INTRODUCTION
Water is an indispensable resource for humans and other lives, and electricity has been a workhorse in the development of modern society since the last century. Both have aroused scientists’ curiosity and interest, and it is not a surprise that their marriage is scientifically interesting and practically significant. Owing to the inherent polarity endowed by the water molecule comprising one hydrogen atom and two oxide atoms, water is easily electrified and therefore, its properties are closely affected by electricity. Over the past decades, there have been extensive achievements in the understanding of basic mechanisms underlying the intriguing interaction of water and electricity. For example, electricity can tune the orientation of water molecules at the microscopic scale, and therefore tailors the phase transition of water at the macroscopic scale. The electrifying ability of water also facilitates many potential applications such as energy harvesting, droplet transport, and so on. Furthermore, electricity, in the form of electric current, electric field, or static charges could also facilitate several applications in which water serves as a medium.
Despite significant progress, the research on the coupling of water and electricity still has many unexplored or not well-understood points. In the basic interaction of water and electricity, most of the previous reviews focus on electrohydrodynamic and hydroelectric effects, both of which mainly involve the interfacial phenomena. To the best of our knowledge, there is still lack of a comprehensive review that focuses on the most direct interaction between water and electricity, that is, how water obtains electric charges. Despite the numerous studies and reports, direct conclusive classification and formulation for many commonly seen electrifying phenomena of water are still missing, for example, why the liquid droplet obtains charges during electrospray. Technically, driven by a basic understanding of the interaction between water and electricity, many applications, ranging from chemistry rection, energy harvesting, and others belonged to diverse research disciplines, have emerged. However, very few previous studies have provided a holistic review of these applications with a broad branch of learning. Thus, a comprehensive review that provides an integrated perspective from fundamentals to applications of the water–electricity connection is urgently required.
In this review, we summarize the recent progress in the coupling of electricity and water from fundamental phenomena and mechanisms to related applications. From the fundamental perspective, we introduce a novel classification for the electrifying methods of water and explain the underlying mechanisms. Regarding the applications, we highlight the cutting-edge applications based on the interplay of electricity and water, including energy harvesting, droplet transport, optical imaging, and so on. This review aims to establish a comprehensive understanding of the water–electricity connection for the readers.
ELECTRIFICATION OF WATER
The inherent molecule structure of water endows it with a strong electrifying ability. The electrification of water, especially liquid water that is stable in an ambient environment, has received considerable attention over the past several decades, but there is still a lack of conclusive work to categorize these electrifying methods. Here, according to the generation methods of charges in water, we summarize the electrifying method of liquid water into several files, mainly including contact electrification, induction electrification, and conduction electrification (Figure ). In addition, we also summarize the common electrifying methods for another form of liquid water, that is, ice (Figure ).
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Contact electrification, a classical physical phenomenon, is a powerful tool to endow water with electrostatic charges. During the contact electrification process, the water acquires the charges with the opposite but same amount of charges as the contacted surface (Figure ). The contact electrification of water has been proven to have good generality from both experimental and theoretical points of view. For example, water flowing in plastic and rubber pipes or sliding on flat surfaces could obtain positive electrostatic charges. In addition, water droplet dropping and then jumping, or directly jumping due to the coalescence of two condensing droplets, from the superhydrophobic surface, could also acquire positive electrostatic charges. Despite these intuitive experimental results, there have been several decades of debate on the mechanism for contact electrification. Contact electrification of water is first regarded as the result of ion transfer. During contact electrification, water droplets leave the OH− on the surface because of the preferred adsorption of OH− on the surface, and therefore, there are net positive electrostatic charges in water due to the residual H+. However, several recent experimental and theoretical studies have proven that both ion transfers and electron transfers occur during contact electrification of water. Note that, in addition to the solid surface, water could also be electrified by contacting with other liquids or gases.
In contrast to contact electrification, which generally positively electrifies the water, induction electrification allows for the on-demand polarity of water's electrification. Figure shows a typical process of induction electrification of water. When the water is closing to a charged object carrying positive charges, it will acquire negative charges because the electrostatic induction imposed by the charged objects results in the charge redistribution in water and its touched surface. If the charged object carries negative charges, water will acquire positive charges. A common example of induction electrification is the Kelvin water dropper (lower part of Figure ), which comprises two metal containers and two metal rings in a unique connection method, developed by William Thomson (Lord Kelvin) in 1867. Due to the infinitesimal and stochastic charges in water, water is gradually electrostatically induced to obtain more negative or positive charges while passing through the metal rings. Similarly, water falling through a metal ring with biased voltage or pre-charged insulators can also acquire the induced electrostatic charges, and the polarity of induced charges in water is always opposite to that of applied voltages. Besides, the induction electrification could also be capable of slitting neutral droplets under the electric field to form two oppositely charged daughter droplets.
