Tag: superhydrophobic

Jumping Off Water
Many insects and arachnids can walk on water by virtue of their hydrophobicity and small size. With their light weight and skinny legs, these invertebrates curve the air-water interface like a trampoline, with surface tension providing the elasticity that keeps them afloat. What’s truly incredible, though, is that many of these creatures, like water striders, can actually jump off the water surface.
The top animation shows high-speed video footage of a water strider leaping off the water. Notice how it distorts the air-water interface but doesn’t break the surface – it makes no splash.
The key is not to push too hard. If the insect exerts a force exceeding the limits of what surface tension can withstand, then its legs will break the water surface and it will lose energy to drag and viscous forces. The insect must generate its jumping force without exceeding a hard limit.
The water strider achieves this feat not by pushing downward but by rotating its middle and hind legs. Rotating its legs allows the insect to maintain contact with the water surface longer and continue deforming the interface as it jumps. This maximizes the momentum it transfers to the water, which, in turn, increases the insect’s take-off velocity. By studying and then emulating this mechanism, scientists were able to successfully create a tiny 68-mg water-jumping robot. (Image credits: J. Koh et al., sources, PDF)
This week FYFD is exploring the physics of walking on water, all leading up to a special webcast March 5th with guests from The Splash Lab.
March 3, 2016
Surface-Tension Supported Walkers
Nature’s smallest water-walkers use surface tension to keep themselves afloat. This includes hundreds of species of invertebrates like insects and spiders as well as the occasional extremely tiny vertebrate, like the 2-4 cm long pygmy gecko shown above. These animals typically have very thin parts of themselves touching the water – like the spindly legs of the water strider. These skinny appendages curve the air-water interface and that curvature, along with the water’s surface tension, generates the force supporting the animal.
Staying afloat on surface tension does little good if a raindrop or passing splash submerges these tiny water-walkers. To avoid that fate, these animals are also hydrophobic or water repellent. This adaptation keeps them from drowning and helps them enhance the curvature where their feet meet the water.
Those tiny indentations can also be important for the animal’s propulsion. Water striders, for example, use their long middle legs like oars to propel themselves. Any rower will tell you that sticks make poor paddles – they’re just not good at transferring momentum to the water. But curving the surface and then pushing off that curvature works remarkably well. It’s how the water strider creates the vortices in its wake in the image above.
For more on water strider propulsion, I recommend this Science Friday video. If you’d like to see the gecko in action, check out BBC Life’s “Reptiles and Amphibians” episode, which is available on Netflix in the U.S. (Image credits: pygmy gecko, BBC; water strider, J. Bush et al.)
This week FYFD is exploring the physics of walking on water, all leading up to a special webcast on March 5th with guests from The Splash Lab. You don’t want to miss it!
March 1, 2016
Trampolining Droplet
Imagine a droplet sitting on a rigid surface spontaneously bouncing up and then continuing to bounce higher after each impact, as if it were on a trampoline. It sounds impossible, but it’s not. There are two key features to making such a trampolining droplet–one is a superhydrophobic surface covered in an array of tiny micropillars and the other is very low air pressure. The low-pressure, low-humidity air around the droplet causes it to vaporize. Inside the micropillar array, this vapor can get trapped by viscosity instead of draining away. The result is an overpressurization beneath the droplet that, if it overcomes the drop’s adhesion, will cause it to leap upward. For more, check out the original research paper or the coverage at Chemistry World. (Video credit and submission: T. Schutzius et al.)
