Draft:Superwettability

Superwettability is a surface phenomenon that describes how gases and liquids interact on a solid surface.. . It is determined by physical and chemical characteristics of both the liquid and gas as well as the solid surface and is partially dependent on the physical composition of the liquid on the surface, which is either greatly adsorbed or greatly repelled.

Superwettability has been found to have applications in areas such as photocatalysis, separation science , water purification technology , printing technology , microfluidics , bioelectronics , cell engineering , and more. Recent studies include investigations of the cooperation of surface energy and structure in the form of multiscale micro-nanostructures, including 0D/1D structures like carbon nanofibers and 3D structures like microscopic pyramids, many of which were based on biological systems. The micro- and nanoscale surface structure of materials, surface-adsorbed molecules, and external factors contribute to the phenomenon. Studies on biological surfaces such as lotus leaves and gecko feet have improved understanding of the mechanisms utilized by biological organisms and has also established that the combination of multiscale surface topography and surface chemical composition and structure including surface energy determine superwettability.

Causes and mechanism
The wettability of a substance is defined by the contact angle between the liquid and surface. The contact angle is dependent on the cohesive, or adhesive, forces between the surface and liquid and the surface tension of the liquid itself. The surface tension is directly proportional to the strength of the cohesive force between the liquid and surface as described by 𝜸=F/L, where 𝜸 is the surface tension (N/m), F is the cohesive force acting on the liquid, and L is the length along which the force is applied. As the length (or area) of interaction between the liquid which meets the surface increases, or as the cohesive forces decrease, the overall surface tension of the liquid decreases, and vice versa.

Superwettability is also dependent on surface energies at each interface between liquid, gas, and the solid surface and can be mathematically described by the contact angle.

There are six primary divisions of superwettability based on the liquid-surface contact angle and the composition of the liquid (see Figure 1):


 * Superhydrophilicity: water on surface with a contact angle <10°,
 * Superhydrophobicity: water on surface with a contact angle >150°,
 * Superoleophilicity: oil on surface with a contact angle <10°,
 * Superoleophobicity: oil on surface with a contact angle >150°,
 * Superamphiphilicity: any liquid on a surface with a contact angle <10° which can exhibit either superhydrophilicity or superoleophilicity, and
 * Superamphiphobicity: any liquid on a surface with a contact angle >150° which can exhibit either superhydrophobicity or superoleophobicity.

Physically, how well the liquid will adhere to the surface determines the contact angle, so the contact angle can be used as a quantitative measure of the wettability of the surface and puts both the cohesive force between the liquid and surface and the liquid surface tension relative to the surface adherence.

The wettability of a surface is determined in part by the “roughness” of the surface, meaning any physical protrusion or adsorbed molecules on an otherwise smooth surface can alter the liquid’s ability to adhere to the surface. Research has shown that the wettability of a surface is affected by factors at both micro- and nanoscale, meaning that one must consider the physical and chemical composition at both levels to design the surface effectively. For example, collections of microscopic hydrophobic bumps can nearly completely repel water as is seen on lotus leaves, such as in a Cassie state. Inversely, especially porous surfaces without hydrophobic qualities can retain water by allowing near total cohesion of water with the surface, such as in a Wenzel state.

Historical significance
The quantitative study of superwettability began in 1804 when Dr. Thomas Young was studying how changing capillary size affects the movement of liquid. In this work, An Essay on the Cohesion of Fluids, Dr. Young first mathematically defined the contact angle between liquid and surface and described how it relates to surface cohesion. The work continued as researchers began to consider the contact angle as the mainstay experimental method to study the effects of wetting on a variety of materials from paraffin to plants.

The next major milestone for superwettability was when Robert Wenzel published a work in 1936 called Resistance of Solid Surfaces to Wetting by Water, where Wenzel first describes the study of, essentially, superhydrophobicity on materials and the replication of the results. He mostly considered the contribution of wetting to the liquid phase, such as that the wetting should be dictated by characteristics of the liquid such as polarity. However, he implied in future studies, further considerations should be given to the characteristics of the surface since there was still an explanation needed for how the same liquid can wet different surfaces in different ways. This provided an avenue for further illumination of the physics behind superwetting as well as the optimization of fabrication of superwettable materials going beyond the Cassie-Baxter and Wenzel models. Following that, the introduction of the Cassie-Baxter model of wetting, in which they described the parameters for contact angles which set the standard for hydrophobicity and hydrophilicity. Today, several methods are used by researchers for studying and fabricating diverse kind of superwettable surfaces. Some of these include:

Superhydrophilic surfaces: coating, etching. More methods can be found in the review articles relating to surfaces.