Different from induction electrification, conduction electrification is a process in which pre-existing charges directly conduct or inject the same polarity of charges into water (Figure ). A well-known phenomenon for conduction electrification of water is electrospray, where water droplets obtain the electrostatic charges from the conductive emitter with biased voltages. In addition, the droplets flowing along with the charged insulators (lower part of Figure ) or passing through charged liquid membranes could also gain the controllable magnitude of charges. Furthermore, water can also be electrified by capturing the charges from electrified air caused by corona discharging.
In addition to the above three methods, water can also be electrified by some trivial methods. For example, water on hot substrates with a temperature higher than the Leidenfrost point can acquire negative charges due to evaporation-induced charge separation. Another example is balloelectrification of water, in which water broken into a swarm of droplets via sonic spray, vibrating, or laser-caused explosive boiling obtains both negative and positive charges, and water droplets with positive charges are much more prevalent than those with negative charges.
Like liquid water, solid water, that is, ice, could also obtain electrostatic charges, which mainly relies on the temperature difference-induced ion-transfer velocity difference. In fact, the research on the electrification of ice initially aimed to understand the formation of thunderstorm. By experimentally simulating the thunderstorms, it was found that graupel ice pellets could be negatively charged during their contact with the smaller ice crystals. After that, Latham and Mason showed that two pieces of ice specimens with different initial temperatures are temporarily contacted and then separated. During this process, the ice specimen with a higher temperature is negatively charged, while the colder specimen is positively charged (Figure ). The mechanism for charge transfers caused by the temperature difference between two ice samples is referred to as the Latham–Mason temperature theory. In detail, there are two kinds of mobile charges in ice, including H+ and OH−. H+ with higher mobility has a higher diffusion rate along the temperature gradient than OH−, causing the net excess of positive charges in the colder area and negative charges in the warmer area. Accordingly, the magnitude of the transferred charges is proportional to the temperature difference of the ice specimen.
Based on the same mechanism, when two ice species are asymmetrically rubbed (schematically shown in Figure ), the ice with a smaller contact area will be warmer due to the friction-induced heat and therefore acquires negative charges, whereas the other ice acquires positive charges. Note that several factors, such as bubbles in the rubbed ice specimens, could affect the magnitude of the transferred charges because of the lower thermal conductivity of the ice with bubbles. Other approaches, including natural ice/frost depositing on the colder surfaces, and using cold or warm air streams to electrify the ice, also rely on the charge separation caused by the temperature difference. For gaseous water, that is, vapor, the electrifying methods are rarely reported, which is not further discussed here.
APPLICATIONS
The understanding of the above fundamental methods and mechanisms for the electrification of water provides essential insights for the applications correlating electricity and water. On the one hand, water possesses the inherent capability to carry energy, momentum, and mass where soluble and insoluble substances are included, which provides an excellent source for a series of applications. In addition, electricity, as the main power of science and technology, is always at the core of electricity-related applications. The unique function of water and electricity promotes their applications in diverse disciplines, including physics, chemistry, microelectronics, and so on. In this part, we present a brief account of recently emerged applications based on the water–electricity connection.
Harvesting electrical energy from water
Energy shortage, as a result of explosive population growth, rapid economic development, and improved standards of living, necessitates new techniques to efficiently harvest and harness energy. Traditional energy supply mainly relies on the use of fossil fuel, which is accompanied by environmental pollution. Therefore, many kinds of sustainable and eco-friendly energy, including but not limited to solar, hydrogen, wind, water, and so forth, have been widely explored. Here, we focus on the energy harvesting from the water due to its green, abundant, renewable properties. Traditional technology to harvest energy from water suffers from several bottlenecks, such as bulky size, geographic constraints, and so on. Thus, to energize some currently-popular portable and miniature devices that are usually decentralized-distributed, several strategies to harvest energy from water at a small scale, including liquid water and vapor, have flourished over the past decade.
