Triple-line dynamics of a soft colloid-laden drop on a hydrophobic surface

Contact Angle Modulation: In Situ Polymer Deposition during Sessile Drop Evaporation

The drying kinetics of a sessile drop on a solid surface are a widely studied phenomenon because of their relevance to various fields such as coating, printing, medical diagnostics, sensing, and microfluidic technology. Typically, the drop undergoes drying either at a constant contact radius (R) with a decrease in the three-phase contact angle or at a constant contact angle (θ) with a reduction in the radius with time. These two drying modes are referred to as CCR and CCA, respectively. It is not uncommon where both R and θ may decrease during drying, especially in the penultimate stage of drying. In this work, we report a scenario wherein the θ increases while R decreases during the drying process of an aqueous polymer solution on a high surface energy substrate. This behavior is observed across different polymer systems (such as poly(ethylene oxide) and polyvinyl pyrrolidine), varying molecular weights, and polymer concentrations. As the drop dries, the polymer gets deposited at the three-phase contact line, thus reducing the surface energy of the substrate and leading to an increase in the contact angle. The drop responds by attempting to reach a new equilibrium contact angle through slipping. The temporal increase in contact angle follows a power law scaling behavior. This study demonstrates an in situ modulation of contact angle facilitated by evaporation and polymer deposition, showcasing unconventional drying dynamics.

Depletion zone in two-dimensional deposits of soft microgel particles

HYPOTHESIS

Microgels are a class of model soft colloids that act like surfactants due to their amphiphilicity and are spontaneously adsorbed to the fluid-air interface. Here, we exploit the surfactant-like characteristics of microgels to generate Marangoni stress-induced fluid flow at the surface of a drop containing soft colloids. This Marangoni flow combined with the well-known capillary flow that arises during the evaporation of a drop placed on a solid surface, leads to the formation of a novel two-dimensional deposit of particles with distinct depletion zones at its edge.

EXPERIMENTS

The evaporation experiments using sessile and pendant drops containing microgel particles were carried out, and the microstructure of the final particulate deposits were recorded. The kinetics of the formation of the depletion zone and its width is studied by tracking the time evolution of the microgel particle monolayer adsorbed to the interface using in situ video microscopy.

FINDINGS

The experiments reveal that the depletion zone width linearly increases with droplet volume. Interestingly, the depletion zone width is larger for drops evaporated in pendant configuration than the sessile drops, which is corroborated by considering the gravitational forces exerted on the microgel assembly on the fluid-air interface. The fluid flows arising from Marangoni stresses and the effect of gravity provide novel ways to manipulate the self-assembly of two-dimensional layers of soft colloids.