Liquid gating technology

Background

Gating strategies for selectively opening and closing pores, which are applicable in the micrometre or nanometre scales, play a fundamental role in everyday technologies such as air purification, industrial separation, and water treatment. Widely adopted approaches for developing such strategies include tailoring inner surface properties as in ion gating (Fig. 1(A)) [1] and using external fields as in semiconductor diode logic gating (Fig. 1(B)) [2]. With the rapid development of materials science, researchers are focusing on building new gating systems by exploiting the inherent properties of materials. Accordingly, the idea of using liquids as structural materials to form responsive gates has been proposed [3]. In the science-fiction film Stargate, a new type of liquid “door” for warping space and time was demonstrated with wormhole material gating (Fig. 1(C)) [4], [5], [6]. Although fictional, the liquid state “Stargate” has inspired us to explore the possibility of using liquids to build a gate. At the macroscale level, liquids are constantly mobile (Fig. 1(D)) and unlikely to be fabricated into fixed shapes. However, at the microscale level, the phenomena driven by capillarity, whereby liquids fill and seal porous media once they are brought into contact, seem to affirm this possibility (Fig. 1(E)).

Fig. 1:

(A) Ion channel. (B) Diode logic gating. (C) Notion of a liquid-like “Stargate” in the science-fiction movie of the same name; it is a type of liquid “door” that distorts time and space. (D) Liquids often exhibit good fluidity. (E) Liquid filling a vertical channel by capillarity. (F) Transport processes in plants: water evaporates through the stomata on the underside of the leaf in the transpiration process, the xylem (blue) mediates the net transfer of water, and the phloem (orange) exports sugars formed by photosynthesis from the leaf. (G) Design of solid and liquid materials of the composite membrane for liquid gating technology.

In nature, liquids are used for gating functions, the examples of which are commonly found in various living organisms. For instance, plants coordinate air, water, and microbe exchange by employing liquids to mechanically reconfigure the pores in the stomata and xylem (Fig. 1(F)) [7]. Drawing inspiration from this phenomenon, Hou and Aizenberg et al. first proposed a liquid-based gating mechanism that uses a capillary-stabilised liquid as a reversible, reconfigurable gate to fill and seal pores in the closed state (Fig. 1(G)) and create non-fouling, liquid-lined pores in the open state [8]. In contrast to solid-based gating materials, gating liquids completely seal pores and form a contiguous coating along the adjacent surface. To enter the pores, transport fluids must deform the liquid interface and require differential pressure thresholds to do so. Thus, the response profiles of threshold pressures can be obtained for a variety of liquids and gases. Owing to the intrinsic variations in its critical pressure for different transport fluids, a liquid-based gated system is an ideal prototype in which a single system is capable of regulating complex, selective multiphase transport.

Subsequently, Hou conceptualised smart gating pore/channel-based membranes in which the transport behaviours across the pores/channels can be controlled through sophisticated design of the membrane properties [3]. Further, he proposed smart liquid gating membranes combined with liquid gating technologies and conducted relevant research from the perspectives of both the porous membrane design and the gating liquid design [9, 10]. Recently, characteristic liquid gating systems targeting different application scenarios have been developed, such as adaptive air/liquid pocket transport systems [11] for controlling microfluids, liquid gating elastomeric porous systems [12] for realising multiphase separation under constant pressure, dipole-induced liquid gating-based porous membrane systems [13] for chemical detection, and mobile liquid gating membrane systems [14] for robotic engineering. The core of liquid gating technologies is that, in contrast to solid-based bare pores, the transport substances contact only the gating liquid when passing through the pores. Thus, in the case of liquid gating technologies, the conventional scientific phenomena occurring between solid–liquid/solid–gas interfaces are transformed into those between solid–liquid–liquid/solid–liquid–gas interfaces. Therefore, in spite of their diversity in applicability, liquid gating systems share common advantages, such as antifouling and energy-saving characteristics as well as stability and transparency, which are realised by the unique combination of dynamic and interfacial behaviours. Therefore, although they are currently in their nascent stages, liquid gating technologies with antifouling and energy-efficient properties in large-scale filtration and separation processes are sufficiently attractive and can potentially accelerate universal accessibility to clean water and sanitation to meet the goals stated in Sustainable Development Goal (SDG) 6 (United Nations Department of Economic and Social Affairs). From a similar perspective, the International Union of Pure and Applied Chemistry (IUPAC) declared liquid gating technology as one of the “Top Ten Emerging Technologies in Chemistry” in 2020, accompanied by a review by Gomollón-Bel [15]. In addition, the recognition of liquid gating technologies can promote research on the applications of liquids as structural materials, which have already been demonstrated in science fiction. With the growing demand for versatile and intelligent materials, liquid-based adaptive materials are expected to play important roles in future applications.