As summarized in Figure (), electricity harvesting from liquid water mainly originates from the variations of interfacial charges, bulk charges, and movable ions. Among these, both interfacial charges and bulk charges are related to the electrical double layer (EDL), a universal phenomenon at the solid/liquid interface, and the variation of EDL yields electricity generation. Regarding the interfacial charges, the variation of EDL can be achieved by changing the contact state (Figure ), contact area (Figure ), or contact line (Figure ) at the solid/liquid interface. Leveraging contact state change of water droplets on solid surfaces to harvest energy is usually by means of contact electrification causing EDL variation. Figure shows a typical process of electricity generation from contact electrification. The droplet dropping on and detaching from the solid surface are accompanied by the establishment and disappearance of the electric potential difference between the droplet and the surface. Such a process drives electron flow through external circuits, thereby generating electricity, and this strategy is usually referred to as droplet-based triboelectric nanogenerators. In addition to the contact state variation-induced contact electrification, the change of contact area or lines can induce the capacitance change of an EDL-based capacitor, and here, EDL is regarded as a capacitor because of the configuration of two parallel layers in EDL formed at the solid/liquid interface with opposite polarity. One typical method to ensure the change of contact area to generate electrical energy is shown in Figure , where one water droplet bridging two parallel surfaces with different wettabilities can be regarded as top and bottom capacitors. By vertically oscillating the top surface, the continuous variation of capacitance on top capacitors can output electrical energy. Similarly, mechanically-actuated change of the contact region at the water/solid interface can result in the disruption of the initial charge balance at the water/solid interface created by an external electric bias, which can output electrical energy. In addition, the capacitance change caused by the varied contact lines can also be harnessed in electrical energy generation. As shown in Figure , by dragging the ionic aqueous droplets on two-dimensional functional materials, a pseudocapacitor stemming from ion adsorption at the liquid/solid interface is charged and discharged at the front and rear of the droplet, which creates a potential difference between the two sides of graphene and therefore output electricity. In addition, dragging water droplets on the semiconductor surface, such as silicon, can also harvest electrical energy based on the pooling of the contact electrification effect and the tribovoltaic effect, in which the bonding interaction between liquid molecules and the semiconductor surface promotes electron transfer between solid/water interfaces.
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Despite the extensive efforts to harvest electrical energy by using the interfacial charges, the conversion efficiency of these methods is relatively low because of their surface reliance posed by the electrode configuration, which has been revolutionized by a droplet-based electricity generator with a transistor-inspired electrode architecture. Such a droplet-based electricity generator is characterized by the formation of a closed loop between the electret PTFE film, two electrodes (ITO and aluminum), and dynamically flowing water (Figure ), achieving a remarkably high-output power density due to its bulk effect. From the electrode architecture perspective, the combination of bottom ITO electrodes and electrified PTFE is analog to the source in a field-effect transistor, whereas the top aluminum electrode behaves like a drain terminal. Water droplet spreading on the PFTE surface and contact with the aluminum can bridge the initially disconnected electrodes (ITO and aluminum) into a closed-looped circuit, and therefore, the droplet can be treated as a gate. This transistor-like electrode design is a universal strategy in electrical energy harvesting, and a series of pioneering studies from our group have proven its effectiveness and efficiency at various interfaces, including solid/solid, gas/liquid, and liquid/liquid interfaces. Figure shows that a transistor-inspired bubble energy generator can transform the initial liquid/solid interface into a solid/gas interface by controlling the bubble spreading and subsequent departure, which yields an output at least one order of magnitude higher than that reported in previous studies. With a similar electrode architecture, a recently developed lubricant-armored transistor-like electricity generator (Figure ) has also shown enhanced energy output performances at the liquid/liquid surface.
In addition to interfacial charges and bulk charges, the movable ions can also be regarded as charge carriers for electrical energy harvesting. Harnessing of the movable ions in an aqueous solution to generate electricity is usually regarded as involving the conversion of chemical energy into electrical energy. Generally, this mode of energy transformation relies on the introduction of a salinity gradient that can trigger the ordered motion of ions. Figure demonstrates a typical conversion process from chemical energy into electrical energy, occurring in two-dimensional MoS2 membrane-separated salt solutions with different concentrations. The salinity gradient and the negatively charged pore of the MoS2 membrane selectively allow the directional positive ion flux from the low salt concentration side to the high salt concentration side, therefore outputting electrical energy with an estimated power density of up to 106 W/m2. Despite outputting high electrical energy, the nanopore MoS2 membrane-based osmotic energy generator cannot be fabricated on a large scale. To resolve this issue, another widely used strategy to achieve directional ion motion between the aqueous solution with a salinity gradient relies on the nanoporous polymers. These polymers have different charge polarities on two sides that can select the ion permeation (Figure ). Besides, other types of materials with nanopores or nanochannels, such as single-walled or multi-walled carbon nanotubes, polymer, silica, and graphene oxide, and boron nitride nanotubes, have also been described to harvest osmotic pressure-induced electrical energy in solutions. Furthermore, the salinity difference on some functional materials, such as metal oxide nanolayers (Figure ), can also cause conversion of chemical energy into electrical energy by means of liquid flow-induced motion of ions.
Different from tangible liquid water, harvesting of electrical energy from amorphous vapor (or moisture) usually relies on the mass and heat exchange between vapor and other surfaces, and this exchange is usually associated with the phase change of vapor. Taking the evaporation process as an example, natural evaporation of water on some material surfaces with hydrophilic pores can induce internal water flow at a nanometer scale, which triggers the occurrence of the streaming potential that is a process of electricity generation by forcing ionic solutions to flow through insulated narrow channel or porous media. Note that the streaming potential effect is positively proportional to flow pressure and slip length of the surface. One pioneering research on streaming potential has shown that the evaporation of water with the immersion of a carbon black sheet (Figure ) can output electrical energy at the same level as that of a standard AA battery. With a similar device configuration but a totally different mechanism, in a recent study, it was reported that evaporation of ethanol on nanostructured carbon films can also generate electrical energy, but its mechanism is not stream potential rather than new-proposed evaporating potential.
