Experimental investigation of non-Newtonian droplet collisions: the role of extensional viscosity

Abstract

We investigate the collision behaviour of a shear thinning non-Newtonian fluid xanthan, by binary droplet collision experiments. Droplet collisions of non-Newtonian fluids are more complex than their Newtonian counterpart as the viscosity no longer remains constant during the collision process. Despite the complex collision dynamics, we are able to present a complete regime map based on non-dimensional Weber (We) number and impact parameter (B). We compare the collision outcomes of xanthan, glycerol and a milk concentrate at similar impact conditions. These experiments reveal very rich and complex collision morphologies for shear thinning xanthan solution, strikingly different from Newtonian droplet collisions. Unlike glycerol and milk, xanthan collisions show no reflexive separation even at very high We number. Instead of breakup, we observe disc-like shapes with an oscillating behaviour of the colliding droplets. A detailed analysis reveals that this outcome is related to increased viscous energy dissipation and extensional effects.

The Impact of Single- and Multicomponent Liquid Drops on a Heated Wall: Child Droplets

“…In contrast to the drop-drop collisions [13][14][15][16][17][18], here we distinguished two regimes. In the first regime, the first drop spreads and all the successively impinging drops merge into a liquid layer on the surface.…”

“…The second regime involves a fine aerosol generated by the drop impact on the wall. When drops collide with each other, four regimes are identified [13][14][15][16][17][18], namely, coalescence, rebound, separation, and breakup. Rebound means that drops approach each other and then move apart without immediate contact because the kinetic energy is not enough for drops to pass across the gas-vapor layer between them.…”

“…Depending on the drop properties, their size and velocity, impingement angle, and heating temperature, the Weber number ranges providing each of the four regimes significantly differed. Therefore, the following Weber number ranges in [13][14][15][16][17][18] can be considered general for the known experimental results on the collision of two drops: 0.5-5 for rebound, 1-20 for coalescence, 15-50 for separation, and 30-100 for breakup. These values considerably differed from the critical ones in the case of a drop impact on a surface [1-10].…”

A Numerical Investigation on the Collision Behavior of Polymer Droplets

“…The research of Pan et al [17] identified the key mechanisms governing the impact dynamics of surfactant-coated droplets in the air and imply the potential of using a small amount of surfactant to manipulate impact outcomes. Compared to the Newtonian liquids, relatively few studies [24][25][26][27][28][29][30] have been conducted for the non-Newtonian liquids, which are of relevance to, for example, polymers. For the non-Newtonian liquid, the effects of the viscosity on the deformation process and the boundary model of the droplets are investigated.…”

“…They found that, in the collision process, the kinetic energy was dissipated due to viscous flow, and thus the critical Weber number for the reflexive separation increased with the Ohnesorge number. By combining the experimental results [24], Finotello et al [25] further experimentally investigated the collision behaviors of binary shear-thinning droplets and presented a complete regime map for collision boundaries based on the Weber number (We) and impact parameter (B).In the experiments, for the formation of high-quality and stable droplets, the transient visualization of the collision droplets is difficult to realize, so numerical simulation work is needed to compensate for this technical deficiency. Amani et al[19] simulated head-on and off-center binary droplet collisions in all the regimes [18] using a conservative level-set method.…”