In this study, we present an overview of liquid gating technology, including its emergence, development, and potential applications. First, we introduce the concept and historical timeline of liquid gating technology, followed by the preparation processes, technical parameters, and theoretical modelling of liquid gating systems. Then, we discuss useful applications of liquid gating technologies, such as microscale flow control, multiphase separation, chemical detection, and biomedical catheters. Finally, we summarise our findings and briefly explore future opportunities for the development of liquid gating technologies.

Historical timeline

Figure 2 shows the development timeline of liquid gating technology in terms of the proposal of mechanisms, material designs, and real-world applications. The concept of a liquid-based gating mechanism was first proposed in 2015, whereby the capillary-stabilised liquid inside the pores of the matrix sealed the pores in the closed state and created completely liquid-lined pores in the open state in response to pressure gradients [8]. In 2016, Hou reported the design and preparation of smart gating pore/channel-based membranes from the perspective of symmetric and asymmetric strategies [3], which would endow these membranes greater functionalization and smarter features with cooperative contributions from various disciplines. Further in-depth exploration of both porous matrix design and gating liquid formulation led to the establishment of smart liquid gating systems. As stated in Background, a typical liquid gating-based system involves both gating liquids, termed as functional liquids in some studies, and transport liquids or gases. In general, preventing the replacement of gating liquids with transport liquids requires special focus on the miscibility between them. Meanwhile, in 2017, Cui and Hou et al. contrived a responsive, reversible liquid-based gating system composed of miscible gating liquids and transport fluids [16]. In this system, a new silicone-based lubricant is used as the gating liquid, which is miscible with water at ambient temperature owing to its lower critical solution temperature (LCST); the lubricant and water separate at temperatures higher than the LCST of the lubricant (LCST_oil). Therefore, the penetration of water into this system is reversible, which is controlled by the temperature. At temperatures lower than LCST_oil, water passes through the membrane. As the temperature rises above LCST_oil, the transport of water is blocked. This design has considerable potential for applications such as multiple extraction and controlled release. In 2018, Hou et al. constructed a dynamic air/liquid pocket system that is responsive to external pressure in order to control microscale fluid flow behaviours in microchannels, with apparent advantages of transparency and stability as well as antifouling and self-recovery characteristics [11]. This system can potentially be used in microscale flow control and microchip sensors. Later that year, an elastomeric liquid gating system was fabricated [12]. With elastomeric materials acting as a membrane matrix, the pore sizes of the membrane can be altered by mechanical stretching to influence the critical pressure for transport of fluids through the system. Therefore, dynamic multiphase separation under constant pressure is easily regulated by mechanical forces. This methodology is beneficial for a wide range of multiphase separation and transport applications, especially those that require constant pressure. Bazyar et al. proposed a liquid–liquid displacement mechanism that extended the basic theoretical models in liquid gating systems [17]. In 2018, Aizenberg’s group provided a research update on the filtration performance of liquid gating membrane for separation of inorganic microparticles from an aqueous feed such as hydrophilic bentonite suspension [18]. They compared cleaning efficiency with repeated cycling between conventional membranes and liquid gating membranes. Compared with conventional membranes, liquid gating membranes shows lower transmembrane pressure, higher throughput, and longer time to foul. They demonstrated the potential of liquid gating membranes for high-impact applications such as water treatment, bioprocessing, and other industrial filtration processes. In 2019, a dipole-induced liquid gating-based porous membrane system was proposed, in which the dipole-induced interfacial molecular reconfiguration, which is sensitive to cations, was adopted to influence the interfacial behaviour of gating liquids and subsequently manifested through the movement of transport fluids [13]. This system can be used to build a new chemical detection platform that does not require electricity. In the same year, a mobile liquid gating membrane system was reported [17]. In this system, the applied pressure can be used to not only control the movement of transport substances but also regulate the displacement of the liquid gating membrane, which makes it a promising candidate for a smart piston and valve. With the increasing diversity of designs proposed both for membrane materials and functional liquids, the family of liquid gating technologies is expanding rapidly. In 2020, metallic liquid gating membranes with antifouling and anticorrosion properties were reported; these membranes enhance the durability of liquid gating systems under extreme conditions such as corrosive and high-temperature environments [19]. Liquid gating technologies have also been demonstrated in biomedical engineering through the development of liquid gating-based biomedical catheters [20]. Compared with conventional catheters, liquid gating-based catheters show improved anticoagulation properties to prevent blood clots owing to their adaptive sizes and slippery surfaces imparted by the liquid linings. Another possible future application of liquid gating is positional drug release through liquid-sealed pores arranged on a catheter. Colloid fluids are attractive materials, and their dynamics have been investigated intensively. In 2021, a colloid-composed liquid gating system was constructed to study the dynamics of colloids in microscale confined spaces for fluid transport [21]. In the absence of a magnetic field, the magnetic colloids distributed in the confined space of pore randomly. While the magnetic field is on, the magnetic colloids could form a chain shape structures along the magnetic direction, and the viscosity of magnetic fluid would increase, resulting in an increase in the threshold pressure of the transport fluid. Moreover, the arrangement of the magnetic colloids in confined space could be controlled by changing the direction of the magnetic field, which could change the flow rate of the transport fluid. Therefore, this system shows potential application in precisely control microfluidic flow. Although they are currently in their nascent stages, liquid gating technologies have been recognised as key innovations owing to their advantageous properties, such as anti-fouling and energy-efficient characteristics, which are highly desirable in many industrial applications. Moreover, the applications of liquid-based materials are expected to bestow liquid gating technologies with considerable versatility, similar to liquids themselves.

