Computational Study of Surfactant-Induced Modification of Droplet Impact Dynamics and Heat Transfer on Hydrophobic and Hydrophilic Surfaces

Download or read book Computational Study of Surfactant-Induced Modification of Droplet Impact Dynamics and Heat Transfer on Hydrophobic and Hydrophilic Surfaces written by and published by . This book was released on 2008 with total page pages. Available in PDF, EPUB and Kindle. Book excerpt: Computational simulations of the spreading, recoil, and rebound/break up of liquid droplet on a horizontal surface were carried out with a finite volume method on a structured grid. A volume-of-fluid (VOF) technique was used to track the deforming liquid-air interface. Simulations were carried out for water and aqueous surfactant solution drops impacting on hydrophilic (glass) and hydrophobic (Teflon) surfaces in the range of Weber numbers between 20 ~ 80. The computational predictions were compared with high-speed visualization of isothermal droplet impact reported by Gatne (2006) to validate the numerical model. The computational scheme based on the VOF method was able to capture the dynamics of this droplet-surface interaction phenomenon observed in the experiments. The numerical treatment included the dynamic surface tension variation for surfactant solutions and different values for the advancing and receding contact angles, which resulted in accurate prediction of the drop impact-spreading-recoil behavior. The computational simulations show that the water droplets spread and then recoil sharply so as to form a vertical column, which breaks up and ejects secondary droplets on a teflon surface but on a glass surface they spread, recoil to a lesser extent and oscillate to rest without rebound. The decrease in surface tension at the liquid-air interface and change in the wetting characteristics of the liquid-solid interface facilitates larger initial spreading and weaker recoil of surfactant solution droplets compared to water drops. The solution of lower molecular weight (higher mobility) surfactant (SDS) showed a higher maximum and final spread with weaker recoil compared to the higher molecular weight (lower mobility) Triton X-100 surfactant. The heat transfer phenomena in droplet impact were studied for both cooling and heating of water and aqueous surfactant solution drops. The results indicated that the wettability of the substrate has the biggest impact on heat transfer. SDS solution droplets resulted in higher heat transfer compared to water droplets due to better wettability and less recoil exhibited by these droplets. Surfactant solutions containing higher concentrations showed higher heat transfer rates as they have lesser recoil which results in more contact surface area with the surface. This study shows that addition of surfactants to water profoundly changes the droplet impact dynamics and concomitant heat transfer and this provides a method to passively modify and control the droplet impact-spreading-recoil processes.