Modeling thermoelastic and photoacoustic generation from polystyrene particles in microfluidic sensing platforms

Abstract On-chip label-free detection of microparticles is a key challenge for developing miniaturized platforms for real-time sensing in liquids. A promising strategy involves exploiting photoacoustic and thermoelastic effects to convert optical absorption into detectable acoustic signals. Understanding these processes at the single-particle level is essential for advancing next-generation diagnostic and analytical platforms. In this work, we present a numerical investigation of the thermal, mechanical, and acoustic dynamics of a dye-free polystyrene microsphere in water under pulsed optical excitation. We developed a fully coupled finite-element model to simulate heat transfer, thermoelastic deformation, and acoustic wave propagation. The effect of optical pulse duration, from nanoseconds to milliseconds, was systematically analysed. This provides a crucial link between the temporal profile of the photonic excitation and the resulting optomechanical response. Simulations reveal a transition from an impulsive thermoelastic regime for nanosecond pulses, characterized by elastic ringing, to a quasi-static regime for longer pulses. While near-field pressure depends on pulse duration, far-field propagation becomes largely independent of excitation conditions due to acoustic attenuation in water. These findings offer key design rules for tailoring laser source specifications to optimize energy conversion efficiency. Ultimately, this work establishes a comprehensive predictive framework for the development of integrated on-chip photonics and photoacoustic sensing architectures.