Conventional superhydrophobic coatings that repel liquids by trapping air inside microscopic surface pockets tend to lose their properties when liquids are forced into those pockets. In this work ("Collapse and Reversibility of the Superhydrophobic State on Nanotextured Surfaces"), extremely water-repellant or superhydrophobic surfaces were fabricated that can withstand pressures that are 10 times greater than the average pressure a surface would experience resting in a room. The surfaces resist the infiltration of liquid into the nanoscale pockets. 
(Top left) Arrays of tapered-cone and (bottom left) cylindrical nanostructures that create superhydrophobic or water-repellant surfaces. Air pockets between the structures give rise to the hydrophobic properties. (Right) High-speed photographs of a falling water droplet on a nanostructured surface (top) before, (middle) during, and (bottom) after impact. The extent to which nanometer-size textured, superhydrophobic coatings can withstand elevated pressures is largely determined by the geometry of the texturing. This work shows that by careful tuning of the nanoscale geometry, substantial gains in the durability and applicability of these structures for solar panels, highly robust, self-healing coatings, and anti-icing applications could be realized. Superhydrophobic coatings repel liquids by trapping air inside microscopic surface textures. However, the resulting composite interface is prone to collapse under external pressure. Nanometer-size textures should facilitate more resilient coatings owing to geometry and confinement effects at the nanoscale. This study uses in situ x-ray diffraction to investigate the extent to which the superhydrophobic state in arrays of ~20 nanometer-wide silicon textures with cylindrical, conical, and linear features persists under pressure. The research revealed that the upper bounds of the superhydrophobic state are reached when the liquid pressure is raised above a critical value, which depends on texture shape and size. This infiltration is modeled quantitatively by accounting for the actual geometry of the texture and macroscopic capillary theory. Another important finding is that the infiltration is irreversible for all but the conical surface textures, which exhibit a spontaneous, partial reappearance of the trapped gas phase upon liquid depressurization. This phenomenon appears to be influenced by the kinetics of gas-liquid exchange. These results have profound implications for the understanding and the design of nanosized multiphase (liquid/vapor) systems, including more effective superhydrophobic coatings.
read more "Super water-repellant nanocoatings can now take the pressure"
(Top left) Arrays of tapered-cone and (bottom left) cylindrical nanostructures that create superhydrophobic or water-repellant surfaces. Air pockets between the structures give rise to the hydrophobic properties. (Right) High-speed photographs of a falling water droplet on a nanostructured surface (top) before, (middle) during, and (bottom) after impact. The extent to which nanometer-size textured, superhydrophobic coatings can withstand elevated pressures is largely determined by the geometry of the texturing. This work shows that by careful tuning of the nanoscale geometry, substantial gains in the durability and applicability of these structures for solar panels, highly robust, self-healing coatings, and anti-icing applications could be realized. Superhydrophobic coatings repel liquids by trapping air inside microscopic surface textures. However, the resulting composite interface is prone to collapse under external pressure. Nanometer-size textures should facilitate more resilient coatings owing to geometry and confinement effects at the nanoscale. This study uses in situ x-ray diffraction to investigate the extent to which the superhydrophobic state in arrays of ~20 nanometer-wide silicon textures with cylindrical, conical, and linear features persists under pressure. The research revealed that the upper bounds of the superhydrophobic state are reached when the liquid pressure is raised above a critical value, which depends on texture shape and size. This infiltration is modeled quantitatively by accounting for the actual geometry of the texture and macroscopic capillary theory. Another important finding is that the infiltration is irreversible for all but the conical surface textures, which exhibit a spontaneous, partial reappearance of the trapped gas phase upon liquid depressurization. This phenomenon appears to be influenced by the kinetics of gas-liquid exchange. These results have profound implications for the understanding and the design of nanosized multiphase (liquid/vapor) systems, including more effective superhydrophobic coatings.