Superhydrophobic surfaces: template-based synthesis, chemical vapor deposition , electrochemical deposition , and electrohydrodynamics. More methods can be found in reviews.

Superoleophilic surfaces: Dip-coating and electrospinning.

Superoleophobic surfaces: Etching and more.

Superamphiphilic surfaces: 2D capillary effect and 3D capillary effect.

Superamphiphobic surfaces: Anodic oxidation, Water-assisted self-organization.

Biomimetic research
Biomimetic research of superwettability aims to mimic the physical properties of some natural surfaces, such as lotus leaves, rice leaves , butterfly wings , and water strider legs to obtain the same wetting ability as is seen in biological systems. These surfaces can exhibit superhydrophobicity, superhydrophilicity, superoleophobicity, or superoleophilicity, depending on their micro/nanostructures and chemical compositions.

Biomimetic research of superwettability can be divided into two aspects: Applications and theoretical aspect involves understanding the mechanisms and principles of natural superwettable surfaces and developing models and simulations to guide the design. The fabrication aspect involves creating artificial superwettable surfaces with different methods, such as chemical modification, physical deposition, laser processing, and self-assembly.

Some of the most important inspirations for the following are the following:

Lotus leaf: The surface of the lotus leaf is made of very small bumps that trap air between them and the water drops. This makes the water drops almost round and very slippery, so they slide off the leaf quickly. As they slide, they also take away any dirt or dust that might be on the leaf, making it stay clean and dry. This is called the lotus effect or self-cleaning effect. The mechanism is based on two factors: the hydrophobicity and the roughness of the leaf surface.

Hydrophobicity means that the leaf surface repels water, which makes it difficult for water droplets to wet the surface. The leaf surface is coated with a layer of wax that is non-polar and does not interact with the polar water molecules. The contact angle between the water droplets and the leaf surface is very high, around 147 degrees, which means that the droplets are almost spherical and have minimal contact with the surface.

Roughness means that the leaf surface has a microscale and nanoscale structure that consists of numerous small bumps and wax-coated hairs. This structure creates air pockets between the water droplets and the surface, which further reduces the contact area and the adhesion force. The water droplets can easily roll off the surface due to gravity or wind, and they can also pick up any dirt or debris that may be on the surface, resulting in a self-cleaning effect.

Shark Skin: Shark skin is a biomimetic model for superwettability research, as it has a textured surface composed of ridges on tooth-like scales (denticles) that reduce drag and enhance thrust in water. Scientists have tried to replicate shark skin’s hydrodynamic properties by designing artificial materials with similar features, such as ribbed swimsuits and boat hulls, but there’s still more study that needs to be done to clearly understand these properties and mimic it better in future.

Pitcher Plant: The ridge structure and the mucus layer create a superhydrophobic and superoleophobic surface preventing the water and oil from sticking to the skin. Inspired by the pitcher plant, researchers have fabricated artificial surfaces with similar properties by infusing porous materials with lubricants. These surfaces have potential applications in oil/water separation, anti-fouling, and sensing

Cactus: Cacti are plants that can survive in arid and desert environments, where water is scarce and precious. Cactus has developed a strategy to collect water from fog, which is a suspension of tiny water droplets in the air, by using conical spine structures on its surface, which are composed of a hydrophilic core and a hydrophobic cuticle. The hydrophilic core can attract water droplets from the fog, while the hydrophobic cuticle can repel water droplets from the spine tip. The conical spine structure creates a gradient of wettability and Laplace pressure along the spine, which drives the water droplets to move from the spine base to the spine tip. The water droplets coalesce and grow at the spine tip, until they reach a critical size and fall off due to gravity. The fallen water droplets are then collected by the cactus body, which has a waxy and hydrophobic surface that prevents water loss by evaporation. The water droplets are stored in the cactus tissue, which can swell and shrink according to the water content.

