Unidirectional Liquid Transport Research Articles

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.

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.

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.

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.

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.

A review on emerging developments in thermal and moisture management by membrane‐based clothing systems towards personal comfort

AbstractEnhanced heat and moisture management has become highly urgent to clothing systems due to the global need to personalize the ventilation process. As a significant sector of apparel industries, membrane‐based clothing systems offer protection and ventilation functionalities. This review aims at providing a new viewpoint on efforts to improve heat and moisture management in membrane‐based clothing systems according to the latest advances in this field. The study terminates with presenting a perspective on current challenges and opportunities for future research directions in personal thermal and moisture management technologies. In fact, membranes with different functionalities used in clothing systems such as photonic membranes with heating and cooling performances, unidirectional liquid transport membranes, waterproof‐breathable membranes, shape‐memory membranes, thermoregulating membranes, thermal conductive membranes and finally protective membranes are investigated. The essential functions associated to different categories of the investigated membranes along with the relationships between membrane structure and properties are presented.

Unidirectional transport of both wettable and nonwettable liquids on an asymmetrically concave structured surface

Unidirectional liquid transport (UDLT) has been widely used in various fields as an important process for transferring both mass and energy. However, UDLT driven by a structural gradient has been witnessed for a long time only in wettable liquids. For nonwettable liquids, UDLT can hardly proceed merely by a structural gradient. Herein, we propose an asymmetrically concave structured surface (AMC-surface), featuring tip-to-base periodically arranged pyramid-shaped concave structures with a certain degree of overlap, which enables the UDLT of both wettable and nonwettable liquids. For wettable liquids, the capillary force along each corner leads to the UDLT pointing toward the base side of the concave pyramid, while for nonwettable liquids, the UDLT is attributable to the static liquid pressure overwhelming the repulsive Laplace pressure induced by the asymmetric grooves and overlapping part. As a result, both wettable and nonwettable liquids transport spontaneously and unidirectionally on the AMC-surface with no energy input. Moreover, the concave structure endows good mechanical stability and can be easily prepared using a facile nail-punching approach over a large area. We also demonstrated its application in a continuous chemical reaction in a confined area. We envision that the unique UDLT behavior on the as-developed AMC-surface will shed new light on the programmable manipulation of various liquids.

Hybrid Janus Membrane with Dual-Asymmetry Integration of Wettability and Conductivity for Ultra-Low-Volume Sweat Sensing.

Highly sensitive and selective analysis of sweat at ultra-low sample volume remains a major challenge in the field of biosensing. Manipulation of small volumes of liquid for efficient sampling is essential to address this challenge. A hybrid Janus membrane with dual-asymmetry integration of wettability and conductivity is developed for regulated micro-volume liquid transport in wearable sweat biosensing. Unlike the uncontrollable liquid diffusion in a conventional porous membrane, the asymmetric wettability of porous Janus membrane leads to unique unidirectional liquid transport with high breakthrough pressure (1737.66 Pa) and fast self-pumping rate (35.94 μL/min) for micro-volume liquid sampling. The asymmetric conductive layer shows excellent flexible conductivity, anti-interference of friction, and efficient electrochemical interface due to the in situ generation of gold nanoparticles on one side of the membrane. The fabricated Pt-enzyme electrodes on the membrane promises effective testing range, great selectivity, and high sensitivity and accuracy (correlation efficiency, glucose: R2 = 0.999, lactate: R2 = 0.997), enabling ultra-low volume (∼0.15 μL) real time measurements on the skin surface. The innovative Janus membrane with unidirectional, self-pumping, and anti-interference performance provides a new strategy for miniaturized wearable microfluidic sweat electrochemical biosensor preparation in athletic performance evaluation, health monitoring, disease diagnosis, intelligent medicine, and so forth.

Liquid-Assisted Single-Layer Janus Membrane for Efficient Unidirectional Liquid Penetration.

Unidirectional liquid penetration plays an important role in many fields, such as microfluidic devices, biological medical, liquid printing, and oil/water separation. Although there are some progresses in the liquid unidirectional penetration using a variety of Janus membranes with anisotropic wettability, it still remains a great difficulty for single‐layer Janus membranes with straight pore to balance spontaneous liquid penetration in positive direction and superior liquid resistance in the reverse direction. Herein, a liquid‐assisted strategy for single‐layer Janus membrane is developed, which can efficiently decrease the critical breakthrough pressure from superhydrophobic side to hydrophilic side and show little influence on that in the reverse direction. Consequently, unidirectional water penetration with high hydraulic pressure difference can be achieved. The Laplace pressure change along the thickness of the single‐layer Janus membranes is further discussed, and the mechanism by which the auxiliary liquid decreases the critical breakthrough pressure is revealed. Furthermore, this Janus membrane with unidirectional water penetration “diode” performance can be used to prevent liquid backflow in intravenous transfusion. It is believed that this work can open an avenue for people to design single‐layer Janus membrane with high pressure difference and find wide applications in unidirectional liquid transport.

