FYFD

Tag: gravity

  • Raindrops on the Windshield

    Raindrops on the Windshield

    When I was a child, I was fascinated by the raindrops that shimmied along the windshield of our car. Some would slide up the glass. Some would run down. And some just seemed to wiggle in place, until the car’s speed changed. As common as this sight is, the physics of these droplets is quite complicated and not completely understood.

    Each droplet has a host of forces on it: gravity flattening it or pulling it down an incline; a drag force from the wind flowing over it; and friction between the drop and the surface trying to pin it in place. Recently, scientists have developed a new mathematical model that captures some of the behaviors behind these drops. The work describes the wind speed necessary to move a drop of a given size sitting on a flat surface. The authors also explored how that critical wind speed changes when a drop sits on a tilted surface aligned or against the wind. (Image credit: P. Gupta; research credit: A. Hooshanginejad and S. Lee; via Science News; submitted by Kam-Yung Soh)

  • What Drives Droplets

    What Drives Droplets

    There’s been a lot of interest recently in what goes on inside droplets made up of more than one fluid as they evaporate. This can be entertaining with liquids like whiskey or ouzo, but it has practical applications in ink-jet printing and manufacturing as well. And a new experiment suggests that we’ve been fundamentally wrong about what drives the flow inside these drops.

    As these drops evaporate, a donut-shaped recirculating vortex forms inside them, as seem in the cutaway views above. Conventional wisdom says that vortex is driven by surface tension. Evaporation of components like alcohol is more efficient at the edges of the drop, and as the alcohol evaporates, it creates a higher surface tension at the drop’s edge than at its peak. Marangoni forces then pull fluid down toward the edges, creating the vortex. That explanation is  consistent with observations of a sessile drop sitting on top of a surface (left side of images).

    But those observations are also consistent with another explanation: evaporating ethanol makes the local density higher, so alcohol-rich parts of the drop rise toward the peak while alcohol-poor regions sink. This difference in density would also create a flow pattern consistent with observations. So which is the real driver, surface tension or gravity?

    To find out, researchers flipped the drop upside-down (right side of images). When hanging, the preferred flow direction due to surface tension doesn’t change; flow should still go from the deepest point on the drop toward the edge. But gravity is swapped; alcohol-rich areas should be found near the edge and attachment points of the drop because buoyancy drives them there. And that is exactly what’s observed. The flow direction inside the hanging droplet is consistent with the direction prescribed by buoyancy-driven flow, thereby upending conventional wisdom. It turns out that gravity, not surface tension, is the major driver of internal flow in these multi-component droplets! (Image and research credit: A. Edwards et al.; via APS Physics; submitted by Kam-Yung Soh)

  • Using Sound to Print

    Using Sound to Print

    Inkjet printing and other methods for directing and depositing tiny droplets rely on the force of gravity to overcome the internal forces that hold a liquid together. But that requires using a liquid with finely tuned surface tension and viscosity properties. If your fluid is too viscous, gravity simply cannot provide consistent, small droplets. So researchers are turning instead to sound waves. 

    Using an acoustic resonator, scientists are able to generate forces up to 100 times stronger than gravity, allowing them to precisely and repeatably form and deposit micro- and nano-sized droplets of a variety of liquids. In the images above, they’re printing tiny drops of honey, some of which they’ve placed on an Oreo cookie for scale. The researchers hope the technique will be especially useful in pharmaceutical manufacturing, where it could precisely dispense even highly viscous and non-Newtonian fluids. (Image and research credit: D. Foresti et al.; via Smithsonian Mag; submitted by Kam-Yung Soh)

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    Flames in Freefall

    Gravity is such an omnipresent force in our lives that we frequently forget how strongly it affects our daily experiences and how differently nature behaves without it. A wonderful example of this is the simple flame of a candle. On Earth, a candle flame is tear-drop-shaped and elongated, burning hotter near the bottom and glowing yellow from soot at the top. But, as Dianna demonstrates with her free-fall experiment, this shape is due entirely to the effects of gravity. Buoyant forces make the hot air near the candle rise, pulling in cooler air and fresh oxygen at the base while stretching out the flame. In microgravity – or free-fall – flames are instead spherical, their shape driven by molecular and chemical diffusion. Check out the full video to see more effects of acceleration on flames. (Video credit: Physics Girl)

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    Coarsening in a Soap Film

    Flow in a soap film is driven by gravity’s efforts to thin the film and surface tension’s attempts to stabilize variations in thickness. Because evaporation guarantees that the soap film will eventually dry out, gravity typically wins the battle and causes a soap film to rupture. This video takes a close look at what happens in the film just before it ruptures. Black dots form in the thinnest region of the flow. These areas are not holes, but they appear black because they are thinner than any wavelength of visible light. Before rupture, the black dots begin coalescing with one another, first due to diffusion and later more rapidly due to convection in the soap film. Ultimately, the black dots are the harbingers of doom for the fragile bubble. (Video credit: L. Shen et al.)

  • Fire in Microgravity

    Fire in Microgravity

    In the movie “Gravity” Sandra Bullock’s character battles a fire aboard the International Space Station. Combustion is a huge concern in space habitats. Microgravity fires are challenging to detect and fight because they behave very differently in the absence of buoyancy. On Earth, buoyancy makes hot air rise from a flame while cooler air is pulled in near the base. This feeds fresh oxygen to the teardrop-shaped flame. In space, there is no buoyancy and flames are spherical. They also burn at lower temperatures and lower oxygen concentrations–so low, in fact, that the oxygen depletion necessary to extinguish a fire is lower than what humans require to survive.

    No buoyancy makes it harder for fires to spread, but it also makes them harder to detect since smoke doesn’t rise toward a detector on the ceiling. Instead, fire detectors aboard the Space Station are housed in the ventilation system that moves air through the modules constantly. In the event of a fire, astronauts use a three-step fire suppression system. First, they shut off the ventilation system to delay the fire’s spread. Then they shut off power to the affected unit, and, finally, they use fire extinguishers on the flames. The Russian module is equipped with a foam extinguisher and the others use CO2 units. (Image credit: Warner Brothers)

  • Hanging Liquids

    Hanging Liquids

    A horizontal filament of viscous liquid hanging between two plates stretches under gravity. The photo above is a composite showing the stretching of a single thread over several time steps. The fluid forms a catenary, the same shape as a hanging chain or cable when supported only at its ends. This behavior is confined to viscous filaments of sufficient length and diameter. Short and thin filaments instead form a U-shape with a thin horizontal filament joined to two thicker vertical threads. This difference in shape occurs due to the drainage of the liquid along the filament’s length. If the viscous thread begins to fall before surface tension drains the fluid from the center toward the ends, then a catenary of essentially uniform diameter forms. If instead the liquid drains before falling, the non-uniform U-shape is observed. (Photo credit: M. Le Merrer et al.)

  • How Cats Drink

    How Cats Drink

    While humans use suction and dogs scoop water using their tongues*, cats use a dainty fluid mechanism to drink. Researchers used high-speed video to find that cats drink by touching the surface of their tongue to the water and drawing their tongue rapidly back into their mouth. Friction between their tongue and the water creates a fluid column about which the cat closes its jaw before gravity breaks off the column. They also built an artificial tongue to test different frequencies and found an optimal lapping frequency dependent upon the mass of the feline.

    *ETA: More recent research show that dogs actually use the same technique as cats, not a scooping method.

    (Image credit: P. Reis et al.)