The sticky colloid science question is now solved

(Nanowerk News) The soft and dense properties of colloid gels are of great importance in fields such as food and medical applications, but how these properties manifest themselves has remained a long-standing mystery. Until recently it was believed that the solid gel properties arose through the formation of the glass.

In a study recently published in Natural Physics (“Hierarchical amorphous ordering in colloidal gelation”), researchers from the Institute of Industrial Science, The University of Tokyo have used a new microscopic technique – in place confocal microscopy – to reveal differences in gel and glass formation.

Colloidal fluids are mixtures consisting of tiny particles scattered throughout other liquids; milk is an example. Understanding how colloidal liquids can gel after phase separation—through what is called a dynamically captured state—is important for optimizing the design of food, cosmetics, and biomedical materials. However, a single particle-level explanation of the dynamically captured state remains elusive.

Efforts in this direction have recently focused on a principle known as local amorphous arrangement, which is concerned with the arrangement of colloidal liquid constituents. However, there is no experimental way to visualize such sequences, at the single-particle level, in the early stages of colloidal gelation. Providing fundamental physical insights into the origin of the amorphous makeup—and thus the dynamically captured state—of colloid gels is a problem researchers are keen to address.

“The pentagonal bipyramid shape has a symmetry that is incompatible with crystallization, and might help prevent colloidal particles from undergoing a gel-to-crystal transition,” explained Hideyo Tsurusawa, lead author of the study. “We developed a in place confocal microscopy method to test this hypothesis in real-time, real-space.”

The researchers report results on aqueous colloid gels composed of “sticky” spherical particles that exhibit short-range, directionless, attractive interactions. They revealed that the arrangement of different localized particles uniquely modulates the properties of the gel. In particular, tetrahedra restrain local particle motion, 3-tetrahedra inhibit crystallization, and clusters of pentagonal bipyramids provide solidity. The researchers propose that minimizing local potential energy is essential for forming a gel state, whereas minimizing free energy (entropy) is essential for forming a glass state.

“A unique feature characterizing the formation of aqueous gels is the ordered hierarchical order from tetrahedra to pentagonal bipyramids and to their clusters,” said Hajime Tanaka, senior author. “It’s not in a glass formation.”

This work proposes a novel mechanism based on sequential amorphous ordering for the dynamic capture of colloidal gels, unlike previous glass-based explanations, via direct access to the gelling process at the single particle level. This work has practical implications in helping to optimize colloid gel materials with desired mechanical properties; for example, in food and medical applications.