Temperature field regulation of a droplet using an acoustothermal heater

Heating a droplet without contamination is desired for the emerging applications of microfluidic devices in life science and materials science, especially in the form of controllable temperature distribution. Microfluidic heaters using surface acoustic waves have been recently demonstrated, which highlights an urgent need for an insight into the detailed heating mechanism to guide the development of temperature regulation methodologies. Here, we show that the temperature field of a droplet on the path of a travelling wave can be regulated by modulating the heat source distribution and thermal conduction inside the target. We model the acoustothermal process of the droplet including the effects of electric dissipation, acoustic dissipation, and acoustic-induced steady flow. The electric-mechanical-acoustic coupling contributes to the dominant heat source, and we call it acoustic heat source. The nonlinear effects of incident waves generate acoustic vortexes with a velocity of up to 20 mm s^-1, inducing forced convection inside the droplet to enhance heat transfer. The equilibrium temperature field of a droplet is determined by a synergy of dissipative acoustic attenuation and acoustic streaming. We demonstrate that the distribution of the acoustic heat source and the patterns of acoustic streaming can be modulated by fluid viscosity and droplet size. Various spatial combinations of the acoustic heat source and steady streaming make different temperature fields in the droplet. We also propose a phase diagram of the temperature distribution in the droplet. This methodology enables opportunities for temperature-related processing inside a droplet bioparticle carrier or microreactor.