Real-Time Acoustic Spectroscopy of Nanoparticle Sedimentation in Microfluidic Systems Using Phononic Crystal-Based Ultrasonic Interferometry

Real-time quantification of nanoparticle sedimentation is critical for predicting colloidal stability, yet most optical or centrifugation-based techniques require dilution, labeling, or long acquisition times. We report a defect-engineered phononic-crystal acoustic spectrometer, called a defect-based acoustic spectrometer (DAS), that tracks the sedimentation dynamics of copper nanoparticles (Cu-NPs) <i>in situ</i> inside a microfluidic channel. A Mach-Zehnder-type interferometer embedded in a two-dimensional solid-liquid phononic crystal produces a sharp absorption resonance at ≈416 kHz for pure acetone, whose frequency is governed by the local mass density of the nanofluid in the microchannel. Upon dispersion in acetone, 25 nm Cu-NPs generated a reproducible down-shift of 0.96 kHz that relaxed to the pure-solvent value within 52 min, whereas 60-80 nm Cu-NPs exhibited a smaller initial shift (0.58 kHz) and faster stabilization, direct evidence of size-dependent agglomeration and gravitational sedimentation. The method resolves collective nanoparticle dynamics with ultrasound wavelengths 5 orders of magnitude larger than the particle diameter, requires no optical transparency, and operates at realistic volume fractions. Beyond validating classical Stokesian behavior at the microscale, the platform is readily adaptable to temperature-, pH-, or biomolecule-mediated stability studies in complex media. The DAS thus offers a label-free route to interrogate interfacial forces and transport processes central to nanofluid formulation, drug delivery, and environmental nanoscience, which are useful in analytical chemical engineering, as well as bio- and environmental applications.