Bacterial Biofilm colonies: Bacterial Biofilms have remarkable nonwetting properties, meaning that they repel water and other liquids, as well as gases. the nonwetting properties of biofilms limit the effectiveness of antimicrobial liquids, as they cannot reach the bacterial cells within the biofilm Understanding the mechanism and composition of biofilms have led to the creation of chemical replicas of non wetting gas impenetrating surfaces.

Recent advances
Some unique aspects of bioinspired superwettable materials is that they can reconfigure their shape and properties under external stimuli, such as light, heat, pH, electric field, or magnetic field.

The biomimetic superwettability systems into four types of superwettable materials : static, dynamic, responsive, and multifunctional.

Static superwettability systems are interfacial materials that exhibit extreme wetting states (superhydrophobicity or superhydrophilicity) without external stimuli, based on the combination of surface chemistry and multiscale structures .They can be used for self-cleaning, oil/water separation, fog collection, and antibacterial application. Some examples of static superwettability systems from nature are:

-Lotus leaves: superhydrophobic and self-cleaning due to micro-/nano-papillae with hydrophobic wax.

-Rose petals: superhydrophobic and adhesive to water droplets due to micro-folds and nano-papillae

Butterfly wings : superhydrophobic and iridescent due to micro-ridges and nano-holes

-Shark skin: superhydrophilic and drag-reducing due to multiscale dermal denticles.

Some examples of static superwettability systems from recent research are:

Superhydrophobic porous membrane: oil/water separation due to micro-/nano-pores with low surface energy.

Underwater superoleophobic porous membrane: water/oil separation due to micro-/nano-pores with high surface energy.

Superhydrophobic 3D oil-absorption materials: oil spill cleanup due to micro-/nano-fibers with low surface energy.

Superhydrophobic/superhydrophilic Janus membrane: directional water transport due to asymmetric wettability.

Superhydrophobic/superhydrophilic patterned surface: microfluidic manipulation due to spatially controlled wettability

Dynamic superwettability : Dynamic superwettability systems are interfacial materials that can switch between extreme wetting states (superhydrophobicity or superhydrophilicity) under external stimuli, based on the modulation of surface chemistry and multiscale structures.

Some examples of dynamic superwettability systems from nature are:

Spider silk: superhydrophobic or superhydrophilic depending on the humidity and temperature.

Cactus-like carbon nanotubes are an example of directional liquid dynamics. Artificial spines can enhance the water collection efficiency by increasing the contact area, reducing the contact angle hysteresis, and facilitating the droplet coalescence and transportation.

Pitcher plants: superhydrophilic or superhydrophobic depending on the presence or absence of nectar secretion.

Some examples of dynamic superwettability systems from recent research are:

Thermoresponsive surfaces: superhydrophobic or superhydrophilic depending on the temperature change.

Photoresponsive surfaces: superhydrophobic or superhydrophilic depending on the light irradiation.

Electroresponsive surfaces: superhydrophobic or superhydrophilic depending on the electric field.

Magneto-responsive surfaces: superhydrophobic or superhydrophilic depending on the magnetic field.

Responsive superwettability systems are interfacial materials that can switch between superhydrophilic and superhydrophobic states in response to external stimuli. These systems have potential applications in smart devices, such as sensors, actuators, and drug carriers, that can adapt to different environments and functions.

Some examples of responsive superwettability systems are:

Bioinspired gel surfaces that can change their wettability and adhesion by altering the fluidity of the trapped solvents

Magnetic-responsive surfaces that can manipulate the droplet shape and motion by applying a magnetic field

Electric-responsive surfaces that can control the droplet spreading and detachment by applying an electric voltage

Smart bionic surfaces that can combine two extreme wetting states and switch between them by using various stimuli, such as temperature, light, pH, and mechanical stress

Multifunctional superwettability systems are interfacial materials that combine extreme wetting behaviors with other functionalities, such as electrical, optical, catalytic, and magnetic properties. They have potential applications in various fields of energy science, such as solar cells, batteries, fuel cells, and water splitting. Some examples of multifunctional superwettability systems are:

Bioinspired superhydrophobic/superhydrophilic surfaces that can harvest solar energy and water simultaneously

Superhydrophobic/superaerophobic electrodes that can enhance the performance of electrochemical water splitting

Superhydrophobic/superoleophilic membranes that can separate oil-water mixtures and catalyze the degradation of oil pollutants.