CrazyNeutrino said:
Microscopic Sonic Screwdrivers Act Like ‘Tornadoes Of Sound’
Scientists created a “microscopic sonic screwdriver” composed of acoustic vortices that grip and spin microparticles suspended in water.
The vortices act like “tornadoes of sound,” and manipulate microscopic particles by drawing them to the core of the vortex, the University of Bristol reported.
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I’ve done some work on this for CSIRO, not microscopic ones but fluid mechanics ones on the scale of cm.
People will think that particles migrate to the centre because the pressure there is a minimum, but that’s not true. There are three mechanisms involved. One is from friction on the surface-fluid interface, the surface friction causes and inward motion of fluid at the interface which drags particles inward. The second is that particles flee from regions of higher turbulence towards regions of lower turbulence, and the turbulence is least when the velocity is least – in the centre. The third is the Saffman lift force, which is like the Magnus lift force (well known for balls curving in flight in sports) except that the spin on the particle that creates the Saffman lift force is created by fluid shear.