Particle behavior in a rotating ultrasonic waveguide

Abstract

An apparatus was designed to allow a suspension of biological cells to be subjected to a well-defined, 160-kHz standing ultrasonic field while being viewed through a stereo microscope. The chamber, which has a square cross section and pressure release walls, acts as a single mode acoustic waveguide. Aqueous metrizamide solution is used to fill the ultrasonic chamber because it has a unique combination of properties including low viscosity, low osmolarity, and high density. The chamber rotates about its axis (whose inclination can be varied) producing the centripetal force necessary to contain the buoyant cells in the axial region. Observations were made on stroboscopically illuminated suspensions both of latex microspheres and of red blood cells. The particles arranged themselves at half-wavelength intervals into axially symmetric formations that became complex and flowering when many particles were present. Some aspects of this behavior are explained by preliminary theory that considers gravitational, rotational, and acoustic radiation forces on the particles. [Work supported by NIH grants GM0729409 (University of Virginia) and GM08209 (University of Vermont).]