5895wileyonlinelibrary.com substrate, owing to the sound-velocity mismatch between substrate and fl uid, SAWs will radiate acoustic energy into the fl uid. This will induce a pressure wave and drive a steady-state fl ow known as acoustic streaming, the basis for the gamut of possible SAW microfl uidic operations. At the same time, however, the viscous dissipation of the acoustic energy into the fl uid can generate a heating effect. [ 31 ] This heating effect is a much-discussed issue in SAW-driven microfl uidics and questions surrounding the problem of fl uid heating are continually arising. How much will the dissipation of the acoustic wave into a droplet raise its temperature? Will any temperature rise be enough to negatively affect biological samples or even evaporation rates? This is especially of concern in the case of free-droplets where a systematic study of fl uid-heating effects would be desirable over the typical SAW-microfl uidic operational frequency and power ranges. At the same time, it is interesting to understand to which extent any microfl uidic heating effects can actually be positively exploited to enhance biological or chemical reactions in lab-on-a-chip devices when desired.To date, only few studies investigated SAW thermal effect in microfl uidics. The fi rst reported characterization of SAWdriven liquid heating effects was carried out by Kondoh et al. in 2005 and further expanded upon in 2009. [ 31-33 ] In these experiments, the authors demonstrated that the primary source of fl uid heating is, in fact, the radiated acoustic wave, with temperature changes being strongly dependent on input power and fl uid viscosity. It was not clear, however, if droplet heating was isolated from heating effects occurring at the interdigital transducer (IDT). The latter can signifi cantly heat up the substrate-and through it the droplet on its surface-if not properly controlled. In 2006, Beyssen et al. reported similar results using comparable experimental conditions, further quantifying the effect of fl uid viscosity on temperature changes and uniformity. [ 34 ] In 2009, Kulkarni et al. showed the possibility to use this heating mechanism as an energy source for synthetic chemistry in digital microfl uidic systems. [ 35 ] More recently, Roux-Marchand et al. further investigated the temperature uniformity within viscous glycerol droplets and found decreased uniformity at high SAW powers but did not discuss absolute temperature changes. [ 36 ] As a recent application of acoustic microdroplet heating, Reboud et al. demonstrated polymerase chain reactions (PCRs) in an oil-covered droplet. [ 37 ] Fast and controllable surface acoustic wave (SAW) driven digital microfl uidic temperature changes are demonstrated. Within typical operating conditions, the direct acoustic heating effect is shown to lead to a maximum temperature increase of about 10 °C in microliter water droplets. The importance of decoupling droplets from other on-chip heating sources is demonstrated. Acousticheating-driven temperature changes reac...