FYFD

Year: 2014

  • The Inside of an Evaporating Drop

    [original media no longer available]

    Evaporating droplets may not look like much to the naked eye, but they contain complicated flow patterns. The type of pattern observed depends strongly on the contact line, the place where the liquid, solid, and air meet. When the contact line is pinned–kept unchanged–during evaporation, any particulates in the drop get pulled toward the edges as the drop evaporates. This is what leaves the classic coffee ring stain. It is also what is shown in the first clip in the video above. Contrast this with the second clip, in which the contact line is unpinned and varies irregularly as the drop evaporates. In the unpinned drop, particles are drawn inward during evaporation. The flow patterns are very different as well, complicated by swirling that is the result of force imbalances caused by the irregularly receding contact line. (Video credit: H. Kim)

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    What Makes Squids Fast

    Cephalopods like the octopus or squid are some of the fastest marine creatures, able to accelerate to many body lengths per second by jetting water behind them. Part of what makes its high speed achievable, though, is the way the animal changes its shape. In general, drag forces are proportional to the square of velocity, meaning that doubling the velocity increases the drag by a factor of four. The energy necessary to overcome such large drag increases generally prevents marine animals from going very fast (compared to those of us used to moving through air!) But drag is also proportional to frontal area. Like the bio-inspired rocket in the video above, jetting cephalopods begin their acceleration from a bulbous shape and then shrink their exposed area as they accelerate. Not only does this shape change help mitigate increases in drag due to velocity, it prevents flow from separating around the animal, shielding it from more drag. The result is incredible acceleration using only a simple jet for thrust. For example, the octopus-like rocket in the video above reaches velocities of more than ten body lengths per second in less than a second. (Video credit: G. Weymouth et al.)

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    Impacts on Sand

    Granular materials like sand are sometimes very fluid-like in their behaviors. The high-speed video above shows a ball bearing being dropped into packed sand. Many features of the splash are fluid-like; the initial impact creates a spreading crownlike splash, followed by a strong upward jet that eventually collapses back into the medium. At the same time, many of the impact characteristics are decidedly non-fluidic. Sand has no surface tension, so both the crown and the jet readily break up into small particles. The granular jet is very narrow and energetic, reaching heights greater than the impacter’s drop height. Interestingly, the column begins collapsing on its lower end before the jet even reaches its highest peak. This may be due to the lower energy of the sand particles that were ejected later in the crater formation process. (Video credit: J. Verschuur, B. van Capelleveen, R. Lammerink and T. Nguyen)