Experimental investigation and comparison of Newtonian and non-Newtonian shear-thinning drop formation

Abstract

The presence of small amount of polymer in Newtonian liquids affects the fluid behavior significantly and creates superficial differences between Newtonian and non-Newtonian drop formation. In the present study, Newtonian and non-Newtonian shear-thinning drop formation in air is examined experimentally and the geometrical parameters, such as drop elongation length, minimum neck diameter and current contact angle are studied for water, glycerin solutions (79, 89 and 94% w/w) and Carboxymethyl cellulose (CMC) solutions (0.5 to 1.5% w/w) drops. The results indicate that at initial stages of the drop growth, Newtonian and non-Newtonian liquids behave similarly. However, at the start of the necking process in which the deformation rate increases, clear differences can be seen between Newtonian and shear-thinning fluids. The most important effects of an increase in the glycerin and CMC concentration, which enlarges the viscosity, are an increase in the time span of the necking process, a decrease in the elongation rate, an increase in the length of the liquid filament and an increase in the probability of the satellite drop formation. On the other hand, due to the shear-thinning properties of CMC solutions, their viscosity decreases in the necking region and as a result, the length of the Newtonian drop at the moment of separation is longer than that of the shear-thinning drop with equal viscosity. The variation of minimum neck thickness with the time remaining until the detachment for the investigated liquids is presented by hmin∝(td-t)α, in which α is 2/3 for water, 1 for viscous Newtonian fluids, and for non-Newtonian shear-thinning fluids it is equal to the index of fluid in the power-law relation.