Title:Oral : Effect of Solute concentration on the deposition pattern in evaporative drying of a Polymer droplet from a rapidly evaporating solvent

Evaporative drying of a polymer solution droplet is an exciting scientific problem that results in many unique non-intuitive deposition patterns, including a thick peripheral deposit known as the coffee ring. Depending on interplay of various forces active during evaporation, such as solutal Marangoni flow, thermo-capillary flow, along with peripheral pinning of the contact line a variety of novel secondary patterns around the primary deposition might be observed. In this work we show how completely novel deposition morphology comprising of “Gear Tooth Patterns” are formed when a dilute solution of Polystyrene (PS) in tetrahydrofuran (TFH) is allowed to evaporate. We report how the deposition morphology changes with change in the PS concentration, which was varied from 0.05% to 1%. The resulting deposition patterns show intricate structures, including elongated, periodic finger-like formations at the edges of the droplet, along with fragmented polymer threads within the droplet's footprint. At lower concentrations, the patterns are marked by a delicate network of polymer threads, while at higher concentrations, the edges reveal more pronounced finger-like formations. The periodicity and structure of these patterns change with increasing PS concentration, as higher concentrations tend to suppress periodic undulations due to greater viscosity and modified flow dynamics. We argue that these finger-like structures form due to strong solutal Marangoni flow, which is driven by the rapid evaporation of THF, creating a strong radial concentration gradient. Additionally, Bernard convection leads to the formation of periodic undulations at the droplet's edge, especially at lower polymer concentrations. This study deepens our understanding of the complex interactions between evaporation, solutal Marangoni effects, and contact line dynamics, illuminating the intricate nature of pattern formation in polymer solutions.