The physics behind gel formation

The soft, solid-like properties of colloidal gels are essential in fields such as food and medical applications, but exactly how these properties manifest themselves has been a longstanding mystery. It was previously believed that the solid nature of gels emerges through glass formation, but now researchers from the Institute of Industrial Science, The University of Tokyo have used a new microscopic technique — in situ confocal microscopy — to reveal the differences between gel and glass formation.

A colloidal liquid is a mixture consisting of small particles that are scattered throughout another liquid substance; milk is an example. Understanding how colloidal liquids can become a gel upon phase separation — through a so-called dynamically arrested state — is important for optimising the design of foods, cosmetics and biomedical materials. However, a single-particle-level explanation of the dynamically arrested state remains elusive.

Efforts in this direction have recently focused on a principle known as local amorphous ordering, which pertains to the order of arrangement of the constituents of colloidal liquids. Nevertheless, there has not been an experimental means of visualising such ordering, on a single-particle level, in the initial stage of colloidal gelation. Providing fundamental physical insights into the origin of amorphous ordering — and thus the dynamically arrested state — of colloidal gels is the problem that the researchers sought to address.

“A pentagonal bipyramid shape has a symmetry that is incompatible with crystallisation, and might help prevent colloid particles from undergoing the gel-to-crystal transition,” said Hideyo Tsurusawa, lead author of the new study. “We developed an in situ confocal microscopy method for testing this hypothesis in a real-time, real-space manner.”

Writing in the journal Nature Physics, the researchers reported on dilute colloidal gels consisting of ‘sticky’ spherical particles that exhibited short-range, directionless, attractive interactions. They revealed that different local particle arrangements uniquely modulated the properties of the gel. Specifically, tetrahedra arrest local particle motion, 3-tetrahedra hinder crystallisation and pentagonal bipyramid clusters impart solidity. The researchers propose that minimising the local potential energy is central to forming a gel state, whereas minimising the free energy (entropy) is central to forming a glass state.

“The unique feature characterising formation of a dilute gel is a sequential hierarchical ordering from tetrahedra to pentagonal bipyramids and to their clusters,” said Hajime Tanaka, senior author of the study. “This is absent in glass formation.”

This work proposed a novel mechanism based on sequential amorphous ordering for the dynamical arrest of colloidal gels, unlike the previous glass-based explanation, through directly accessing the gel formation process at a single-particle level. This work has practical implications in helping to optimise colloidal gel materials with desired mechanical properties; for example, in food and medical applications.

Image credit: Institute of Industrial Science, The University of Tokyo.