Influence of the interfacial tension on the microstructural and mechanical properties of microgels at fluid interfaces

TL;DR

This study investigates how different interfacial tensions affect the shape, structure, and mechanical properties of microgel monolayers at fluid interfaces, revealing new ways to control soft particle assemblies.

Contribution

It compares microgel behavior at two organic fluid interfaces with different interfacial tensions, elucidating how tension influences microgel deformation and monolayer mechanics.

Findings

Lower interfacial tension results in less deformed microgels.

Microgel monolayers become softer with decreasing interfacial tension.

Surface energy tuning allows control over particle assembly and properties.

Abstract

Microgels are soft colloidal particles constituted by cross-linked polymer networks with a high potential for applications. In particular, after adsorption at a fluid interface, interfacial tension provides two-dimensional (2D) confinement for microgel monolayers and drives the reconfiguration of the particles, enabling their deployment in foam and emulsion stabilization and in surface patterning for lithography, sensing and optical materials. However, most studies focus on systems of fluids with a high interfacial tension, e.g. alkanes/ or air/water interfaces, which imparts similar properties to the assembled monolayers. Here, instead, we compare two organic fluid phases, hexane and methyl tert-butyl ether, which have markedly different interfacial tension ($\gamma$) values with water and thus tune the elasticity and deformation of adsorbed microgels. We rationalize how $\gamma$ controls the single-particle morphology, which consequently modulates the structural and mechanical response of the monolayers at varying interfacial compression. Specifically, when $\gamma$ is low, the microgels are less deformed within the interface plane and their polymer networks can rearrange more easily upon lateral compression, leading to softer monolayers. Selecting interfaces with different surface energy offers an additional control to customize the 2D assembly of soft particles, from the fine-tuning of particle size and interparticle spacing to the tailoring of mechanical properties.