Fig. 2:

Development timeline of liquid gating technology, including the proposed mechanisms, material designs, and real-world applications.

SDBS, sodium dodecyl benzene sulfonate; Mⁿ⁺, cation. Reproduced from Ref. [3, 8, 11], [12], [13], [14, 16, 17, 20, 21]. Copyright 2015 and 2018 Springer Nature, 2016 Walter de Gruyter GmbH, 2018 and 2020 American Association for the Advancement of Science, 2018 Royal Society of Chemistry, 2019 Wiley-VCH, 2017 and 2019 American Chemical Society, 2021 Oxford University Press.

Preparation and parameters

Gated transport systems based on liquid gating technologies are compatible with a wide variety of pore sizes, pore geometries, surface chemistries, and gating liquids; liquids as dynamic structural materials can provide active gating mechanisms. In the case of a transporting substance for which the applied pressure exceeds the critical pressure, instead of being expelled from the gated pore, the gating liquid performs structural reconfiguration by forming a liquid lining along the adjacent surface. The pores will remain open as long as the transport fluid is flowing. Once the applied pressure falls below the threshold, the gating liquid will be thermodynamically primed to recover and close the pores immediately [5, 8]. Such a gating rationale is significant in the fields of membrane science, and liquid gating membranes have been proposed accordingly [9, 15]. By simply combining proper gating liquids with suitable membrane matrices, liquid gating membranes can be conveniently fabricated with reversible, reconfigurable, non-fouling, and energy-saving properties. The essential components for preparing liquid gating membranes are gating liquids, which are also called functional liquids, and porous membrane materials. Through careful selection and pairing, these two components can reinforce each other to achieve multiple functions (Fig. 3). In this section, we describe how to pair gating liquids and membrane matrices as per different requirements and applications.

