Acoustic radiation force


Acoustic radiation force is a physical phenomenon resulting from the interaction of an acoustic wave with an obstacle placed along its path. Generally, the force exerted on the obstacle is evaluated by integrating the acoustic radiation pressure over its time-varying surface.
The magnitude of the force exerted by an acoustic plane wave at any given location can be calculated as:
where
The effect of frequency on acoustic radiation force is taken into account via intensity and absorption. As a reference, water has an acoustic absorption of 0.002 dB/. Acoustic radiation forces on compressible particles such as bubbles are also known as Bjerknes forces, and are generated through a different mechanism, which does not require sound absorption or reflection. Acoustic radiation forces can also be controlled through sub-wavelength patterning of the surface of the object.
When a particle is exposed to an acoustic standing wave it will experience a time-averaged force known as the primary acoustic radiation force . In a rectangular microfluidic channel with coplanar walls which acts as a resonance chamber, the incoming acoustic wave can be approximated as a resonant, standing pressure wave of the form:
.
where is the wave number.
For a compressible, spherical and micrometre-sized particle suspended in an inviscid fluid in a rectangular micro-channel with a 1D planar standing ultrasonic wave of wavelength, the expression for the primary radiation force (at the far-field region where ）becomes then :

where