Another strategy to harvest electrical energy from moisture relies on the moisture adsorption-induced motion of charge carriers. The key elements in moisture-based energy harvesting are the rational design of moisture-sensitive and -responsive materials that are capable of releasing movable ions as well as the electrode morphology. Taking a recently published work as an example (Figure ), use of two gold electrodes with different sizes sandwiching electrically conductive protein leads to a moisture gradient, and this moisture gradient is associated with the gradient of charges carriers directly from moisture or ionized moisture adsorbed on the nanowire surface, contributing to the generation of closed-loop current of two electrodes. This kind of moisture gradient-driven nanogenerator can also be obtained using functional electrodes or materials with various affinities to water molecules, including carbon material, polymers, and so on. Figure shows electrical generation from bilayer polyelectrolyte films with a heterogeneous distribution of charged mobile ions (Cl− and H+) in moist conditions.
In fact, the existing water-based generators are far beyond the above-mentioned types. For example, a recently reported triboelectric nanogenerator first uses another phase of water, that is, ice, to generate electricity, and such an ice-based generator demonstrates an advantage over other kinds of triboelectric nanogenerator solid generators due to the intrinsic self-healing properties of ice. Due to space constraints, we cannot carry out and present more detailed reviews of these studies.
Droplet transport driven by electricity
Droplet transport plays a crucial role in multidisciplinary applications, which is mainly achieved by introducing energy gradients on the substrate or asymmetric force on the free surface of the droplet by external electrical, thermal, optical, magnetic fields, and so on. Among these external sources, electrical field stands out because of its simplicity, versatility, scalability, and intrinsic property, as most types of droplets and conductive substrates are naturally responsive to electricity, which makes electricity to be powerful driving force in droplet transport. In addition, the electricity generated from the stimulation of other external forces, such as light and thermal fields, can also drive droplet motion. In this section, we review the recent progress in electricity, as a single driving force or cooperating with other forces, driving droplet transport.
Among various electricity-based droplet manipulation methods, electrowetting-on-dielectric (EWOD) has been widely explored. EWOD works on the principle of an external electric field reducing the liquid/solid interfacial surface energy, enabling droplets to move toward the energized electrodes. Notably, the EWOD has two different electrode configurations, including the coplanar electrodes (Figure ) and sandwiched electrodes, and the former is an updated version of the latter due to its less friction force for droplet motion. Despite its efficacy in droplet transport, EWOD is also associated with several disadvantages. For example, EWOD is susceptible to contact line pinning at the solid/liquid interface. One potential solution to this dilemma is electrowetting on a liquid-infused film, which prevents contact angle hysteresis owing to the introduction of a smooth liquid/liquid interface. In addition, EWOD is also limited by the need for high actuation voltage due to the introduction of an insulating layer (~ 100 V), which could pose certain drawbacks, such as electric discharging. Moreover, EWOD also suffers from contact angle saturation at a high voltage, a daunting challenge that needs to be solved. Excitingly, a recently developed new technology, so-called electro-dewetting, only needs 2.5 V to change the wettability of droplets with the addition of an ionic surfactant, therefore driving the droplet with a proper electric circuit control (Figure ). In contrast to the EWOD and electro-dewetting that transport the droplet by changing the interfacial energy, dielectrophoretic methods for droplet manipulation use the Coulomb attraction force originating from the inherent polarizable properties of water when sensing the energized electrode (Figure ). Note that the net charges in the droplet are zero because the polarization of water only causes the redistribution of charges. By timely switching the electrode status, the droplet floating on the oil can continuously move along the electrode arrays. In addition to the alternating electric field, direct electricity from the power supply or triboelectric nanogenerator can also transport droplets by the dielectrophoretic force. However, these droplet transport strategies have a complex electrode pattern, which significantly hinders their feasibility, scalability, and functionality.
