Researchers have discovered how certain colloids can form a solid-like gel and revealed how this mechanism differs from glass formation. Image: Institute of Industrial Science, The University of Tokyo.The soft, solid-like properties of colloidal gels are essential in fields such as food and medical applications, but how these properties manifest themselves is a long-standing mystery. Until now, it was believed that the solid nature of gels emerges via glass formation.
In paper in Nature Physics, researchers from the Institute of Industrial Science at the University of Tokyo in Japan report using a new microscopic technique – in situ confocal microscopy – to reveal the differences between gel and glass formation.
A colloidal liquid is a mixture comprising small particles scattered throughout a liquid substance; milk is an example. Understanding how colloidal liquids can become a gel upon phase separation – via a so-called dynamically arrested state – is important for optimizing 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 arrangement of the constituents of colloidal liquids. Nevertheless, there has not to date been an experimental means of visualizing 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 crystallization and might help prevent colloid particles from undergoing the gel-to-crystal transition," explains Hideyo Tsurusawa, lead author of the paper. "We developed an in situ confocal microscopy method for testing this hypothesis in a real-time, real-space manner."
The researchers used this method to study dilute colloidal gels consisting of ‘sticky’ spherical particles that exhibited short-range, directionless, attractive interactions. This revealed that different local arrangements of the particles uniquely modulated the properties of the gel. Specifically, tetrahedra-shaped particle clusters arrested local particle motion; 3-tetrahedra clusters hindered crystallization; and pentagonal bipyramid clusters imparted solidity. The researchers propose that minimizing the local potential energy is central to forming a gel state, whereas minimizing the free energy (entropy) is central to forming a glass state.
"The unique feature characterizing formation of a dilute gel is a sequential hierarchical ordering from tetrahedra to pentagonal bipyramids and to their clusters," says Hajime Tanaka, senior author of the paper. "This is absent in glass formation."
These findings led the researchers to propose a novel mechanism based on sequential amorphous ordering for the dynamical arrest of colloidal gels, unlike the previous glass-based explanation. This work has practical implications in helping to optimize colloidal gel materials with desired mechanical properties, such as for food and medical applications.
This story is adapted from material from the University of Tokyo, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.