Breakup dynamics of dilute polymer solution droplet moving down a grooved hydrophobic surface

In this study, we investigated the behavior of aqueous polymer solution droplets moving down an inclined, grooved hydrophobic surface using a shadow imaging system. Several aqueous polyethylene oxide (PEO) solutions were prepared by adding different amounts of PEO with molecular weights of M_w = 1 × 10⁶ g/mol in water. Rectangular grooves were fabricated on the polydimethylsiloxane (PDMS) surface using standard lithography. The receding dynamics of droplets at various surface-inclination angles were captured using high-speed imaging. The increase in viscosity and elasticity of the droplets significantly delayed the retraction of the liquid on top of the grooves, leading to the formation of several liquid films between the grooves. These films, which resembled planar surfaces, evolved into cylindrical filaments, with the filament diameter decreasing exponentially over time. As the polymer concentration and surface-inclination angle increased, the lifetime of the liquid ligaments increased owing to the increased elasticity of the liquid. The extensional relaxation times of all the dilute polymer solution droplets used in the experiment were calculated by evaluating the slope of the filament decay in the elasto-capillary regime, and they closely matched the values reported in the literature. Our findings not only elucidate the receding dynamics of dilute polymer solution droplets but also open up the possibility of using the sliding droplet method (SDM) to quantify the extensional properties of these solutions.