Experiment on liquid spreading

• Main Author: Haryono.
• Other Authors: Chen Yan
• Format: Final Year Project
• Language: English
• Published: 2011
• Subjects: DRNTU::Engineering::Mechanical engineering::Fluid mechanics
• Online Access: http://hdl.handle.net/10356/46087
• Institution: Nanyang Technological University

The development in the study of liquid spreading process has been remarkable because it is of utmost importance in many practical industrial applications such as ink-jet printing, surface coating, and painting. Besides, there are also many applications in biomedical engineering. In this research project, the objectives is to investigate the spreading behaviours of liquid droplets on solid substrates by varying the surface tension of liquid droplets and tuning the surface features of the solid. The influence of surface roughness and wrinkled wavelength on the spreading behaviour is experimentally studied. Liquid droplets of different surface tension were used in this project. The solid substrates were prepared through buckling or wrinkling process from shape memory polymers (SMPs). They are characterized as smooth substrates, isotropic substrates and anisotropic substrates with different surface roughness and wrinkle wavelength. The spreading process is found to be dependent on both the surface tension of the liquid and the surface properties of the solid. When the surface tension of the liquid is lowered, the liquid droplet will require longer time to reach equilibrium, the contact angle (from when it touches to its equilibrium state) becomes smaller and the contact area gets bigger. On smooth and isotropic substrates, the droplets show shape of part of a sphere and the equilibrium contract angles are similar. The equilibrium contact angles on isotropic substrates are comparable with that on smooth substrates because the roughness factor of the isotropic substrates is still insignificant to cause significant changes in equilibrium contact angles. When the surface roughness increases, the equilibrium contact angle is found to be lower which is expected based on Wenzel law. Droplets on anisotropic substrates are elongated along the wrinkled grooves and some of the droplets are pinned by wrinkled grooves. The contact angle measured from in the direction perpendicular to the wrinkled groove was found to be higher because there is energy barriers that the droplets need to overcome before they can spread further. On anisotropic substrates, the higher the wavelength, the higher the equilibrium contact angle will be because the wrinkled grooves will be larger and the droplet needs more time to cover it. Hence, the spreading is observed to take longer time to reach equilibrium and the contact angle is found to be larger. From numerical simulation, the whole spreading process could be shown and the mobility of the contact line can be determined by comparing the simulation results and experimental results.