  Acoustic microfluidics (or acoustofluidics) provides a non-contact and
label-free means to manipulate and interrogate bioparticles. Owing to their
biocompatibility and precision, acoustofluidic approaches have enabled
innovations in various areas of biomedical research. Future breakthroughs will
rely on translation of these techniques from academic labs to clinical and
industrial settings. Here, accurate characterization and standardization of
device performance is crucial. Versatile, rapid, and widely accessible
performance quantification is needed. We propose a field quantification method
using motile Chlamydomonas reinhardtii algae cells. We previously reported
qualitative mapping of acoustic fields using living microswimmers as active
probes. In the present study, we extend our approach to achieve the challenging
quantitative in situ measurement of the acoustic energy density. C. reinhardtii
cells continuously swim in an imposed force field and dynamically redistribute
as the field changes. This behavior allows accurate and complete, real-time
performance monitoring, which can be easily applied and adopted within the
acoustofluidics and broader microfluidics research communities. Additionally,
the approach relies only on standard bright-field microscopy to assess the
field under numerous conditions within minutes. We benchmark the method against
conventional passive-particle tracking, achieving agreement within 1 % for
field strengths from 0 to 100 J m-3 (0 to ~1 MPa).