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Recently, new advancements that eliminate the need for pre-designed cumbersome electrode patterns have emerged, and their key feature lies in the direct electrostatic interaction between charged droplets and a freely movable driving electrode or surface. For example, on the basis of the electrostatic field-evoked electrostatic induction effect of liquid droplets on conductive superhydrophobic surfaces, our group has developed a versatile droplet manipulation strategy that can use a tweezer-functional-like electrode to electrostatically trap or guide liquid droplets under diverse conditions, such as in open and closed spaces, on flat and tilted surfaces, and in oil medium, resulting in droplet transport with high velocity, unlimited distance, and agile direction, steering in remote and programmatical ways (Figure ). Note that the water droplet on conductive superhydrophobic surfaces gains temporary and adjustable induced charges, which is different from the aforementioned dielectrophoretic method. The charges used to mediate droplet transport can also be generated through contact electrification. In an example shown in Figure , a water droplet is positively charged by dragging it on insulating substrates and therefore the droplet could be repulsively driven by a positively charged glass rod. Similarly, the droplets, charged by means of conduction electrification, can also be attractively or repulsively manipulated by a guiding electrode with different polarities of applied voltages. Additionally, through the elegant design of the surface charge distribution, a surface charge printing technology, eliminating the need for an electrode, could transport water droplets in a directed, long-range, and self-propelled fashion on superhydrophobic surfaces (Figure ).
Despite the achievement of versatile droplet transport by the application of electricity alone, the seamless combination of electricity with other physical effects provides a high level of flexibility and freedom for droplet transport, which can markedly extend the scope of applications and perform tasks that may be otherwise impossible with the use of electricity alone. For example, harnessing the combination of electricity and an optical field, optoelectrowetting is widely used to manipulate droplets by illuminating optical-responsive materials to induce changes in the conductivity of photoconductive surfaces, which serves as the initiator of electrowetting for droplet manipulation. In detail, patterning light to achieve different conductivities of the surfaces can tailor different degrees of electrowetting for the water droplets on surfaces, and therefore, droplets could move from the light to the dark area (Figure ). Different from optoelectrowetting, in the photopyroelectric effect, a light illumination-induced electrostatic field is used to manipulate droplets. As shown in Figure , one single beam of light could trigger the heating of the irradiated spot on pyroelectric crystal substrates and therefore generate an electric field, which further exerts a dielectrophoretic force on the droplet. As a result, the water droplet can be maneuvered by following the motion of the laser spot on the surface (Figure ). In addition, the pyroelectric effect can also be directly activated by the thermal field. When a colder or hotter liquid droplet deposits on pyroelectric crystal-supported substrates (Figure ), an electric potential difference develops on the droplet, which results in a unidirectional, bifurcated, and trifurcated self-propulsion of the droplet according to the crystalline structure of the pyroelectric crystal.
The application of external electric voltage also leads to property variations of the droplet-deposited surface, which offers the possibility to transport droplets by the proper choice of substrates. One example is shown in Figure , in which electricity-generated Joule heat transforms the solid paraffin into the liquid state, facilitating the sliding of the droplet due to the reduced adhesion force between the droplet and substrates. Similarly, the slippery property of the droplet on the surface can also be achieved by electricity-induced mechanical deformation. Figure shows that by applying the voltage, the deformation of two flexible electrode-sandwiched dielectric elastomers leads to the stretching of an oil-infused poroelastic film, which pins the droplet due to the adhesion of the exposed solid structure to the droplet. When the voltage is off, the original shapes of the dielectric elastomer and the poroelastic film are restored, and the droplet can slide again on the oil surface.
Electricity-promoted phase transition-related applications
The understanding of electricity-induced water droplet motion provides us with a toolkit to develop applications that mainly rely on droplet kinetic motions. By further combining with the understanding of electrification of various water phases, a new path for us to understand and practice some phase transition-related applications that are strongly related to human well-being, such as fog collection and anti-icing, is paved. Fog collection is a process where freshwater is collected from environmental sources, which is especially vital for arid regions to mitigate the water shortage crisis. Anti-icing is an effective means to prevent property losses from undesired ice formation on vehicles, refrigeration systems, power grids, and so on. Here, we mainly focus on the electricity-induced water droplet kinetic motions in these applications, rather than the conventional ones, such as electric heating de-icing. In addition, we also present a brief review of the studies reporting on how electricity affects the microscopic phase change of water.
The past several decades have witnessed the booming development of fog collection technology, some of which mimic natural organisms with ingenious surface properties that enable fog collection. Fog collection is a multiscale process involving the microscopic condensation of vapor and the macroscopic collection of the liquid state of fog. In the condensation step, electricity can enhance the condensation of vapor due to decreased critical size of nuclei, and promote condensation nucleation and chemical potential per molecule in water. However, improved fog collection mainly involves the electricity-induced directional transport of fog droplets. Based on the aforementioned electrification methods of water droplets, several electricity-induced fog collection methods have been developed recently. For example, a recent study reveals that electrospun polycarbonate fibers with lower surface potential show a superior water collection rate, due to the electrostatic attraction force to smaller droplets than that with high surface potential, which is achieved by changing the voltage polarity during electrospinning. Apart from charging the collector, charging the droplets could also promote fog collection. Using a corona emitter to spatially electrify the fog droplets (i.e., conduction electrification), the charged droplets are repulsed by the emitter to overcome the aerodynamic drag force (Figure ) and directly propelled toward the collector. Such a method could result in the collection of approximately 30 mL of water within 30 min, which is in striking contrast to its counterpart without the use of electricity (Figure ). Besides, the charge density gradient on superhydrophobic surfaces could sweepingly remove the dropwise condensates. By using corona emitter to positively charge the center of surfaces, the charge gradient is fabricated on surfaces where the charge density decreases from the center to the surrounding area. As a result, spontaneously positively charged condensate droplets at a small size would experience snowball-like growing and chase-like propelling due to the repulsive forces from the center of the surface. Such a process enables a continuous and fast removal of condensates on superhydrophobic surfaces, offering a promising solution to the fields of fog harvesting or removal, as well as anti-icing.