November 13, 2015
Controlling Droplet Bounce
Water repellent, or hydrophobic, surfaces are common in nature, including lotus leaves, many insects, and even some geckos. These hydrophobic surfaces typically gain their water-repelling ability from extremely tiny nanoscale structures in the form of tiny hairs or specially textured surfaces. But, while the nanoscale structures impart superhydrophobicity, researchers have found that larger macroscale structures can improve water-repellent characteristics by reducing a drop’s time of contact with the surface. A smaller contact time means less chance of contamination on self-cleaning surfaces. It’s also helpful in preventing water from freezing on contact to cold surfaces – valuable, for example, in protecting airplane wings’ leading edges from icing over. This combination of nanoscale and macroscale, water-repelling structures can be found in nature, too, such as on the wings of butterflies, which must quickly shed water in order to fly. (Image credits: K. Hounsell et al.; A. Gauthier et al., source video)
August 17, 2015
Weaponizing Water-Repellency
St. Pauli, a neighborhood in the German city of Hamburg, has demonstrated one of the most unusual applications of superhydrophobicity I’ve ever heard of. St. Pauli is known as a party district, and the residents of the area have grown understandably frustrated with inebriated visitors publicly urinating on their buildings and, yes, playgrounds. When fines failed to curb the issue, they took to treating walls chemically to make them superhydrophobic. As the targeted audience has discovered, water repellency tends to make liquid jets bounce off rather than run down a surface. Well played, St. Pauli. (Video credit: IG St. Pauli; submitted by entropy-perturbation)
August 7, 2015
Laser-Made Superhydrophobics
Superhydrophobic surfaces are so repellent to water that liquids often cannot wet them. Today these surfaces are usually created with chemical coatings or deliberate manufacturing to create micro- and nanoscale structures that trap air between the drop and the surface in order to prevent adhesion. Researchers recently announced they’ve made metals superhydrophobic with laser treatments. The process is still time-consuming, but they hope it can be scaled up for wider applications. Because drops bounce so readily off the treated surfaces, it takes very little water to clean them, which may be especially useful for sanitation purposes in the developing world. Superhydrophobic materials are also good for preventing icing on aircraft wings. To learn more about the research, check out the University of Rochester’s video explanations. (Image credit: C. Guo et al., source videos 1,2; submitted by entropy-perturbation and buckitdrop)
January 23, 2015
Manipulating Fluids
Combining water-repelling superhydrophobic surfaces with water-loving hydrophilic surfaces allows scientists and engineers to manipulate common fluids. Here a hydrophilic track surrounded by a superhydrophobic background collects and distributes drops of dyed water. The wetting characteristics of the surface combined with surface tension in the liquid drives the flow. No pumping or power input is necessary. This kind of manipulation of droplets can be especially useful in biomedical applications where fast-acting, low-cost devices could be used to diagnose diseases or measure blood glucose levels. (Image credit: A. Ghosh et al., via NSF; see also source video)
December 29, 2014
Jumping Droplets
When droplets on a superhydrophobic surface coalesce with one another, they jump. Individually, each drop has a surface energy that depends on its size. When two smaller droplets coalesce into a larger drop, the final drop’s surface energy is smaller than the sum of the parent droplets. Energy has to be conserved, though, so that excess surface energy gets converted to kinetic energy, causing the new droplet to leap up. Smaller droplets have higher jumping velocities. For more, see the original video. (Image credit: J. Boreyko and C. Chen, source video)
December 16, 2014
Bounce or Freeze?
Icing is a major problem for aircraft. When ice builds up on the leading edge of a wing it creates major disruptions in flow around the wing and can lead to a loss of flight control. One of the important factors in predicting and controlling ice building up is knowing when and where water droplets will freeze. The video above shows how surface conditions on the wing affect how an impacting droplet freezes. On a subzero hydrophilic surface, a falling droplet spreads and freezes over a wide area, which would hasten ice buildup. A hydrophobic surface is slightly better, with the droplet freezing over a smaller area, whereas a superhydrophobic surface shows no ice buildup. Unfortunately, at present superhydrophobic surfaces and surface treatments are extremely delicate, making them unsuitable for use on aircraft leading edges. (Video credit: G. Finlay)
October 20, 2014
Hydrophobicity and Viscous Flow
Hydrophobic surfaces are great for creating some wild behaviors with water droplets, but they make neat effects with other liquids, too. The viscous honey in the first segment of this Chemical Bouillon video is a great example. Because the honey doesn’t adhere to the hydrophobic surface, the viscoelastic fluid does not maintain the form it had when drizzled on the surface. Instead, the honey contracts, with surface tension driving Plateau-Rayleigh-like instabilities that break the contracting ligaments apart to form nearly spherical droplets of honey on the surface. (Video credit: Chemical Bouillon)
August 19, 2014