Influence of rheology and micropatterns on spreading, retraction and fingering of an impacting drop

Rheology and surface microstructure affect many drop impact processes, including in emerging printing and patterning applications. This study reports on experiments systematically addressing the influence of these parameters on drop impacts. The experiments involved drop impacts of water, glycerol, and shear-thinning carbopol solutions on ten different microstructured surfaces, captured using high-speed photography. The impact Weber number (We) was varied from 70 to 350, and the microstructures consisted of 20 mm wide pillars with circular and square cross sections arranged in square arrays. The data focus on maximum spreading, retraction rates, threshold conditions for asymmetric (non-circular) spreading, and fingers protruding from the spreading rim. The extent of spreading was reduced by the presence of micropillars, and was well-explained using a hybrid scaling model. The drop retraction rate (_e) showed moderate agreement with the inertial regime scaling e_ pWe(-0.50), but did decrease with effective viscosity. Retraction was slower when the contact line was pinned on surfaces that were flat or had relatively tall or closely-spaced pillars, and was disrupted by drop break-up at We \ 250 for low-viscosity fluids. Impact velocities at the onset of asymmetric spreading had weak dependence on viscosity. Fingers were more numerous for greater We, lower effective viscosity, lower pillar height, and for pillars with square cross-sections. Fingers were favoured in directions parallel to the rows of the pillar array, especially near the onset of finger formation. Consistent comparisons between Newtonian and non-Newtonian fluids were enabled by calculating an effective Reynolds number.

Automated summary

This study investigates the effects of rheology and surface microstructure on drop impact processes. Experiments were conducted using water, glycerol, and shear-thinning carbopol solutions on microstructured surfaces. The findings show that the presence of micropillars reduces the extent of spreading, and the retraction rate decreases with effective viscosity. The onset of asymmetric spreading is weakly dependent on viscosity, and the number of fingers protruding from the spreading rim is influenced by several factors including the Weber number, effective viscosity, pillar height, and cross-section shape.