Particle size effects on stable levitation positions in acoustic standing waves

Schlieren images can show a two-dimensional representation of pressure distributions. Using this method, we have demonstrated that there is a particle size effect for levitating solid elastic Styrofoam spheres in an ultrasonic acoustic standing wave: (1) spheres of density 13.3 kg/m(3) and diameters less than 0.58 ? levitate at pressure nodes, and (2) spheres larger than 0.66 ? levitate with their centers at pressure antinodes. Pressure measurements of ultrasonic standing waves made in conjunction with their schlieren images to identify pressure nodes and antinodes are presented. These observations contribute to prior experimental and theoretical research concerning the influence of particle size on levitation.