The soft and hard properties of colloidal gels are important in areas such as food and medical applications, but how these things manifest themselves has been a mystery for a long time. Until now, it is believed (The sticky questions about colloidal science are now settled) that the solid nature of gels is due to the structure of the glass.
In a study recently published in Nature Physics, researchers from the University of Tokyo’s Institute of Industrial Sciences used a new microscopic technique – in situ confocal microscope – to reveal the differences in the structure of gel and glass.
A colloidal liquid is a mixture of small particles that are dispersed in another liquid; milk is an example. Understanding how a colloidal liquid can turn into a gel over a period of time – through the so-called trapped energy state – is important for improving the processing of food, cosmetics and nutrients.
However, the one-dimensional level definition for the power-out condition remains relatively simple. Recent efforts have focused on a principle known as locally amorphous systems, which is related to the structure of colloidal liquid compositions.
Nevertheless, there has been no experimental method to visualize such a structure, at the level of one part, in the initial stages of colloidal gelation. Providing the necessary physical information about the origin of the amorphous phase – and therefore the energy-holding state – of colloidal gels is the problem that the researchers want to solve.
“The pentagonal bipyramid shape has an inconsistent crystal structure and can help prevent colloidal particles from entering the gel-crystal transition,” says Hideyo Tsurusawa, lead author of the study. “We have developed a technique in confocal microscopy to test this hypothesis directly and in real-time.”
The researchers report results on dilute colloidal gels containing spherical particles that exhibit nondirectional short-range interactions. They revealed that the different localization of the molecules changes the properties of the gel. Specifically, tetrahedra arrest local motion, 3-tetrahedra impede crystallization, and pentagonal bipyramid clusters provide strength. Researchers suggest that reducing the local energy is important for the formation of the gel state, while the reduction of free energy (entropy) is important for the formation of the glass state.
“The unique feature of the dilute gel formation is the structural arrangement from tetrahedra to pentagonal bipyramids and their clusters,” says author Hajime Tanaka. “This is not in the glass configuration.”
This work has presented a new method based on the amorphous process for the determination of the strength of colloidal gels, in contrast to the explanation based on the previous glass, by obtaining the gel structure directly at the one-dimensional level. This work has a beneficial effect in helping to develop a colloidal gel with the required mechanical properties; for example, in food and medical equipment.
Source: University of Tokyo
