Heated droplet impact onto a heated substrate: Effect of temperature-dependent viscosity on the lamella thickness

As a droplet impacts onto a substrate, it transits from a kinematic to an inertia-dominated phase, followed by a viscosity-dominated phase toward the end of its spreading. A thin lamella is formed in the viscosity-dominated phase, and its thickness mainly depends on the liquid viscosity. Recent studies have proposed the measurement of viscosity from this lamella thickness. For the case of heated substrates, the lamella formation is more complex as the heating of the lamella is restricted to the thermal boundary layer. The heat transfer from the substrate inhibits the evaluation of the effect of temperature-dependent viscosity on the lamella parameters as the lamella is not at uniform temperature. A possible way to overcome this limitation is to preheat the droplet to the substrate temperature and thus to avoid heat interaction between the two. With this view, in the present work, an experimental study is conducted to evaluate the effect of temperature on lamella thickness through the impact of heated water droplet onto a heated substrate. The substrate and droplet temperatures were in the range of 30–60 °C, and the Weber numbers varied between 232 and 561. The evolution of lamella thickness has been measured using a chromatic confocal sensor. The results show that the effect of temperature-dependent viscosity on the lamella thickness can be captured using this experiment of heated droplet impact onto heated surfaces. The results also show promise for finding temperature-dependent viscosity from the measured lamella thickness.