtual reality technology. In this work, a bionic fibrous membrane (BFM) integrated with the function of electrostatic field accelerated evaporation (EFAE) is applied for realizing the virtual olfactory generation (VOG) system. The BFM is capable of self‐driven unidirectional liquid transmission and EFAE process is induced on BFM by using an ultrafast voltage‐elevation triboelectric nanogenerator (UVE‐TENG). The output voltage from UVE‐TENG can reach 8 kV within 40 ms, which is enough to maintain all of the functions of VOG system. Meanwhile, the output current from UVE‐TENG is smaller than 1 μA, leading to a safe and human compatible system. The flow rate of the volatile liquid spray from the system emitter can reach 0.1 μl/s, and the average evaporation rate of the BFM device with integrated EFAE function is 0.12 mg/s. Accordingly, the user of this VOG system can feel the generation of odor within 3 s, while the switching of different odor channels can be wirelessly controlled by a mobile phone. This VOG system enhances the user's immersive and interactive experience for virtual reality technology. The similar system composed of UVE‐TENG, reed switch, BFM and EFAE devices also has potential application value in assisted breathing treatment and nasal delivery.image

Ultrastable Super-Hydrophobic Surface with an Ordered Scaly Structure for Decompression and Guiding Liquid Manipulation.

Directional droplet manipulation is very crucial in microfluidics, intelligent liquid management, etc. However, excessive liquid pressure tends to destroy the solid-gas-liquid (SAL) composite interface, creating a highly adhesive surface, which is not conducive to liquid transport. Herein, we propose a strategy to enhance the surface durability, in which the surface cannot withstand liquid pressure only by "blocking" but must instead guide liquid transport for "decompression". Learning from the water resistance of water strider legs and the drag reduction of shark skin, we present a continuous integrated system to obtain an ultrastable super-hydrophobic surface with a highly ordered scaly structure via a liquid flow-induced alignment method for lossless unidirectional liquid transport. The nonwetting scaly structure can both buffer liquid pressure and drive droplet motion to further reduce the vertical pressure of the liquid. Moreover, droplets can be manipulated unidirectionally using a voice. This work could aid in manufacturing scalable anisotropic micro-nanostructure surfaces, which inspires efforts in realizing lossless continuous liquid control on demand and related microfluidic applications.

An Investigation of Unidirectional Liquid Moisture Transport Performance Evaluation of Textile Materials Based on Double-Sided Imaging

Abstract The interaction between water and fabric is critical in the physiological comfort of garments. However, it is still practically challenging to evaluate textiles with continuous directional liquid moisture transport and outstanding prevention of water penetration in the reverse direction. The traditional liquid moisture transmission test method has its shortcomings. In order to better evaluate the liquid water transport behaviors of fabric, a new method is proposed in this paper. A series of images of fabric wetting are acquired from two different perspectives through a self-made double-sided image acquisition device. At the same time, a suitable segmentation algorithm for the image sequence is proposed in this study. Finally, the index parameters of fabric wettability can be obtained, and the wettability process of fabric can be analyzed. The experimental results show that the method of double-sided imaging can obtain more comprehensive fabric wetting information. Combined with the existing image-processing algorithms, the double-sided imaging image acquisition system proposed in this paper can be used to form a complete evaluation system of fabric liquid moisture transport behaviors.