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Corona discharging-induced motion of droplets can also be fully utilized in anti-icing applications. As shown in Figure , by applying a voltage that is higher than the threshold of corona discharging, the water droplet is charged and then detached from the cold superhydrophobic surface. Due to the high droplet detaching velocity, the droplet leaves the substrate before freezing, achieving anti-icing performance. Similarly, the use of a positive electric field could also induce the exist of condensates from the chilled superhydrophobic surface. The condensate droplets gain positive electrostatic charges during the coalescence of smaller condensates and eventually accelerate away from the surface under an external positive electric field. Different from application of an external electric field, the spontaneous charge separation of ice during its growth could also lead to the de-icing on surfaces. As shown in Figure , when frost grows on a chilled surface, the temperature gradient of frost induces an accumulation of positive charges on the bottom side but negative charges on the top side of frost due to the different diffusion rates of the two kinds of charges. After the placement of a water-soaked paper, the negative charges in frost could electrostatically induce water in the paper, therefore generating an attraction force between them. As a result, the frost could jump away from the substrates, preventing the continuous frost accretion on chilled surfaces.
In addition to the electricity-induced macroscopic kinetic motions of the water droplet, electricity also plays a crucial role in the microscopical phase change behavior between liquid and ice, including water freezing and ice melting, which is termed as electrofreezing and electromelting, respectively. In electrofreezing, electricity usually stimulates heterogeneous ice nucleation of supercooled water by affecting the orientation of water molecules. It is worth noting that heterogeneous ice nucleation of water is dictated by several factors, such as droplet size, molecular orientation, surface energy or wettability, confinement, cooling rate, surface modulus, and so on, which is in contrast to homogeneous ice nucleation, where the temperature of homogeneous ice nucleation is usually fixed at approximately −38°C. A common electrofreezing phenomenon is where power cables are covered with ice in cold environments due to the preferred ice nucleation of water caused by the electric field around the cable. Besides, surface charges can also trigger electrofreezing. A reported simple experimental phenomenon is where the droplets on a triboelectrified polyethylene plate can freeze at a higher temperature than that on an uncharged polyethylene plate. In a more systematic investigation, Lubomirsky and collegues found that a negatively charged pyroelectric surface could suppress ice nucleation, while a positively charged surface had a contrasting effect. Also, the initial nucleating points of water are also different on two kinds of charged surfaces, in which water nucleates at the solid/water interface on the positively charged surface but at the air/water interface on a negatively charged surface. This electrofreezing is attributed to the influence of the charged surface on the orientation of water molecules. Similarly, electromelting, a reverse process of electrofreezing, could promote ice melting at below 0°C by orientating the water dipoles.
Electricity-tailored optical imaging applications
The electricity-based optical imaging application generally relies on the configuration change of the liquid droplet. For example, electrowetting could change the wettability of water on the solid surface or water/oil interface, leading to a change of profile or position of the droplet, therefore providing the possibility for use in optical applications. In this section, we mainly focus on electricity-based optical applications, including liquid lenses, light steering, and video display.
Optical lenses are one of the most critical components in diverse optical devices, which revolutionizes the way and scale range we see. The traditional optical lenses are usually made of some rigid materials, including glass or plastic, suffering from several limitations compared to the lenses comprising soft materials, such as liquid crystal, polyelectrolyte gel, and liquid. Taking liquid lenses as an example, they have various advantages, including intrinsic smoothness, variable focus, vibration tolerance, and flexible and adaptable focuses; of these, liquid lenses are the simplest and show the highest-quality performance due to the deformative and fluidic nature of the liquid. To obtain adaptable focuses in liquid lenses, several kinds of electric-related strategies, including electrowetting (Figure ) and dielectrowetting, have been widely investigated. These variable-focus lenses mainly rely on the electricity-stimulated wettability change of liquid, several of which have been integrated into commercial devices, such as cameras and smartphones. With a similar mechanism, electricity can also be used to change the direction of light beam that passes through two kinds of liquids sandwiched by two non-parallel electrodes. In detail, the exertion of electricity imposes different forces on two kinds of liquids because of their different dielectric constants, which changes the position of liquid-liquid interface and thereby deflects the light beam (Figure ).
