Core-Shell Microgels at Air/Water Interfaces: Role of Interfacial Tension in Monolayer Evolution

Core-shell microgels with rigid cores and soft, deformable hydrogel shells can assemble at air-water interfaces, forming freely floating monolayers. The strong adsorption at such interfaces is related to the reduction in interfacial tension, which also causes the microgels to deform laterally. The degree of this deformation is typically controlled through applied surface pressure. Until now, surprisingly little has been known about the impact of interfacial tension imbalances between interfacial areas covered with a microgel monolayer and microgel-free areas in the surroundings. In this work, we systematically study the monolayer evolution at air/water interfaces in dependence of interfacial tension controlled by the addition of sodium dodecyl sulfate or linear poly-N-isopropylacrylamide homopolymer to the free area. We do this by globally monitoring the evolution of the area of freely floating monolayers. Macroscopic changes are also related to the local microstructure studied by atomic force microscopy (AFM). Depending on the interfacial tension imbalance, the monolayer either expands, shrinks, or maintains its conformation. The kinetics of monolayer expansion is compared for core-shell microgels with the same silica core and varying cross-linker densities. Our study reveals the impact of interfacial tension on the behavior of microgel monolayers at liquid interfaces and also provides useful insights into controlling the two-dimensional (2D) microstructure without the need for a Langmuir trough.