FYFD

Tag: slip

  • Rolling Down Soft Surfaces

    Rolling Down Soft Surfaces

    Place a rigid ball on a hard vertical surface, and it will free fall. Stick a liquid drop there, and it will slide down. But researchers discovered that with a soft sphere and a soft surface, it’s possible to roll down a vertical wall. The effect requires just the right level of squishiness for both the wall and sphere, but when conditions are right, the 1-millimeter radius sphere rolls (with a little slipping) down the wall.

    Rolling requires torque, something that’s usually lacking on a vertical surface. But the team found that their soft spheres got the torque needed to roll from their asymmetric contact with the surface. More of the sphere contacted above its centerline than below it. The researchers compared the way the sphere contacted the surface to a crack opening (at the back of the sphere) and a crack closing (at the front of the sphere). That asymmetry creates just enough torque to roll the sphere slowly. The team hopes their discovery opens up new possibilities for soft robots to climb and descend vertical surfaces. (Image and research credit: S. Mitra et al.; via Gizmodo)

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  • Thermal Slipping

    Thermal Slipping

    A particle suspended in a liquid typically jitters haphazardly about as it’s struck randomly by nearby liquid molecules. But when a temperature gradient is applied to the liquid, that random motion instead becomes directional. In a recent study, researchers directly mapped the motions underlying this thermophoresis.

    In their experiment, the team placed a 7-micron sphere in water laced with 500-nanometer fluorescent tracers. Using a laser, they optically trapped the sphere, pinning it in place. Then, with a second laser, they heated the water on one side of the sphere and observed, under a microscope, what happened. After a few seconds, the tracers began moving toward the hot region, creating a slip flow along the surface of the sphere. Had the sphere been able to move freely, they found, the flow would have been strong enough to move it. (Image and research credit: T. Tsuji et al.; via APS Physics)

  • When Shear Meets Slip

    When Shear Meets Slip

    One of the classic concepts students learn early in their fluids education is the no-slip condition. In essence, this idea says that friction between a solid object — say, a wall — and the fluid immediately next to it is such that no movement is possible where they meet. The fluid cannot “slip” along the surface, hence “no-slip”. It’s a simple concept, but one that can create a lot of complexity in practice.

    Imagine, for example, a fluid sandwiched between two surfaces: one stationary and one moving at a constant speed. This movement creates a shear flow, in which the velocity of the fluid varies from the speed of the moving plate all the way down to zero, the speed of the stationary plate. If we placed a little platelet in the middle of this flow, we’d expect it to rotate because of the faster flow on one side.

    But a new paper finds something rather different, at least when considering an extremely small nanoplatelet. With a tiny enough plate, individual molecules can slip along the surface, and when that happens, instead of rotating, the nanoplatelet aligns itself with the flow. That alignment means the added particle would disturb the flow less, creating a lower viscosity and better flowability. (Image and research credit: C. Kamal et al.; submitted by Simon G.)