Hydrodynamics of droplet impingement on heated surfaces: Effects of nanofluid and nano-structured surface

This study investigates the hydrodynamic characteristics of droplet impinging on heated surfaces using High Speed High Resolution (HSHR) imaging and evaluates the effect of surface temperature and using water and a nanofluid on a polished and a nano-structured surface. Three types of surfaces are used in the study: polished silicon, nano-structured porous silicon and gold coated polished silicon. Seven different surface temperatures including single phase (non-boiling) and two phase (boiling) conditions are studied. Droplet impact velocity, transient spreading diameter and dynamic contact angle are determined using image processing.;Five stages are observed during impingement: initial impact, boiling (if the surface temperature is high enough), near constant wetting diameter evaporation, fast receding contact line evaporation and final dry-out. Results of water and a water based single-wall-carbon-nano-tube (SWCNT) nanofluid impinging on a polished silicon surface are compared to determine effects of nano-particles on impinging dynamics. The data show that the nanofluid exhibits larger spreading velocities, larger spreading diameters and an increase in early stage dynamic contact angle. The results of water impinging on both polished silicon and nano-structured silicon disks are compared to determine effects of the nano-structured surface on impingement dynamics. It is found that the nano-structured surface enhances the heat transfer for evaporative cooling at lower surface temperatures which is indicated by a shorter evaporation time. Ultimately, using nanofluid and nano-structured surface can reduce the total evaporation time up to 37% and 20%, respectively.;Experimental data are compared with models that predict dynamic contact angle and non-dimensional maximum spreading diameter. Results show that the molecular-kinetic theory's dynamic contact angle model (M-K model) agrees well with current experimental data at low velocities range corresponding to later times during impingement, but over-predict at high velocities range corresponding to early times of initial impact. Predictions of maximum spreading diameter based on surface energy analysis are compared with current experimental data. Results indicate that all the four models over-predict unless empirical coefficients are adjusted to fit the test conditions.