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This study seeks to understand the role of photodegradation of aqueous polymers in drop splashing. Polymer drop impact commonly occurs in various industrial applications such as inkjet printing, spray coating, and agrochemical sprays. In agrochemicals, the various constituent components (e.g., adjuvants) imparts multiple changes to the fluid dynamics and the wetting behaviors of the drops, as well as interacting with the environmental conditions. The environmental conditions (e.g., thermal-degradation, photo-degradation, and oxidation) of the chemicals affect the shelf stability of the intended physicochemical properties of the chemicals, which are added to stabilize drift from the spray nozzles and minimize drop bouncing from leaves. The aging effects of the adjuvants in tandem with the already low pesticide delivery efficiency has an unknown effect on the agrochemical delivery efficiency and the related environmental burden from the increased run off. Herein, we systematically photo-degraded polyethylene oxide (PEO) to probe the drop splashing behavior as a result of the simulated aging conditions. Dye was added to accelerate the degradation of the polymers and caused drops to splash at Weber numbers where the pure PEO case did not, confirming the need to consider environmental factors which contributes to adjuvant aging in agrochemical applications. We have also conducted experiments with various concentrations of PEO to probe the changes in the splash dynamics as well as including surfactants, which played a marginal role in altering the splash dynamics under our parameter space. The significance of the study is that the degradation of the polymers influences the splashing and increases the amount of splashed droplets, indicating the importance of controlling the environmental conditions under which polymer solutions are stored. 

Understanding the peripheral capillary wave propagation during droplet impact is crucial for comprehending the physics of wetting onset and droplet fragmentation. Although Newtonian droplets have been extensively studied, we show how capillary waves deform non-Newtonian droplets in such a way that rheological features, such as the critical concentrations for the overlap (c*) and entangled polymer molecules (c**), may be directly obtained from the deformation history. Determining these critical concentrations is essential as they mark transitions in the rheological behavior of aqueous polymeric solutions, influencing viscosity, elasticity, and associated fluid dynamics. We have also compared capillary waves among Newtonian, shear-thinning, and Boger fluid droplets and found that although the fluid kinematics appear to be purely biaxial extensional flow, the infinite-shear properties of the droplets dominate the physics of capillary wave formation and propagation. 

Recent studies have revealed the air-cushioning effect of droplet impact upon various surfaces and although pure water droplets have extensively been studied, the air entrainment dynamics for aqueous polymeric droplets was the focus of this study. Herein, droplets of low to moderate Weber numbers, [Formula: see text], displayed air film thickness gradients which was strongly influenced by the viscoelastic properties of the aqueous polymeric droplets in the dilute to the semidilute unentangled regimes. Aqueous polyethylene oxide droplets impacting a smooth thin oil film surface formed a submicrometer air layer, moments prior to impact, which was tracked by a high-speed total internal reflection microscopy technique. The radial changes in the air film thickness were related to the polymer concentration, thus providing an alternative tool for comparing the rheometer-derived overlap concentrations with a contactless optical technique. 

Marangoni flow is the motion induced by a surface tension gradient along a fluid–fluid interface. In this study, we report a Marangoni flow generated when a bath of surfactant contacts a pre-wetted film of deionized water on a vertical substrate. The thickness profile of the pre-wetted film is set by gravitational drainage and so varies with the drainage time. The surface tension is lower in the bath due to the surfactant, and thus a liquid film climbs upwards along the vertical substrate due to the surface tension difference. Particle tracking velocimetry is performed to measure the dynamics in the film, where the mean fluid velocity reverses direction as the draining film encounters the front of the climbing film. The effect of the surfactant concentration and the pre-wetted film thickness on the film climbing is then studied. High-speed interferometry is used to measure the front position of the climbing film and the film thickness profile. As a result, higher surfactant concentration induces a faster and thicker climbing film. Also, for high surfactant concentrations, where Marangoni driving dominates, increasing the film thickness increases the rise speed of the climbing front, since viscous resistance is less important. In contrast, for low surfactant concentrations, where Marangoni driving balances gravitational drainage, increasing the film thickness decreases the rise speed of the climbing front while enhancing gravitational drainage. We rationalize these observations by utilizing a dimensionless parameter that compares the magnitudes of the Marangoni stress and gravitational drainage. A model is established to analyse the climbing front, either in the Marangoni-driving-dominated region or in the Marangoni-balanced drainage region. Our work highlights the effects of the gravitational drainage on the Marangoni flow, both by setting the thickness of a pre-wetted film and by resisting the film climbing.