Fig. 3:

Design of gating liquid and porous matrix according to liquid gating technology and their operational scenarios.

Reproduced from ref. [8, 9, 11], [12], [13], [14, 19], [20], [21], [22]. Copyright 2015, 2018, and 2020 Springer Nature, 2018 and 2020 American Association for the Advancement of Science, 2019 Wiley-VCH, 2019 and 2020 American Chemical Society, 2020 Royal Society of Chemistry, 2021 Oxford University Press.

For both liquids and gases to pass through a liquid-gated system, the applied pressure of the transport substances must exceed the critical pressure, which is the pressure required to deform the surface of the pore-filling liquid. The critical pressure varies owing to the capillarity mechanism. Therefore, the variables that affect capillaries play an important role in influencing the technical parameters of liquid gating membrane systems.

Here, the system design and preparation processes can be categorised into three parts. Polymers, ceramics, metals, and composite materials have all been employed as porous membrane materials. As liquids constantly flow and it is difficult for them to remain stationary, porous matrices with liquid gating technologies are expected to provide confinement and mechanical strength to the system. Thus, different membrane materials can be selected according to the operational scenario. For example, under demanding conditions such as high temperature or pressure, materials with superior stability, such as metals or ceramics, are ideal candidates. Membrane materials commonly used in many studies include silica gel, PDMS, and foamed copper (Table 1) [12, 14]. In addition to sustaining mechanical stability, the design philosophies of porous matrices also focus on enhancing the affinity between the porous materials and the gating liquids; therefore, factors such as pore size, surface roughness, porosity, tortuosity, and pore geometry are routinely considered. For instance, in general, the appropriate size range of a liquid-gated pore is 1–100 μm [9]. If the pore is larger, the threshold pressure of the liquid gating system would become relatively low, leading to instability of the gating liquid inside the porous matrix [12]. By contrast, at smaller pore sizes, the threshold pressure of the system is expected to increase sharply, causing higher energy consumption. Therefore, to further process the membrane materials for the preparation of liquid gating membranes with proper pore sizes or geometries, various methods, such as sintering technology, freeze-drying technology, 3D printing technology, electrochemical technology, electrospinning technology, template method, projection microstereolithography, and laser drilling technology (Fig. 4) [12, 19, 20, 23], [24], [25], [26], can be adopted. Moreover, with the emerging research trends in smart gating membranes, responsive and adaptive materials can also be considered.

Fig. 4:

Preparation methods of porous matrix: sintering technology, freeze-drying technology, 3D printing technology, electrochemical technology, electrospinning technology, template method, projection microstereolithography, and laser drilling technology.

Reproduced from Ref. [12, 19, 20, 23], [24], [25], [26]. Copyright 2014, 2018, 2019, 2020 American Association for the Advancement of Science, 2020 American Chemical Society, 2018 Royal Society of Chemistry.

As mentioned above, one of the most important novelties of liquid gating technologies is the introduction of solid–liquid–liquid/solid–liquid–gas interfaces into the gating mechanisms. The formulation or selection of the gating liquid is the key factor in the successful realisation of liquid gating technologies [9, 12, 27]. Once the membrane materials are set, the baseline for the selection range of the gating liquids can be established. To prevent the replacement of gating liquids by transport fluids, the affinity of the selected porous matrix to the gating liquid must be greater than that to the transport fluid. To this end, the chemical composition, viscosity, volatility, and conductivity of the gating liquid are important parameters to be considered. When the transport fluid is a gas, a water-based or oil-based liquid can be used as the gating liquid [9]. When the transport fluid is a liquid, careful selection of an immiscible liquid as the gating liquid is necessary [9]. Silicone oil, liquid paraffin, and perfluorinated liquids (Krytox-grade oils) have experimentally been shown to be the most suitable gating liquids. The specific parameters of commonly used gating liquid materials are listed in Table 2. And the experimental results for various porous matrix/gating liquid/transport liquid combinations are listed in Table 3.

