The benefits of being repellent
The accumulation of foreign substances on a surface, whether it is dirt on a window or ice on an airplane wing, can lead to hazardous conditions. Many surfaces have been engineered to resist the accumulation of one type of fluid or matter in a particular state, but engineering broader resistivity has remained a challenge. For example, surfaces that repel water droplets may still be susceptible to fog accumulation. Dhyani et al. review the wetting performance and fouling resistance of different liquid-repellent coatings, focusing on superhydrophobic, superomniphobic, lubricant-infused, and liquid-like surfaces. Two key aspects are the performance of the surface to different foulants and the relevance of considering different length scales.
Science, aba5010, this issue p. eaba5010
Structured Abstract
BACKGROUND
Surfaces that control solid, liquid, or vapor accretion have numerous applications, including self-cleaning windows and solar panels; water and fog harvesting; antimicrobial coatings; ice-shedding coatings for airplane wings, automobiles, or wind turbine blades; and enhancing phase-change heat transport during boiling or condensation. The design of such surfaces has been influenced in part by numerous natural surfaces that can direct the accretion of different states of matter. Examples include water-harvesting cactus spines, self-cleaning superhydrophobic leaves and feathers, and prey-trapping slippery surfaces on carnivorous pitcher plants. Engineered liquid and solid repellent surfaces are often designed to impart control over a single state of matter, phase, or foulant length scale. However, surfaces used in different real-world applications need to effectively control the accrual of matter across multiple phases and fouling length scales. For example, ice-shedding surfaces must reduce the accretion of foulants ranging from frost to large blocks of ice; coatings for reducing marine fouling must control the sequential attachment of soft proteins, bacteria, algae, mussels, and barnacles; and medical implant coatings need to prevent fouling from complex bodily fluids, proteins, and bacterial biofilms. These challenging operational requirements cause many traditional surface design strategies for controlling the accretion of a single state of matter to have limited practical impactтАФconsider superhydrophobic surfaces, which, though effective at repelling liquid water droplets, are easily fouled by water vapor or frost in cold environments.
ADVANCES
Over the past two decades, surface design approaches in liquid repellency have moved from controlling the wetting of a single highтАУsurface tension liquid, such as water, to other singular but more challenging phases, such as lowтАУsurface tension organic liquids. More recently, surfaces have been developed to manifest control over dual-phase mixtures, such as water-oil mixtures, and complex fluids, such as blood. Similarly, surface design strategies to control the accretion of different individual solid foulants have moved from modifying surface chemistry and texture alone to varying other material properties, including surface modulus, mobility, and charge. Solid foulants display considerable disparity in terms of composition, chemical structure, modulus, and the length scale of deposition, making it challenging for a single surface design strategy to be effective against multiple foulants or even a single foulant under different environmental conditions. For example, depending on the environmental conditions, ice displays a wide disparity in terms of its structure, modulus (1.7 to 9.1 GPa), density (0.08 to 0.9 g/cm3), and length scales of fouling (approximately square nanometers to square meters). Recently, strategies have been introduced to control ice accretion across different environmental conditions and fouling length scales. These strategies can also be used to control the attachment of a myriad of other solid foulants, such as scale, wax, clathrates, marine foulants, and bacterial biofilms.
OUTLOOK
Major strides have been made in understanding the surface design principles that control the accretion of specific states of matter and regulate their phase transitions. However, in numerous real-world applications, synergistic accumulation of multiple states of matter across a wide range of length scales is common. Overlap in surface design strategies to control collective solid, liquid, and vapor accretion is limited, although strategies based on surfaces with high interfacial mobility, such as tethered polymeric chains above their glass transition temperature, and lubricant-infused surfaces have shown promise in repelling multiple foulants across different fouling lengths scales. One of the primary challenges associated with the large-scale adoption of the different surfaces discussed here is their mechanical durability, as many of the coatings developed in the field thus far utilize materials that can be easily damaged through abrasion or have poor adherence to underlying substrates owing to their low surface energy. Additional challenges remain in the use of specific chemistries or coating methods that can restrict scale-up, as well as the difficulty of directly comparing the performance of different coatings. Current research is aimed at addressing these challenges and promises a new generation of surfaces that improve our quality of life, offer innovative solutions to some of the most important challenges facing society, and have a substantial commercial impact, measured in billions of dollars every year.
Such surfaces, either individually or in combination, have a range of commercial applications, including anti-fog surfaces, surfaces that enhance condensation heat transport, omniphobic surfaces that repel almost all contacting liquids, antimicrobial surfaces, surfaces that reduce marine fouling, and those that facilitate passive ice-shedding. Such surfaces can be used in diverse operating environments, including oil pipelines, automotive vehicles, eyewear, marine vessels, hospital beds, and aircraft. Image created with BioRender.com.
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Such surfaces, either individually or in combination, have a range of commercial applications, including anti-fog surfaces, surfaces that enhance condensation heat transport, omniphobic surfaces that repel almost all contacting liquids, antimicrobial surfaces, surfaces that reduce marine fouling, and those that facilitate passive ice-shedding. Such surfaces can be used in diverse operating environments, including oil pipelines, automotive vehicles, eyewear, marine vessels, hospital beds, and aircraft. Image created with BioRender.com.
Abstract
Surfaces that provide control over liquid, solid, or vapor accretion provide an evolutionary advantage to numerous plants, insects, and animals. Synthetic surfaces inspired by these natural surfaces can have a substantial impact on diverse commercial applications. Engineered liquid and solid repellent surfaces are often designed to impart control over a single state of matter, phase, or fouling length scale. However, surfaces used in diverse real-world applications need to effectively control the accrual of matter across multiple phases and fouling length scales. We discuss the surface design strategies aimed at controlling the accretion of different states of matter, particularly those that work across multiple length scales and different foulants. We also highlight notable applications, as well as challenges associated with these designer surfacesтАЩ scale-up and commercialization.