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Similar to liquid lenses, the electricity-driven display works through the deformation of colored liquid, which affects the color of reflective light. The deformation of liquid generally results from the electricity-stimulated wettability change of the droplet or electromechanical pressure. Figure shows a typical sketch of an electricity-driven display consisting of the arrays of a switchable optical imaging pixel. Based on the oils with the colors of cyan, magenta, and yellow (CMY), the single pixel comprising three layers of oils with CMY colors could reflect the light of various colors by programmatically controlling voltages to achieve the reversible control of the beading state and spreading state of droplets. This electrowetting-based display has a wide viewing angle, small thickness, and low energy consumption owing to its light-reflecting nature rather than light emission, with a fast response time of approximately 10 ms, which facilitates its application in video plays.
In fact, in addition to the liquid lens and video display, electricity can also be used in other optical imaging applications, such as optical switches, optical fibers, and so on. Owing to similar mechanisms, these applications will not be described further here.
Electrostatic charges-evoked chemical reactions
The coupling of water and electricity also paves the way for a branch of chemistry, that is, electrochemistry, in which water and electricity have different roles and merits. Water is an excellent solvent for lots of chemicals due to its cost-effective and eco-friendly properties, especially serving as a micro-reactor to save reactants. Also, electricity, in the form of electric fields or electrostatic charges, is the main driving force for diverse chemical reactions in the water/aqueous solutions involved. Because the electric field-driven reactions, such as electrolysis and electrochemical cells, have been intensively reported in other studies, here, we mainly focus on electrostatic charge-evoked chemical reactions.
The electrostatic charges in both aqueous solutions and the solid surface can drive chemical reactions. As mentioned, the negative charges are spontaneously generated in aqueous solutions at Leidenfrost states, which could serve as reducing agents to synthesize nanoparticles. Figure shows the gradual formation of gold nanoparticles in an aqueous tetrachloroauric acid droplet at the Leidenfrost state, which is reflected by the color change of the droplet. Similarly, charges generated from underwater Leidenfrost are also harnessed to yield a size-tailored zinc peroxide nanostructure.
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In addition to spontaneously generated electrostatic charges within water droplets, the surface electrostatic charges originating from external stimuli, such as rubbing, could also dominate electrochemistry reactions. Liu and colleagues reported a series of aqueous reactions, metal deposition, hydrogen formation, and chemiluminescence, which are driven by triboelectrified insulated substrates, and proposed that these reactions are essentially induced by the electrons (also called cryptoelectrons) generated during contact electrification. However, Baytekin and colleagues reported that the reaction is driven by the mechanoradicals arising from contact electrification and the accompanying material transfer during materials' contact and separation, rather than electrons. In addition to the insulated surface, electrically conductive gold surfaces with electrostatic charges could also trigger such mechanoradical-driven reactions. For example, the gold surface, contact-electrified by the PDMS, could induce silver ions to silver nanoparticles (Figure ). Note that such an electrochemical reaction caused by surface electrostatic charges is governed by the stability of charges, rather than the charging magnitude. Recently, Wang et al. reported that the electron exchange during contact electrification of the fluorinated ethylene propylene (FEP)–water interface induced reactive oxygen species that led to the degradation of an aqueous methyl orange solution, as shown in Figure .
Electrospray-related applications
Based on the conduction electrification, emitted droplets from an electrically powered nozzle can gain electrostatic charges, which is referred to as electrospray. The electrospray process electrifies the droplet and then affects the aerodynamical motion of the droplet, and it can be widely used in many applications such as coating fabrication, reaction acceleration, quantitative chemical analysis, and fiber fabrication.
The process for electrospray of a solution containing a reactant is also referred to as ionization, which generates naked charged reactants for reaction acceleration. Figure shows a typical electrospray ionization process. First, the charged droplet containing a reactant is emitted from a high-voltage nozzle. Then, the continuous evaporation of the solvent leads to the enrichment of higher charges in the shrinking droplet, until reaching a Rayleigh limit. At the Rayleigh limit, supersaturated charges electrostatically promote the fission of droplets and the formation of a gas-phase charged reactant (i.e., aerogel). The electrosprayed microdroplet could markedly reduce the critical energy for reaction due to the solvation of the reagents at the solution–air interface, showing orders of magnitude enhancement in the reaction rate than their conventional bulk-phase counterparts. The reaction rate in charged droplets can be tailored by several parameters, such as droplet evaporation, droplet confinement, droplet traveling distance, and droplet size. In addition to reaction acceleration, the charged microdroplet can also activate some chemical reactions that are thermodynamically and kinetically unfavorable in bulk solution.
