Substrate-Confined Lamb Waves Induced Thermal Field Modulation in Microscale Droplet

Precise and noncontact temperature control within microfluidic droplets is critical for microreactor applications. However, conventional acoustothermal platforms heavily rely on expensive, patterned piezoelectric substrates. To address this limitation, our study introduces a novel and cost-effective methodology for droplet thermal regulation utilizing laterally excited Lamb waves (driven at 1 MHz) on universal, nonpiezoelectric glass substrates. We demonstrate that droplet temperature elevation fundamentally arises from the conversion of input acoustic power into localized heat, primarily via the viscous dissipation of radiated longitudinal waves dynamically coupled with acoustic streaming-induced convection. Systematic quantitative analysis of glycerol-water droplets (20-50 μL) revealed that a rapid, stable thermal plateau of 31.4 °C is achieved within 100 s at 1.10 W input power. Furthermore, increasing fluid viscosity (from 10% to 100% glycerol) significantly enhances viscous dissipation, leading to a continuous increase in temperature (at 1.0 W) and improving spatial thermal homogeneity. Coupled numerical simulations validated the experimental thermal fields, revealing that an initial peak acoustic streaming velocity of 0.12 m/s effectively redistributes thermal energy, which subsequently suppresses the streaming intensity to 0.08 m/s due to acoustic-to-thermal energy conversion. By decoupling acoustic generation from the microfluidic substrate, this lateral-excitation methodology provides a highly tunable, low-cost platform for precise spatiotemporal thermal management, significantly advancing the scalable deployment of droplets as isolated bioreactors and bioparticle carriers.