The study of impact, spreading, and recoil of droplets of aqueous solutions of polymeric and surfactant additives on horizontal hydrophobic (Teflon) and hydrophilic (glass) surfaces is presented in this thesis. The non-Newtonian aqueous polymer solutions are prepared by mixing the water-soluble hydroxyethyl cellulose (HEC) with varying degree of polymerization (QPC 300, 250 HR, and 250 HHR) at different concentrations. The solution rheological and interfacial properties are characterized to understand the role of wettability, surface tension, and viscosity on the droplet surface interactions. For each polymer solution, the surface tension is measured by the maximum bubble pressure method and the static contact angle is measured using a contact angle/wettability analyzer. Intrinsic viscosities of the three polymers are determined from the viscosity measurements of their dilute solutions carried out using a capillary tube viscometer.
A high speed digital camera is used to capture the droplet impact behavior at 4000 frames per second. The captured images of the droplet are analyzed using an image-processing software and the temporal variations of the spreading factor and the flattening factor of the droplet are determined. Results show that the higher viscosity coupled with lower surface tension of the polymer solutions leads to larger spread compared to a water droplet and inhibits strong recoil on a hydrophobic surface.
Computational simulations of the fluid flow and heat transfer during spreading, recoil, rebound/break up of hot droplets of water and aqueous solutions of two surfactants (SDS and Triton-X 100) on a Teflon surface have been carried out at We ∼ 28. The continuity, momentum conservation, and the energy conservation equations are solved simultaneously using a finite volume method to determine the drop shape evolution and the drop-substrate heat transfer. The Volume-of-fluid or VOF method is used to track the liquid-gas interface deformations during spreading and recoil. By comparing the water and surfactant solution droplet impact dynamics at the same Weber number, it is evident that the surfactant solution drops produce larger initial spread and weaker recoil due to the reduction in surface tension at the liquid-air interface and the change in the wettability of the liquid-solid interface. The increased contact area leads to higher rate of heat transfer for a surfactant solution droplet compared to that for a water drop. The Triton X-100 with its lower mobility is less effective in increasing the drop-substrate heat transfer compared to SDS which has a higher mobility.