Theoretical modelling

In contrast to membranes, where the transport behaviours are driven by concentration or potential gradients, the operation of liquid gating systems is driven by pressure. For liquid gating systems, when the transport fluid is a gas—subject to the resistance of the liquid gating caused by capillary force during the transport process (Fig. 5(A))—its transmembrane pressure ΔP (which is the difference between the inlet pressure and the outlet pressure of the transport fluid.) is affected by the Laplace pressure [8, 29]:

where γg−l denotes the gas–liquid surface tension, and R1 and R2 represent the radii of curvature of the surface. When the curved surface is spherical, R1=R2=R, and the transmembrane pressure is

Fig. 5:

Theoretical mechanism and modelling of liquid gating technology.

(A) The transport fluid is a gas. (B) The transport fluid is a liquid. Bottom: The pore size distribution and its effective part. (C) Macroscopic and microscopic mechanisms of liquid transportation from the porous matrix. (D) Energy of different configurations and criterion for stable liquid-based porous system. (E) Different configurations of the droplet on the liquid-based porous matrix based on the wetting behaviour. Reproduced from Ref. [13, 23, 37], [38], [39]. Copyright 2011 Springer Nature, 2018 American Association for the Advancement of Science, 2013 Royal Society of Chemistry.

When the transport fluid is a liquid (Fig. 5(B)), the influencing factors are more complex. When the liquid passes through the porous matrix, the transmembrane pressure ΔP depends on the flow rate Q according to Darcy’s law [8, 12, 30], [31], [32]:

where k denotes the permeability, Atot denotes the cross-sectional area, μ denotes the dynamic viscosity, and L represents the thickness of the porous matrix.

A microscopic model is used to further interpret k. Assuming that the pore is in a cylindrical tube—to allow the transport liquid to flow through the pore—a minimum pressure is required because of the capillary effect (Fig. 5(C)). The minimum pressure depends on the pore size and interfacial tension. Meanwhile, the transmembrane pressure ΔP depends on the flow rate Qi of the Hagen–Poiseuille law [8], given by

where A denotes the cross-sectional area, di denotes the microscopic pore size, μ denotes the dynamic viscosity, and La denotes the actual length of the pore.

The flow rate of the entire porous matrix is obtained by accumulating the flow rate of a single microscopic channel as follows:

Thus, the permeability is given by

For a porous matrix, the porosity is given by

Further, the curvature of the pore is given by

For a porous matrix, the pore sizes of all the micropores are equal to the average pore size.

By substituting the above-mentioned equations into Equation (7), the permeability can be expressed as follows:

During the transportation process, the permeability does not affect the transportation pressure; it remains constant. Equation (10) is no longer applicable to liquid gating systems, i.e., the average pore diameter cannot be used for the calculation. The probability distribution of the pore diameter after the liquid door is opened is related to the transport pressure [8] as follows:

Meanwhile, because of the presence of capillary force, the penetrated pores satisfy the following equation (Fig. 5(B)) [8]:

where γl−l denotes the surface tension of the liquid–liquid interface. The smaller pores remain closed. Thus, d‾2 in Equation (11) can be calculated using the expected square (assuming a normal distribution) as follows:

where σ denotes standard deviation.

Consequently, the permeability is related to the transmembrane pressure as follows:

For liquid gating systems, it is necessary to ensure that the transport fluid does not replace the gating liquid in the porous matrix during the gating process. To ensure the stability of the liquid gating system, the following issues should be considered (Fig. 5(D)) [12]:

1. the porous matrix is filled with the transport liquid (interfacial energies: E1).

