Impact Dynamics of Newtonian and Non-Newtonian Fluid Droplets on Super Hydrophobic Substrate

Book excerpt: Conventional rheological methods such as viscometry to characterize linear and nonlinear viscosity behavior requires an excessive amount of sample liquid, which is practically impractical due to the time and cost constraints, for instances, in blood, and other scarce and expensive bio-fluids. There is an urge for a handy tool to quickly evaluate the intrinsic properties of a liquid. In this thesis, droplet of Newtonian (e.g. water) and non-Newtonian liquids (e.g. shear thinning blood, shear thickening starch solution) with a desired dimension is released by a pipette from a vertical distance. Gravitational attraction gives rise to an impact velocity, and the droplet hits a rigid hydrophobic surface with a liquid-substrate contact angle of 150o. An experimental set up equipped with a manual pipette, high speed camera, and substrate holder etc. is constructed to capture the geometrical change over the sequential the impact-spread-recoil-rebound process. Upon impact, the droplet turns into an expanding pancake geometry with coronal spires developing over time. In extreme conditions of high impact velocity, splashing or fragmentation is observed. The video records are analyzed in terms of the classical dimensionless Weber number (We) which comprises impact velocity and surface tension. Other measurements are made: (i) duration of droplet on the substrate, and the change in contact area at droplet-substrate interface, prior to rebounce, (ii) critical droplet dimension and impact velocity leading to fragmentation, (iii) jet formation at rebounce, (iv) maximum number of spires, and (v) wavelength of radial Rayleigh wave. Comparison between Newtonian and non-Newtonian liquids are made, and non-linear behaviors are observed. Weber number is shown to be insufficient in describing spire formations. Non-linear viscosity, playing an indispensable role in droplet geometric deformation, must be incorporated in droplet dynamics.