Design of Open Systems using Aqueous Polymer Solutions causing Meniscus Splitting Phenomena

Self-organization is a process commonly observed in soft matter and in spontaneous phenomena of life. Extracting and reproducing in vitro the physicochemical conditions is an important step beyond designing "life-like" materials having smart responses to environmental change. The behavior of polymeric solutions is known as viscous fingering. We have reported "meniscus splitting"^[1], in which the pattern formed on a substrate is developed into a millimeter-scale spatiotemporal structure. This is a non-equilibrium phenomenon in the controlled evaporation of polymer solution that splits the air-water interface into multiple regions. Drying an aqueous polymer dispersion from one interface in a limited space causes the polymer to precipitate at a specific position, driven by capillary forces. Also, the deposited polymer membrane has an oriented structure and becomes a functional material such as a humidity actuator^[2]. However, the specific spatiotemporal changes at the interface observed in the meniscus-splitting phenomenon have been only shown using some biopolymers, and the regulatory factors toward the universalization are unclear.<br/>In this study, we demonstrate that meniscus splitting can happen with chemical species independence, and it is a universal phenomenon, through the experiment. Polyacrylic acid (PAAc), polyacrylamide (PAAm), and polyvinyl alcohol (PVA) were used as synthetic vinyl polymers for the demonstration. The effect of different molecular weights on the interfacial behavior was pronounced, indicating that molecular weights with appropriate viscosity are necessary for membrane formation. PAAc and PAAm exhibited a splitting pattern like that of the polysaccharides reported previously. On the other hand, PVA did not form vertical membranes, but rather horizontal ones. This difference is thought to be due to the retentivity of water, and PVA's propensity to form intra- and intermolecular hydrogen bonds. When the relative humidity was adjusted, vertical membrane was also formed for PVA as well as for the other chemical species. Furthermore, we attempted a mathematical analysis of the spatiotemporal changes in the specific interfacial shape observed during the drying process. To understand the transformation of the interface the characteristic interface curves were fitted by a hyperbolic curve or an elliptic curve. Through the fitting evaluation, it was suggested that the interface curves transformed from a hyperbolic curve to an elliptic curve during the splitting.<br/><br/>Reference: [1] K. Okeyoshi, et al, <i>Sci. Rep</i>. <b>2017</b>, 7, 5615; <i>Polymer J</i>. <b>2020</b>, 52, 1185. [2] K. Budpud, et al., <i>Small</i>, <b>2020</b>, 16, 2001993.