FYFD

Tag: floating

  • Floating in Levitating Liquids

    Floating in Levitating Liquids

    When it comes to stability, nature can be amazingly counter-intuitive, as in this case of flotation on the underside of a levitating liquid. First things first: how is this liquid layer levitating? To answer that, consider a simpler system: a pendulum. There are two equilibrium positions for a pendulum: hanging straight down or pointing straight up. We don’t typically observe the latter position because it’s unstable; the slightest disturbance from that perfectly vertical situation will make it fall. But it’s possible to stabilize an inverted pendulum simply by shaking it up and down. The vibration creates a dynamic stability.

    The same physics, it turns out, holds for a layer of viscous fluid. With the right vibration, the denser fluid can levitate stably over a layer of air. Inside this vibrating layer, the rules of buoyancy are a little different because the vibration modifies the effects of gravity. As a result, bubbles deep in the liquid layer sink (Image 1). The researchers used this behavior to create their levitating layer (Image 2). The shaking also serves to stabilize objects floating on the underside of the liquid layer, allowing the boat in Image 3 to float upside down! (Image and research credit: B. Apffel et al.; via NYTimes; submitted by multiple sources)

  • Swirling the Wrong Way

    Swirling the Wrong Way

    When you swirl wine, you create a rotating wave that travels in the direction that you’re moving the glass. You would expect that anything floating atop that fluid would travel in the same direction of rotation. But it turns out, for a large, thin raft floating atop the rotating fluid, that’s not the case.

    Above you can see a swirling container, rotating counter-clockwise, with a raft of foam. This is from a timelapse where only one photo is taken per rotation, so that it’s easier to see how the foam is rotating relative to the container. And, once enough foam covers the surface, it starts rotating in a clockwise direction – opposite the container! It works for more than foam, too. The researchers show that the same holds for powders or beads. The key to the counter-rotation is that the raft needs to be coherent; it has to be able to transmit friction and internal stress among its constituents. Otherwise, the raft will just drift along with the swirling wave. (Image and research credit: F. Moisy et al., source, arXiv; via Improbable Research; submitted by David H. and Kam-Yung Soh)