Spreading and receding characteristics of a non-Newtonian droplet impinging on a heated surface

The present study aims to investigate the influence of the Weber number and surface temperature on the spreading and receding characteristics of Newtonian (DI-water) and non-Newtonian (xanthan gum solution) droplets impinging on heated surfaces. The surface temperature was in the range from 25 degrees C to 85 degrees C, which is below the Leidenfrost temperature (similar to 300 degrees C). Using high-speed camera images, this study measured the dynamic contact angle as well as spreading and receding diameters. It also used a modified model to predict the maximum spreading diameter by using the effective viscosity. From the results, the modified model using the effective viscosity was in good agreement with the experimental data in predicting the maximum spreading diameter. In addition, the maximum spreading diameter for a DI-water droplet was larger than that of a non-Newtonian droplet because of the difference in liquid viscosity. In particular, for the Newtonian and non-Newtonian droplets, the dynamic contact angle was almost similar in the spreading regime, but in the receding regime, it substantially changes with temperature owing to the variation of viscosity with temperature. Moreover, the spreading diameter rapidly decreased with the increase in surface temperature in the receding regime in which the change in viscous dissipation energy would be important for the receding motion. Finally, the retraction rates of the Newtonian droplet remained constant with temperature, whereas those of the non-Newtonian droplet increased with temperature, thereby supporting the assertion that the viscosity effect is dominant in the receding characteristics after impact. (C) 2014 Elsevier Inc. All rights reserved.