(Image: Chan Lei and Keith Schwab/Caltech) Researchers have known for years that in classical physics, physical objects indeed can be motionless. Drop a ball into a bowl, and it will roll back and forth a few times. Eventually, however, this motion will be overcome by other forces (such as gravity and friction), and the ball will come to a stop at the bottom of the bowl. "In the past couple of years, my group and a couple of other groups around the world have learned how to cool the motion of a small micrometer-scale object to produce this state at the bottom, or the quantum ground state," says Keith Schwab, a Caltech professor of applied physics, who led the study. "But we know that even at the quantum ground state, at zero-temperature, very small amplitude fluctuations—or noise—remain." Because this quantum motion, or noise, is theoretically an intrinsic part of the motion of all objects, Schwab and his colleagues designed a device that would allow them to observe this noise and then manipulate it. The micrometer-scale device consists of a flexible aluminum plate that sits atop a silicon substrate. The plate is coupled to a superconducting electrical circuit as the plate vibrates at a rate of 3.5 million times per second. According to the laws of classical mechanics, the vibrating structures eventually will come to a complete rest if cooled to the ground state. But that is not what Schwab and his colleagues observed when they actually cooled the spring to the ground state in their experiments. Instead, the residual energy—quantum noise—remained. "This energy is part of the quantum description of nature—you just can't get it out," says Schwab. "We all know quantum mechanics explains precisely why electrons behave weirdly. Here, we're applying quantum physics to something that is relatively big, a device that you can see under an optical microscope, and we're seeing the quantum effects in a trillion atoms instead of just one." Because this noisy quantum motion is always present and cannot be removed, it places a fundamental limit on how precisely one can measure the position of an object. But that limit, Schwab and his colleagues discovered, is not insurmountable. The researchers and collaborators developed a technique to manipulate the inherent quantum noise and found that it is possible to reduce it periodically. Coauthors Aashish Clerk from McGill University and Florian Marquardt from the Max Planck Institute for the Science of Light proposed a novel method to control the quantum noise, which was expected to reduce it periodically. This technique was then implemented on a micron-scale mechanical device in Schwab's low-temperature laboratory at Caltech. "There are two main variables that describe the noise or movement," Schwab explains. "We showed that we can actually make the fluctuations of one of the variables smaller—at the expense of making the quantum fluctuations of the other variable larger. That is what's called a quantum squeezed state; we squeezed the noise down in one place, but because of the squeezing, the noise has to squirt out in other places. But as long as those more noisy places aren't where you're obtaining a measurement, it doesn't matter." The ability to control quantum noise could one day be used to improve the precision of very sensitive measurements, such as those obtained by LIGO, the Laser Interferometry Gravitational-wave Observatory, a Caltech-and-MIT-led project searching for signs of gravitational waves, ripples in the fabric of space-time. "We've been thinking a lot about using these methods to detect gravitational waves from pulsars—incredibly dense stars that are the mass of our sun compressed into a 10 km radius and spin at 10 to 100 times a second," Schwab says. "In the 1970s, Kip Thorne [Caltech's Richard P. Feynman Professor of Theoretical Physics, Emeritus] and others wrote papers saying that these pulsars should be emitting gravity waves that are nearly perfectly periodic, so we're thinking hard about how to use these techniques on a gram-scale object to reduce quantum noise in detectors, thus increasing the sensitivity to pick up on those gravity waves," Schwab says. In order to do that, the current device would have to be scaled up. "Our work aims to detect quantum mechanics at bigger and bigger scales, and one day, our hope is that this will eventually start touching on something as big as gravitational waves," he says.
Schematic showing a new engineered surface that can repel liquids in any state of wetness. “This represents a fundamentally new concept in engineered surfaces,” said Tak-Sing Wong, assistant professor of mechanical engineering and a faculty member in the Penn State Materials Research Institute. “Our surfaces combine the unique surface architectures of lotus leaves and pitcher plants, in such a way that these surfaces possess both high surface area and a slippery interface to enhance droplet collection and mobility. Mobility of liquid droplets on rough surfaces is highly dependent on how the liquid wets the surface. We have demonstrated for the first time experimentally that liquid droplets can be highly mobile when in the Wenzel state.” Liquid droplets on rough surfaces come in one of two states, Cassie, in which the liquid partially floats on a layer of air or gas, and Wenzel, in which the droplets are in full contact with the surface, trapping or pinning them. The Wenzel equation was published in 1936 in one of the most highly cited papers in the field; yet until now, it has been extremely challenging to precisely verify the equation experimentally. “Through careful, systematic analysis, we found that the Wenzel equation does not apply for highly wetting liquids,” said Birgitt Boschitsch Stogin, a graduate student in Wong’s group and coauthor on a paper titled “Slippery Wenzel State”, published in the August 28 online edition of the journal . “Droplets on conventional rough surfaces are mobile in the Cassie state and pinned in the Wenzel state. The sticky Wenzel state results in many problems in condensation heat transfer, water harvesting and ice removal. Our idea is to solve these problems by enabling Wenzel state droplets to be mobile,” said Xianming Dai, a postdoctoral scholar in Wong’s group and the lead author on the paper. 
In conventional superhydrophobic rough surfaces, tiny liquid droplets in the Wenzel state will remain pinned to the surface textures. In contrast, the new slippery rough surface enables high mobility for Wenzel droplets. In the last decade, tremendous efforts have been devoted to designing rough surfaces that prevent the Cassie-to-Wenzel wetting transition. A key conceptual advance in the current study is that both Cassie and Wenzel state droplets can retain mobility on the slippery rough surface, foregoing the difficult process of preventing the wetting transition. In order to make Wenzel state droplets mobile, the researchers etched micrometer scale pillars into a silicon surface using photolithography and deep reactive-ion etching, and then created nanoscale textures on the pillars by wet etching. They then infused the nanotextures with a layer of lubricant that completely coated the nanostructures, resulting in greatly reduced pinning of the droplets. The nanostructures also greatly enhanced lubricant retention compared to the microstructured surface alone. The same design principle can be easily extended to other materials beyond silicon, such as metals, glass, ceramics and plastics. The authors believe this work will open the search for a new, unified model of wetting physics that explains wetting phenomena on rough surfaces such as theirs.