FYFD

Tag: evaporation

  • Sliding Along

    Sliding Along

    Robust, self-cleaning surfaces are a holy grail for many engineers, but they’re tough to achieve. One necessary ingredient for a self-cleaning surface is the ability to shed water, which is why superhydrophobic coatings and surface treatments are popular. Here, researchers prompt their droplets to move at speeds up to 16 cm/s by dropping them onto a thin layer of heated oil.

    Longtime readers will no doubt be reminded of self-propelling Leidenfrost drops, but this situation is not quite the same. In general, the oil layer suppresses the Leidenfrost effect. Instead, the oil heats the drop, evaporating its vapor. A bubble of vapor will nucleate at a random location in the droplet and eject itself, pushing the drop in the opposite direction. Because of the disruption caused by that ejection, new bubbles will preferentially form at the same spot, providing an ongoing supply of vapor that keeps the drop sliding in the same direction. It’s like a miniature rocket zooming along the oil film! (Image and research credit: V. Leon and K. Varanasi; via APS Physics)

  • Programmable Capillary Action

    Programmable Capillary Action

    Capillary action combines the cohesive forces within a liquid and the adhesive forces between a liquid and solid to enable a liquid to fill narrow spaces, even against the force of gravity. To control capillary action, researchers are 3D-printing what they call “unit cells,” tiny structures that water and other liquids can climb. There’s no pump raising the liquid through these structures, just capillary action.

    In a particularly neat demonstration of the technology, the researchers built a tree-like structure out of many open-walled unit cells and placed the “root” system in a closed reservoir. Capillary action drew liquid up the structure to the tips of its branches, where the dyed water evaporated. The process is similar to transpiration in trees, though in trees, capillary action provides much less of the lift. (Image and research credit: N. Dudukovic et al.; via Nature; submitted by Kam-Yung Soh)

  • Snapping When Swollen

    Snapping When Swollen

    The Venus flytrap snaps shut on its hapless prey by swelling cells in its leaves with water. Under the added pressure of a fly’s footstep, the leaves’ snapping instability triggers, trapping the insect. Researchers are using similar physics to create jumping and snapping polymer gels, like the one seen below.

    This jumping polymer shell exploits snapping that occurs as it dries out.

    To trigger the behavior, researchers soaked their polymer-based gel strips and shells in a solvent of n-hexane, which easily permeated the material and made it swell up. As the solvent evaporates from the swollen gel, the polymer material changes shape, sometimes in smooth bends and sometimes in abrupt snaps. The group was able to harness those snaps to have their materials descend slopes and climb ladders — all without motors, batteries, or external sources of energy. (Image credit: plant – A. Dénes, shell – Y. Kim et al.; research credit: Y. Kim et al.; via Physics World)

  • Whiskey Webs

    Whiskey Webs

    Unlike scotch whisky, when American bourbon whiskeys are diluted, they form unique web-like evaporation patterns. These differences arise in part from the way the liquors are aged: scotch is aged in re-used barrels, whereas bourbons require aging in a new, charred American white oak barrel*.

    During aging, the whiskey picks up water-insoluble chemicals from the barrel. When water is added to the bourbon, it helps transport those insoluble components to the surface of a droplet, where they form a monolayer of fatty acid chains (Image 2; in green). As evaporation continues and the droplet gets smaller, the molecules at the shrinking surface collapse inward, forming the rigid web structure we see left behind. The patterns that form act as a kind of fingerprint for the bourbon. Check out some of the brand-to-brand variations over at the researchers’ Whiskey Webs site. (Image and research credit: S. Williams et al.; via Physics Today)

    * In case you were wondering, this is actually a legal requirement in order to be considered bourbon. Bourbons must also be made from a grain mixture that is >50% corn.

  • Spiderwebs and Stratocumulus Clouds

    Spiderwebs and Stratocumulus Clouds

    Stratocumulus clouds cover about 20% of Earth’s surface at any given time, and they form distinctive patterns of lumpy cells separated by thin slits. Because of their interconnectedness, researchers nicknamed these narrow regions spiderwebs. New simulations show that evaporative cooling along the cloud tops drives the formation of these spiderwebs (Image 2). Without it (Image 3), the cloud pattern looks very different. (Image credits: featured image – L. Dauphin/MODIS, others – UConn ME 3250; research credit: G. Matheou et al.)

