Light and dye-mediated polymer drop splashing*

Aqueous polymer drop impact is of relevance to a variety of applications such as in agriculture, printing, and coating where dyes and additives are also commonly introduced for various functionalities. Although polymer solutions are widely used, various physicochemical pathways towards aging can inhibit the polymer performance over time (e.g., decrease in the molecular weight). We show that even a laboratory LED is capable of causing sufficient polymer (e.g., polyethylene oxide) degradation allowing a polymer drop which has previously not splashed to suddenly begin to splash in the presence of dissolve dyes (e.g., fluorescein) – which is commonly used to enhance contrast during high-speed imaging. We have also investigated the time dependence of the degradation by irradiating the solution with an LED with validated spectral output and show how the light-mediated splashing is akin to the molecular weight decrement of the polymer chains as drops with low molecular weight (e.g., polyethylene glycol) is known to have the same effect (without light and dye). We have varied the drop impact velocity, polymer molecular weight, polymer concentration, substrate type, and polymer irradiation time in a custom-built light irradiation chamber. We have also demonstrated how the impact outcomes fit the splashing parameter and provide scaling arguments to describe the physics of the light-mediated polymer drop impacts.