Polydimethylsiloxane-decorated magnetic cellulose nanofiber composite for highly efficient oil-water separation

Developing three-dimensional porous hydrophobic and oleophilic materials (3D-PHOMs) for efficient and selective oil-water separation is important to clean up oil spills and organic pollutants. However, 3D-PHOMs are still confined to lab-scale research due to several crucial drawbacks. Herein, a hydrophobic oil-water separation composite, containing cellulose nanofiber (delignificated porous wood, PW) substrate, magnetic nickel (Ni) layer and hydrophobic polydimethylsiloxane (PDMS) coating, is prepared using electroless deposition (ELD) and surface modification techniques. Owing to the porosity, hydrophobicity (>130° of water contact angle), lipophilicity, convenient magnetic collection and high cycle compressibility, the as-fabricated PDMS-Ni-PW exhibits excellent oil adsorption capacity (>60% of the volumetric absorption capacity) and outstanding cyclic stability (>80% of the adsorption capacity after 200 cycles). Thanks to the low surface energy and rough surface structure, the adsorbent demonstrates superior oil-retention ability (>80% at 200 rpm). Also, the oil-collecting apparatus is successfully designed to continuously separate various oils, e.g., n-hexane and dichloromethane, from water due to the unidirectional liquid transport of the adsorbent. These excellent properties make PDMS-modified cellulose nanofiber a promising candidate for oil-water separation.

Stretchable unidirectional liquid-transporting membrane with antibacterial and biocompatible features based on chitosan derivative and composite nanofibers

Unidirectional liquid transport is critical in achieving high-performance moisture-management fabrics for medical care. However, realizing unidirectional liquid transport while simultaneously satisfying other requirements, such as antibacterial function, adhesiveness, low cytotoxicity, and adequate mechanical strength remains a challenge. In this study, Janus nanofibrous membranes exhibiting both unidirectional liquid transport and antibacterial activity were fabricated via electrospinning and a mild crosslinking procedure. This membrane provides continuous and spontaneous unidirectional water transport with a high one-way transport value (R) of 1483%. The membrane achieved antibacterial rates of 99.2% and 98.7% against E. coli and S. aureus, respectively, without leaching antibacterial agents. In addition, it has high elasticity and self-adhesive properties, which facilitates its use in a range of applications. The design of this versatile Janus nanofibrous membrane provides a new strategy for developing novel moisture-wicking systems, particularly in the field of medical dressings.

Nature-inspired moisture management fabric for unidirectional liquid transport and surface repellence and resistance

Humans sweat to cool down and dissipate heat in indoor and outdoor environments. The evaporation of liquid sweat absorbs body heat and reduces skin temperature. However, when the wearer sweats a lot or is fully wet by external liquids such as rain, even the best moisture management fabrics can be soaked and saturated, becoming unbreathable, heavy, and clingy, and imposing significant reduction in wearer’s comfort and performance. Herein, inspired by liquid collection and release processes on the desert beetle’s skin, we develop an innovative moisture management fabric, transferring excessive sweat from the skin side to the outer surface of the fabric and dissipating it in terms of droplets, while blocking and repelling the external liquids. The completely unidirectional liquid transport is achieved in the developed nature-inspired fabric, which is treated with global hydrophobicity and discrete localized wettability gradient by a sustainable plasma treatment method. Moreover, the proposed fabric shows improved thermal comfort with high breathability, reduced clinginess, and dry skin surface. Specifically, a highly directional water transport capability of 1647.9% is obtained using a Moisture Management Tester, while the water resistance from the external side is equivalent to a water column of 15 mm height. The water flow rate from the inner skin side to the environment is equal to 91.44 g/min∙m2, which is much higher than that of the high sweating rate of an adult (i.e., 15 g/min∙m2). Regulation of liquid movement in the proposed fabric will attract wide potential applications, including sportswear, worker uniform, casualwear, protective clothing, firefighter apparel, and military clothing. Maintaining moisture and thermal comfort while adapting to the expanded ambient temperature range of the air-conditioning system will also be desirable in building energy conservation.

Directional Liquid Transport from the Cold Region to the Hot Region on a Topological Surface.

Manifested from the "tears of wine" to the "coffee-ring effect", the directional transport of a liquid governed by the Marangoni effect is highly pervasive in our daily life and has brought a great number of applications. Similar to this surface tension gradient-dominated process, the fluid preferentially flows from the hot region to the cold region. In contrast to this perception, in this study, we report that water liquid deposited on a specially designed topological surface can flow from the low-temperature region to the high-temperature region in a spontaneous, long-range, and unidirectional manner. We show that such a behavior is mainly owing to a strong topological effect that outweighs the thermal gradient imposed along the surface. Moreover, the specific temperature range applied on the topological surface for the occurrence of such a unidirectional liquid transport phenomenon is also identified. Our findings would find important insights for developing next-generation cooling devices where a rapid flow from the condensation region to the evaporation/boiling region is preferred.

Simply Adjusting the Unidirectional Liquid Transport of Scalable Janus Membranes toward Moisture-Wicking Fabric, Rapid Demulsification, and Fast Oil/Water Separation.

Inspired by nature, Janus membranes with unidirectional liquid transport (ULT) were developed to be used in the fields of fog collection, moisture-wicking fabrics, demulsification, etc. However, the obtained Janus membranes are often unifunctional, and it is still a great challenge to adjust the ULT of Janus membranes for multifunctional applications. Herein, a scalable, low-cost, and machine-washable Janus membrane was developed by combining the cyclic self-assembly of phytic acid and FeIII and a one-side spraying coating of poly(dimethylsiloxane) (PDMS), featuring adjustable ULT upon challenge for multifunctional applications. By controlling the amount of PDMS, the Janus membranes exhibit two different performances, ULT and switchable permeation. The prepared Janus membranes achieved an excellent moisture-wicking fabric (1.6× the water evaporation rate of cotton), fast water collection under oil, rapid demulsification, and the efficient separation of an oil/water mixture. The separation efficiency of a light or heavy oil from water was higher than 99.9% even after 10 separation cycles, and the flux of the separation was up to 2.55 × 104 or 2.38 × 104 L m-2 h-1, respectively. This study could provide an idea for the development of more Janus membranes with adjustable performances to realize multifunctional applications.