  • Inside Drying Wood

    Inside Drying Wood

    Wood must dry before it can be used in most applications, but with its complex internal structure exactly how wood dries out has been unclear. New experiments combining MRI and x-ray imaging reveal a process quite different than expected.

    Inside hardwoods like poplar — the species studied here — wood contains both solid structures and pores where water can gather. The pores do not form a fully interconnected network, so capillary action alone is unable to carry water through the pores and out to a surface where it can evaporate.

    Instead, researchers found that water evaporating at the surface came from so-called “bound water” in the wood’s solid structures. As the bound water evaporated, it caused water in the wood pores to diffuse into the solid walls, becoming bound and continuing to feed the evaporation. (Image and research credit: H. Penvern et al.; via APS Physics)

  • Vanishing Spirits: Aging

    Vanishing Spirits: Aging

    The necessary ingredients for scotch whisky’s evaporation patterns are alcohol, surfactants, and polymers; some of those components are absorbed during the spirit’s aging in oak casks. Photographer Ernie Button explored how long it takes for whisky to absorb enough of these chemicals by photographing the stains left by samples aged between 1 and 5 weeks in an oak cask. He found that it takes about 5 weeks for the scotch patterns to begin emerging.

    The aging process for scotch and other cask-aged spirits depends on the fluid’s flow through the porous grain of the oak. Evaporation plays a significant role in the process, so the aging process differs based on environmental conditions. For example, distillers in the dry, high-altitude climate of Colorado must use climate-controlled storage, whereas Scottish distillers use a more humid natural climate to their advantage.

    Another major factor in the aging process is the charred oak cask itself. Some whiskys, like American bourbon, always use a brand new barrel, whereas scotch is often aged in a previously-used cask. With older casks, absorption of molecules from the wood takes longer, which is why scotch is typically aged for much longer than some other types of whisky. (Image, research, and submission credit: E. Button; see also)

  • Vanishing Spirits: Rice-Based Whisky

    Vanishing Spirits: Rice-Based Whisky

    In yesterday’s post, photographer Ernie Button showed us that barrel-aged gin can leave behind an evaporation pattern remarkably similar to scotch whisky. But even among whiskys not every spirit uses the same grains.

    Here we see patterns left behind by a 10-year-aged, rice-based whisky. The stains are entirely different than those of (barley-based) scotch. The rice leaves behind stains with distinct regions, including a radially uniform rim and an interior reminiscent of satellite photos. Presumably the interaction of rice and the cask leaves the whisky with surfactants and polymers that behave rather differently than those of scotch.

    It takes time for spirits to take on character from the casks they’re aged in. Tomorrow we’ll take a look at just how much aging is necessary for scotch’s patterns to emerge. (Image, research, and submission credit: E. Button; see also)

  • Vanishing Spirits: Gin

    Vanishing Spirits: Gin

    Photographer Ernie Button has spent years exploring the patterns left by evaporating scotch. A team of researchers found that the uniformity of scotch whisky’s stain requires three ingredients: alcohol to drive concentration gradients, surfactants to pull particulates away from the drop’s edge, and polymers to help stick particles to the glass.

    Button wondered whether other spirits might produce similar patterns, and, indeed, some do. The photos above are stains left behind by evaporated gin that’s been aged for a year in oak casks. The patterns are extremely similar in appearance to those from aged scotch whiskies, suggesting that the same fluid dynamical effects are at play here, despite the difference in liquor. But do all grain spirits make these patterns? Check back tomorrow to find out. (Image, research, and submission credit: E. Button; see also)

  • Featured Video Play Icon

    Storm Eyes and Mushrooms in a Drop

    In industry, drying droplets often have many components: a liquid solvent, solid nanoparticles, and dissolved polymers. The concentration of that last component — the polymers — can have a big effect on the way the droplet dries, as seen in the video above.

    Without polymers, the droplet dries similarly to a coffee ring stain. But at moderate concentration, we see something very different. The droplet forms an eye in the middle, similar to a hurricane’s, and the edges of the droplet sprout mushroom-shaped plumes that grow and merge with one another along the edge. With even larger polymer concentrations, the mushrooms sweep their way inward, leaving a feathery stain behind. (Video, image, and research credit: J. Zhao et al.)