Description
Microfluidic systems enable precise fluid handling at the microscale and offer huge potential for integrating multiple functions for rapid, on-chip analysis (Petruzzellis et al., 2024). In this context, while numerous studies have successfully focused on device fabrication and particle manipulation within microchannels, the downstream detection often still relies on off-chip tools such as microscopes and particle counters (Shi et al., 2024). Embedding photoacoustic (PA) detection into microfluidic platforms for particle detection and counting could represent an effective strategy to integrate sensing functionalities directly onto the microfluidic chip (Kishor et al., 2017). PA detection is a label-free technique based on the optical absorption of short laser pulses, which induce pressure waves through thermoelastic expansion. Then, it is inherently background-free, as signals originate exclusively from absorbing analytes, and it overcomes the limitations of conventional absorption detection associated with weak absorption, light scattering, and opaque samples. Moreover, PA detection enables multiparameter analysis by probing the optical, mechanical, and thermal properties of the particles in the fluid (Barbosa & Mendes, 2022), providing information on cell size, morphology, and internal structures (Yao & Wang, 2013).
Despite this potential, a critical gap persists at the single-particle scale: the intrinsic thermoacoustic response of micrometer-sized polymer particles—especially polystyrene (PS), widely used as calibration standards and microplastic surrogates—has not been systematically characterized. Most existing studies focus on ensemble behavior, leaving unresolved the fundamental mechanisms governing heat deposition, elastic deformation, and acoustic emission from individual particles, as well as their propagation in the fluid (Schnepf, von Moers-Meßmer, & Brümmer, 2023).
To address this gap, we present a first-principles, time-resolved multiphysics finite-element framework implemented in COMSOL Multiphysics as a digital twin of photoacoustic excitation of a single particle in a microfluidic environment. The model couples transient heat transfer, thermoelastic deformation of a dye-free PS microsphere, and acoustic wave generation and propagation in water under pulsed excitation at the PS intrinsic absorption band (around 3400 nm), where water absorption is limited. By varying the pulse duration from nanoseconds to milliseconds, we identified the transition from stress-confined impulsive excitation with elastic ringing to a quasi-static thermomechanical regime, and quantified how propagation reshapes signal amplitude and spectrum at standoff distances relevant for on-chip detection. For validation of the digital twin, we assembled a basic experimental setup consisting of a microfluidic device and a photoacoustic excitation system based on a Fabry–Perot interband cascade laser to excite PS microspheres in distilled water. This setup was used to experimentally verify the predicted pulse regimes and to analyze the resulting signal spectra.
Looking ahead, this workflow could be readily extended to biological specimens (cells, vesicles, aggregates) by adapting material parameters and geometries without altering the modeling architecture. In this way, it is expected to bridge controlled studies on PS particles with label-free assays on real samples, advancing computational and data-driven design, optimization, and interpretation of next-generation microfluidic platforms integrated with PA detection.