Experimental investigation of spreading dynamics of glycerol droplets on a heated surface

The dynamics of droplets impacting on solid surfaces are widely encountered in industrial applications. This paper presents an experimental study of the spreading dynamics of droplets of liquids with a temperature-dependent viscosity on a heated horizontal sapphire surface. This surface can be heated between 40 and 90°C and allows for usual observation from different directions. The used liquids are glycerol water solutions with a volume percentage of 80%, 85.2%, and 92.8% glycerol, which feature a large dependency of the viscosity on the temperature. Using high-speed imaging and infrared thermography, the shape of the droplet and the temperature of the droplet surface in contact with the surface are measured, respectively. Our experiments reveal that the spreading of the droplets increases with an increase in the surface temperature, which is also expected as the effective viscosity of the liquid will decrease with an increase in the droplet temperature. However, the heating of the droplets ensures that these effects only are apparent after sufficient time to heat the droplet. In addition, the amplitude of the oscillations in the spreading of the surface will increase when the temperature of the surface is increased, which is also related to the decreased viscosity. Finally, the effects on the local spreading cannot be directly correlated to the temperature of the droplet in contact with the surface, which indicates that the viscosity in the droplet is not homogeneous during the experiment and local gradients in the viscosity are important in the overall spreading behaviour of the droplet.