2. the porous matrix is filled with the gating liquid (interfacial energies: E2).

3. the transported liquid is above the gating liquid (interfacial energies: E3).

To ensure that the affinity of the porous matrix to the gating liquid is higher than that to the transported liquid, ΔEⅠ=E1−E2>0 and ΔEⅡ=E1−E3>0. Further, ΔEⅠ and ΔEⅡ are interpreted as follows [11, 18, 19, 22].

where R denotes the roughness factor of the porous matrix (R=2), which is determined as the ratio between the actual and projected surface areas. Further, γA and γB represent the surface tensions of the transport and gating liquids, respectively, while and γAB represents the interfacial surface tension between the transport liquid and the gating liquid. In addition, θA and θB denote the equilibrium contact angles of the transport and gating liquids on a flat solid membrane surface, respectively.

Hou et al. used an elastomeric porous membrane (EPM) as the porous matrix and Krytox^®103 as the gating liquid, and they subsequently used water, silicone oil, and liquid paraffin as the transport liquids [12]. The stability of the liquid gating system was analysed by testing the contact angles, surface tension, and interface tension. When ΔE_I and ΔE_II are both positive, the liquid gating elastomeric porous membrane (LGEPM) is a stable system; however, when both are negative, the LGEPM tends to be unstable. Krytox^®103 has theoretically and experimentally been shown to ensure a stable LGEPM system for the transport of deionised water; however, it cannot guarantee the stability of the system when transporting silicone oil and liquid paraffin, as indicated in Table 4.

The research of surface wetting behaviour is very important for liquid-based porous materials, such as liquid-infused porous surfaces [33], [34], [35], and liquid gating porous materials. For the liquid gating technology, it is necessary to consider the wetting state of two characteristics when the transport liquid interacts with the gating liquid: the wetting ridge and the shielding layer (Fig. 5(E)) [36], [37], [38]. The wetting ridge is caused by surface tension at the three-phase contact. The annual ridge is formed around the transport liquid because the vertical component acts on the pipeline of the gating liquid. The formation of the masking layer is determined by the diffusion parameter S of the gating fluid on the transport fluid. When there is only a porous matrix and transport fluid, this parameter can be calculated from the interface energy as follows [36], [37], [38]:

where γSV and γLV denote the surface tensions of the transport and gating liquids, respectively, and γSL denotes the interfacial tension between the porous matrix and the gating liquid.

When there is a porous matrix, gating liquid, and transport fluid, this parameter can be calculated from the interface energy as follows [36], [37], [38]:

where γT and γG denote the surface tensions of the transport liquid and gating liquid, respectively, and γGT denotes the interfacial tension between the transport liquid and the gating liquid. When S>0, a gating liquid layer spontaneously forms on the transport liquid. Therefore, an appropriate gating liquid/transport liquid pair must be selected to prevent the loss of the functional liquid. According to the spreading coefficient of mixing and separation, the critical value for the selection of the transport and gating liquids is γGT>0.

Emerging applications

Microscale flow control

The manipulation of fluids at the microscale level is a promising application for medical devices, biological assays, and environmental analyses. Therefore, microscale flow control remains a research hotspot. The microscale flows are usually transported through solid-based channels or between liquid–liquid interfaces, and fouling issues during the processes seem inevitable, especially when handling viscous fluid samples. To mitigate these effects, researchers tend to coat the solid inner surface or employ materials with low surface energy [39, 40]. Although these approaches achieve some improvement, the durability of these systems is limited. Furthermore, liquid–liquid interfaces are relatively fragile and only suitable for low-pressure applications.

