First‐order torques and spin velocities of solid bodies in intense sound fields

Time‐average forces, torques, and velocities in sound fields have always been associated with second‐ or higher‐order effects. We've identified, for the first time so far as we know, phenomena which are first order in the acoustic particle velocity. In a rectangular box of square section the (1,0,0) and (0,1,0) modes are degenerate and when equally excited the particle motion is in general elliptical; it is circular if the phase difference of the modes is 12π and this is the case we'll have in mind. A long, small diameter, solid cylinder with its axis normal to the square cross section and at its center is then surrounded by a fluid whose particles are undergoing rotational motion (neglecting the scattering field of the cylinder). The cylinder experiences a torque due to viscosity and will rotate if free to do so. Moreover if the particle displacement is significantly greater than the viscous penetration depth the torque and velocity of rotation will be first order in the particle velocity. Measurements m...