Controlling the volume fraction of glass-forming colloidal suspensions using thermosensitive host `mesogels'

TL;DR

This paper introduces a novel colloidal suspension system with thermosensitive mesogels that enable reversible control of the volume fraction of silica nanoparticles, facilitating the study of the glass transition.

Contribution

The study presents a new system using non-deformable silica nanoparticles and thermosensitive mesogels to control volume fraction via temperature, overcoming limitations of traditional thermosensitive particles.

Findings

Mesogels retain size change ability in suspension

Size of mesogels is independent of thermal history

System exhibits diverse dynamical behaviors across glass transition

Abstract

The key parameter controlling the glass transition of colloidal suspensions is $\varphi$, the fraction of the sample volume occupied by the particles. Unfortunately, changing $\varphi$ by varying an external parameter, \textit{e.g.} temperature $T$ as in molecular glass formers, is not possible, unless one uses thermosensitive colloidal particles, like the popular poly(N-isopropylacrylamide) (PNiPAM) microgels. These however have several drawbacks, including high deformability, osmotic deswelling and interpenetration, which complicate their use as a model system to study the colloidal glass transition. Here, we propose a new system consisting of a colloidal suspension of non-deformable spherical silica nanoparticles, in which PNiPAM hydrogel spheres of ~$100-200 \mu m$ size are suspended. These non-colloidal `mesogels' allow for controlling the sample volume effectively available to the silica nanoparticles and hence their $\varphi$, thanks to the $T$-induced change in mesogels volume. Using optical microscopy, we first show that the mesogels retain their ability to change size with $T$ when suspended in Ludox suspensions, similarly as in water. We then show that their size is independent of the sample thermal history, such that a well-defined, reversible relationship between $T$ and $\varphi$ may be established. Finally, we use space-resolved dynamic light scattering to demonstrate that, upon varying $T$, our system exhibits a broad range of dynamical behaviors across the glass transition and beyond, comparable with those exhibited by a series of distinct silica nanoparticle suspensions of various $\varphi$.