Extension of acoustic levitation to include the study of micron-size particles in a more compressible host liquid

Abstract

The development of a new technique for measuring the mechanical properties, namely density, compressibility and sound velocity, of biological materials is discussed. The technique makes use of acoustic levitation, a phenomenon resulting from the balance of the gravitational force and a time averaged radiation pressure force acting on an object subjected to a sufficiently energetic sound field. The levitation technique is especially suited for studying biological materials, because it requires only very small samples (< 1 .mu. 1) and it employs a host liquid which acts as a container whose properties can be controlled to accomodate the material under consideration. Previous applications of acoustic levitation have been concerned with single millimeter diameter size droplets in a less compressible host liquid. Through this study, application of this technique has been extended to include levitation of samples consisting of a large number of micron-size particles in a more compressible host liquid. Application of the technique to this new class of materials required the development of a new apparatus, as well as greater attention to phenomena previously neglected in the theory describing the forces acting on a compressible sphere in a standing wave field. The technique was used to measure the compressibility of human red blood cells in several different host environments. Results from the normal red blood cell compressibility measurements were in good agreement with those obtained using a more traditional technique.