In 2018, researchers developed a liquid gating-based adaptive air/liquid pocket composed of an interconnected porous matrix partially infused with a functional liquid and a microchannel constructed inside the materials [11]. In this system, the gating liquid acts as a dynamic liquid gate for fluid transport. The fluid is transported when the applied pressure exceeds the critical pressure, which depends on the interfacial tension between the transport fluid and the gating liquid as well as and on the dimensions of the microchannel (Fig. 6). Meanwhile, the applied pressure should be lower than another threshold pressure determined by the interfacial tension and the average pore size to prevent the replacement of the gating liquid in the pores with the transport fluid. This system prevents the fouling issues that occur in solid matrices, and it can be extended to metal-based systems with superior mechanical and thermal stability. Moreover, this platform with a unique adaptive pressure capability may offer potential solutions to flow control in microfluidics, medical devices, microscale synthesis, and biological assays.

Fig. 6:

Microscale flow control.

(A) An adaptive air/liquid pocket transport system guides the microscale flow in the square microchannel. (B) Dynamic liquid–liquid interfaces in the microchannel. Reproduced from Ref. [11]. Copyright 2018 Springer Nature.

Multiphase separation

The separation of fluids and gases is of great significance to various research areas and industrial applications, including biphasic catalysis, oil pipeline transportation, and colloidal particle synthesis [41], [42], [43], [44]. To realise such separation, the surface properties of the porous matrix should be modified for efficient permeation of one phase while blocking the other phases [45]. Nevertheless, during these processes, achieving multiphase separation under constant pressure in an energy-saving manner remains a challenge, while the fouling issues of materials also persist.

To overcome these issues, an elastomeric liquid gating system composed of an elastomeric porous matrix was proposed in 2018 [12]. The pore sizes on the membrane can be dynamically altered through mechanical stretching. Along with the pore size alteration of the membrane, the critical pressure of the gating system for transmembrane substances can also be modulated. The changes in the critical pressure induce differential transport behaviours of the transport fluids. In multiphase transport, when the pressure applied to the transport substances is lower than both critical pressures (ΔP < P_{critical (gas)} < P_{critical (liquid)}), the system is “closed” for both gases and liquids; neither fluid passes through the system. After stretching the membrane, the pore sizes increase, and the critical pressures for both gases and liquids decrease; then, the applied pressure remains lower than the critical pressure of the liquid but exceeds the critical pressure of the gas (P_{critical (gas)} < ΔP < P_{critical (liquid)}); hence, the gas passes through the system whereas the liquid is blocked (Fig. 7). Consequently, the gas and liquid are separated. Thus, the elastomeric liquid-gated membrane can dynamically separate and transport multiphase substances under mechanical forces without changing the applied pressure. In addition, the elastomeric liquid-gated membrane with dynamic tunabilities will be of benefit in the fields ranging from gas-involved chemical reactions, fuel cells, medical devices, multiphase reactions and beyond.

Fig. 7:

Multiphase separation at constant pressure.

(A) Gas and liquid separation at a steady-state pressure. (B) Threshold pressure of the gas and liquid under straining of the matrix. Reproduced from Ref. [12]. Copyright 2018 American Association for the Advancement of Science.

Chemical detection

The physical and chemical properties of functional liquids, such as surface or interfacial tension, affect the critical pressure in a liquid gating system. Hence, the threshold pressure for transport substances can be modified to reflect the variations in the gating liquids. Accordingly, a liquid gating technology that allows visual chemical detection was proposed in 2019 [13]. This system was prepared by infusing a nylon porous matrix with sodium dodecyl benzene sulfonate (SDBS) surfactants aqueous solutions. After introducing cations into the gating liquid, the distribution of surfactants at the gas–liquid interface is rearranged because of the dipole-induced interfacial molecular reconfiguration (Fig. 8(A)). This results in a decrease in the critical pressure. Thus, different cations lead to different pressure distributions, resulting in differential outlet pressures of the transmembrane substances, indicated by the distances travelled by the marker droplet placed in the tube (Fig. 8(B) and (C)). Moreover, with the incorporation of chemical, physical, and biological interaction mechanisms in gating liquids, the selectivity and sensitivity can be better achieved, and the liquid gating-based detection can be expanded to various substances, including food, medicines, and chemicals.

