Impact Dynamics of Complex Fluid Drops on Heated Flat Surfaces
Creators: Ma, Xiaotian
Publisher: name: University of Alberta Library
Year: 2024

<p>Complex fluid droplet impact on heated surfaces has gradually attracted the attention of worldwide researchers due to its significant implications for various industrial applications,<br>
including spray cooling, spray coating, and inkjet printing. This thesis work experimentally examines the impact dynamics of two types of complex fluid droplets on heated flat surfaces: SiO2/ethylene glycol nanofluid and Carbopol/water solution.</p>

<p>Our first study involves the impact dynamics of nanofluid droplets onto flat heated surfaces, using ethylene glycol droplets laden with both hydrophilic and hydrophobic SiO2<br>
nanoparticles. In particular, we obtain a phase diagram and droplets’ spreading profile to categorize and analyze the droplet impact outcomes. Our results show that nanoparticle addition promotes droplets’ atomization and suppresses droplet spreading by increasing liquid viscosity. Moreover, the spreading suppression is only observed above a certain nanoparticle concentration threshold (11.9 wt% for hydrophilic nanofluid; 4.30 wt% for hydrophobic nanofluid). Furthermore, for the first time, we analyzed the nanoparticles’ wettability effects on droplet impact dynamics. Our results imply that the hydrophobic nanofluid tends to have smaller βmax than hydrophilic nanofluid regardless of surface temperature. In the Leidenfrost regime, characterized by the formation of an insulating vapor layer underneath the droplet, the Leidenfrost temperature increases with a higher nanofluid concentration. However, we observed that this Leidenfrost temperature increase is more significant for the nanofluid laden with hydrophobic nanoparticles.</p>

<p>To further understand the complex fluid droplet impact mechanisms, we performed the droplet impact on the heated surface experiment of another complex fluid: aqueous Carbopol, a yield-stress polymer solution. By comparing the phase diagram of water and the aqueous Carbopol solution, we observed that the Carbopol addition suppresses droplets’<br>
atomization and partial rebound, which might be due to a higher energy dissipation. A modified βmax scaling law is applied to describe the maximum spreading profile of the Carbopol droplets, which suggests that the Carbopol droplet spreading is viscosity dominant. Due to the higher viscosity, the more concentrated Carbopol droplet shows a smaller maximum spreading diameter. Interferometry analysis is used to study the Leidenfrost state of Carbopol droplets. The results indicate that the Carbopol droplet generates a thicker<br>
vapor film than the similar-sized water droplet, promoting droplet rebound and reducing the Leidenfrost temperature. Specifically, as the Carbopol concentration increases from 0.05 wt% to 0.25 wt%, the droplets’ Leidenfrost temperature reduces from 400 ○C to 150 ○C.</p>

<p>In summary, our experimental results contribute to a better understanding of the complex fluids droplets impact outcomes on heated surfaces. More specifically, for the first time, we show the profound effect of nanoparticles’ wettability and a large amount of nanoparticle addition on the droplet impact dynamics. We also capture the impact dynamics of Carbopol<br>
droplets from the non-boiling regime to the Leidenfrost regime and interferometry measurements for the first time. Moreover, several distinct droplet behaviour and impact dynamics compared to their base fluids for various non-Newtonian fluids (e.g., the decrease/increase in Leidenfrost temperature, atomization promotion/suppression) reveal the droplet impact<br>
dynamics are significantly influenced by complex additives. Our results highlight the complexity of the complex fluid droplet impact dynamics on heated surfaces and advocate for<br>
further investigation in this intriguing area of research.</p>