Measurement of Acoustic Radiation Force Acting on Small Particles in a Progressive Wave

Abstract

Nowadays according to developments of micromachine, biotechnology and micro-fabrication technology, manipulation of small objects has become more and more recognized as an important technology. Manipulation technology using an acoustic radiation force by ultrasound has a number of advantages such that wide handling range is available, and has been studied by many researchers. However, for small objects with the size of μm order which are considered to be remarkably influenced by viscosity of the surrounding fluid and are the target of the recent micromanipulation, little studies have been carried out to date. There still remain several questions how viscosity of the surrounding fluid, elasticity of the object influence the acoustic radiation force.

In this study, measurements are made for acoustic radiation force acting on small particles in a progressive wave with a frequency of 400 kHz which is not plane. In order to see the influence of elasticity of the particles, three kinds of material with different properties are used and the experiment is made in the condition of ka « 1, where k is the wave number of sound waves and a is the radius of a particle on which the radiation force is exerted. It is well known that acoustic field interacts not only with objects as the acoustic radiation force but with the surrounding fluid through which the sound is carried to cause acoustic streaming. A small particle in the acoustic field is affected by the radiation force and the streaming. Acoustic streaming is numerically calculated and its effect is attempted to be eliminate in the experiment.

From experimental results, it is found that the acoustic streaming couldn't be neglect to estimate the acoustic radiation force and very larger forces were exerted on small particles than some theories of the radiation force.