Fig. 8:

Liquid gating-based visual chemical detection.

(A) Schematic mechanism of chemical detection based on liquid gating technology. (B) Illustration of open and closed states of the system, the marker is made up of dyed water drop. (C) The threshold pressure changes with the addition of different cations. Reproduced from Ref. [12]. Copyright 2019 Wiley-VCH.

Biomedical catheter

Biomedical catheters are widely used in routine medical treatments for administering fluids and medicine as well as for draining fluids from the body. However, existing catheters have disadvantages, such as limited functionality, inadaptability to environmental changes, and thrombus formation. In addition, the current catheters can only release drugs at the port, which limits flexibility and controllability during drug releasing. In 2020, researchers reported a biomedical catheter using porous polyvinylidene fluoride (PVDF) materials filled with a biocompatible functional liquid (Fig. 9(A)) [20]. As the gating liquid is dynamically distributed through the porous membrane, which can be used as a specific mass transfer pathway on the catheter wall to positionally control drug release properties. In addition, the biomedical catheter can be tunable in sizes with the changes of environmental pressures, which is considerably important to reduce the incidence of thrombosis. As shown in Fig. 9(B), compared with bare PVDF, the liquid gating membrane-based catheter exhibits better anticoagulation properties owing to its ultra-smooth liquid interface. This system provides a novel solution for more complex catheter applications. With these unique properties, the liquid gating membrane provide a good platform for the catheter application, and it can be extended to be the liquid-based specific mass transfer pathway on the catheter wall to be used further in positionally drug release applications.

Fig. 9:

Liquid gating membrane-based catheter.

(A) Design of bioinspired liquid gating-membrane based catheter. (B) Anticoagulation property. Reproduced from Ref. [20]. Copyright 2020 American Association for the Advancement of Science.

Summary and outlook

This paper surveyed recent advancements in liquid gating technologies, including the proposal and development of the related mechanisms, smart designs, and emerging applications. Liquid gating technology comprises a gating liquid and porous matrix that can be used in various fields with the development of responsive porous matrices and functional liquids. Smart designs impart liquid gating technologies with desirable properties, such as antifouling and energy-saving characteristics as well as durability, adaptivity, stability, and reversibility. Existing liquid gating technologies have shown considerable potential and adaptability in a wide range of applications, including multiphase separation, chemical detection, biomedical applications, water treatment, and microfluidics.

In particular, liquid gating technologies are envisaged to provide significant benefits in filtration and separation processes, although such technologies remain in their nascent stages. The potential challenges and opportunities with regard to promoting further research on liquid gating technologies are as follows.

1. Further in-depth exploration of the theoretical fundamentals of liquid gating technologies, especially quantitative studies of the parameters, can facilitate optimal operation of systems. For example, the thickness range of the gating liquid during dynamic operation can be characterised to achieve the optimal thickness according to various pore geometries and porous matrices in the transportation mechanism.

2. The development of responsive materials for building smart liquid gating systems is under way, and it can meet the growing demand for intelligent applications. To this end, the selection of smart materials, including liquid materials, requires further attention to meet future requirements.

3. The development of a detection platform based on liquid gating technologies is a promising research direction, because properties such as visualisation, reading convenience, and electricity-free operation are highly desired, especially in developing countries. To improve these methodologies, the capability of quantitative chemical and biological detection requires further exploration.

Drawing inspiration from nature, the development of liquid gating technologies is expected to promote sustainability and protect the environment by incorporating reusability as well as energy-saving and antifouling properties into conventional filtration and separation processes. In terms of answering the universal call for sustainable development, liquid gating technologies are expected to mature rapidly and allow universal access to clean water and sanitation. In addition, they are expected to be applied in novel applications in oil-water separation in the petroleum process, environmental analysis, chemical separation, energy storage and conversion, etc.