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Electrospray ionization can also be coupled with a mass spectrometer, an important tool for chemical or biological analysis. The ionization process transforms the analyte in a condensed phase into a gas phase (or aerogel) in reaction zones (Figure ), and the subsequent analysis of the mass-to-charge ratios of aerogel in a mass spectrometer is capable of characterizing chemical composition, species, surface ligand, biomolecules interaction, photoelectric performance, and so on. In electrospray ionization mass spectrometry, the electrospray emitter distance from the mass spectrometer inlet and emitter tip diameter play dominant roles in the measured results. Notably, in addition to electrospray, the mass spectrometer can also be coupled with other kinds of electric-based technologies for sample preparation, such as capillary electrophoresis.
Another form of electrospray widely used in coating fabrication is electrospinning. Electrospinning is a process in which the electrified liquid jet is emitted from a powered spinneret and then evaporates to form fine fibers and finally solidifies on a target collector (Figure ). Electrospinning is an advanced technology for the fabrication of fibers with the desired shape, morphology, and composition by regulating the solution properties, process parameters, and ambient conditions. For example, use of a coaxial emitter could generate cable-like core–shell nanofibers of two kinds of materials. Note that some materials used in electrospinning, such as organic polymers, small molecules, and so on, are usually dissolved by an organic solvent, rather than water. Still, the electrospinning process and mechanism for both aqueous and organic solvents are the same. By integrating with other functions, electrospun coating has been a workhorse in many fields, such as smart fabrics, biomedical engineering, environmental engineering, and so on. Besides, electrospinning with a controllable ink-jet velocity can also be used in electrohydrodynamic printing techniques to obtain nanostructures. In addition to electrospinning, there still are many advanced electricity-auxiliary coating fabrication technologies, such as electrolytic deposition for metallic or alloy coatings, electrophoretic deposition, anodic oxidation, plasma electrolytic oxidation, electropolishing, electropolymerization, and so on, which will not be discussed further due to space constraints.
SUMMARY AND OUTLOOK
The past several decades have witnessed considerable progress in the research on water, electricity, and their coupling due to the inherent importance of water and electricity, as well as their vital roles in diverse applications. In particular, during the last ten years, there has been explosive advancement in research focusing on water and electricity; indeed, approximately three-quarters of all the cited references in this review have been published after 2012. In this review, we discuss research on the marriage of water and electricity from basic phenomena and underlying mechanisms to emerged applications. First, we classify numerous electrifying phenomena of water into three main types of electrification methods, including contact electrification, induction electrification, and conduction electrification, which enables a broader readership to easily understand the electrification of water or other liquids. We then present several important applications centering on electricity and water. However, some trivial applications, such as the electrophoretic ink used in the electronic paper, electrowetting-based heat management, moisture-assisted electrostatic dust removal, electricity-tailored switchable adhesion, electric field-induced high elastic ice microfibers, and so on, are not discussed in this review due to space constraints.
There are still lots of spaces in the fundamental studies and practical applications of water–electricity coupling. In future, significant efforts should first be devoted to determining the currently pending arguments, for example, what really drives the chemical reactions that occur on charged surfaces. Furthermore, recent advancements of charge fluctuations in nanoscale liquids that induce quantum friction between the water–carbon interface also pose some new possible questions, for example, whether such quantum friction would lead to a different interaction between electricity and water in an open or confined space. More efforts are also required to improve the performances of existing applications, such as efficient water-based electrical energy harvesting. In addition, it is also of interest and essential to explore potential pioneering applications by consistently unraveling the unknown secrets of water and electricity.
ACKNOWLEDGMENTS
We acknowledge financial support from the National Natural Science Foundation of China (no. 51975502) and the Research Grants Council of Hong Kong (no. C1006-20WF, CityU no. 11213320, and CityU no. 11201020).
CONFLICT OF INTEREST
The authors declare no conflict of interest.
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Abstract
As the most common but indispensable matter to humankind, water usually stays in a macroscopically electric neutral state. Due to its inherent molecular polarity, however, water can be easily electrified, which builds a connection between water and electricity. Such a coupling of water and electricity abounds deep scientific basics and technological applications. The past several decades have witnessed extensive progress in studying the mutual effects between electricity and water, but a comprehensive review of its fundamentals and applications is still largely missing. In this review, we first reassess and classify the basic electrifying methods of water according to their mechanisms, then highlight how to leverage the bond nexus of water and electricity to achieve promising technical applications. We envision that this review will inspire multidisciplinary scientific communities to think and innovate more on the research of water, electricity, and their marriage.
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Details
; Wu, Chenyang 1
; Sun, Pengcheng 1 ; Wang, Mingmei 1 ; Cui, Miaomiao 1 ; Zhang, Chao 1
; Wang, Zuankai 2 1 Department of Mechanical Engineering, City University of Hong Kong, Hong Kong, China
2 Research Center for Nature‐Inspired Engineering, City University of Hong Kong, Hong Kong, China