Acoustophoresis has become a powerful tool to separate
microparticles
and cells, based on their material and biophysical properties, and
is gaining popularity in clinical and biomedical research. One major
application of acoustophoresis is to measure the compressibility of
cells and small organisms, which is related to their contents. The
cell compressibility can be extracted from the acoustic mobility,
which is the main output of acoustic migration experiments, if the
material properties and sizes of reference particles, the size of
the cells, and the surrounding medium are known. Accurate methods
to measure and calibrate the acoustic energy density in acoustophoresis
systems are therefore critical. In this Perspective, polystyrene microparticles
have become the most commonly used reference particles in acoustophoresis,
due to their similar biophysical properties to cells. We utilized
a two-step focusing method to measure the relative acoustic mobility
of polystyrene beads of various sizes and colors and present a quantitative
analysis of the variation in acousto-mechanical properties of polystyrene
microparticles, showing a large spread in their material properties.
A variation of more than 25% between different particle types was
found. Thus, care is required when relying on polystyrene particles
as a reference when characterizing acoustofluidics systems or acousto-mechanical
properties of cells.