Particle dispersion in an ultrasonic standing wave field via node-skipping

Measurement of particles typically requires some method for dispersing particles in space according to their diameter. The acoustic radiation force scales with particle diameter, however at the nanometer scale, dispersing particles using this force alone leads to small spatial separations. By combining a lateral convective flow with an ultrasonic standing wave field, significant particle dispersion is obtained. The force from a standing wave field is bipolar, varying sinusoidally in space, while the drag force from convection is unipolar. The net result is particle motion that skips across nodes in the standing wave field, resulting in dispersions under typical conditions on the order of centimeters for nanoparticles. It is shown that using either convection, or the acoustic radiation force alone results in an order of magnitude lower dispersion of particles. Methods for employing this result in particle sensing applications is discussed.