Clusters of particles determine gel elasticity

Scientists have discovered that the elasticity of gels arises from the packing of clusters of particles, dubbed locally glassy clusters, in the gels

Original source: Materials Today

From the toothpaste you squeeze on your brush first thing in the morning to the yogurt you slurp down to the fabric softener that keeps your pajamas cozy and soft, gels are ubiquitous in consumer products and foods, as well as industrial applications.

But scientists have been unable to explain the microscopic structures within gels that impart their elasticity, or springiness, or how these structures form. A team of scientists from the University of Delaware (UD), Massachusetts Institute of Technology (MIT), North Carolina State University and University of Michigan have now discovered that the elasticity of gels arises from the packing of clusters of particles in the gels, which the group dubbed locally glassy clusters.

This research, reported in a paper in Nature Communications, could help researchers engineer better materials and products at the microscale, assisting companies in the consumer products, biotechnology and agriculture sectors, and beyond.

Many companies formulate and sell gel products, and sometimes the stiffness of the gels changes as a result of instability. Eric Furst, professor and chair of UD’s Department of Chemical and Biomolecular Engineering and one of the paper’s corresponding authors, keeps an old bottle of fabric softener on a shelf in his office and uses it to demonstrate what happens when gels separate or ‘collapse’. The product is supposed to be easy to pour, but when it goes bad, it becomes gloppy and unappealing.

“Our results provide insight into how to engineer cluster size distribution to control stiffness, flow and stability of gel materials,” said Furst.

Gels are semi-solid materials that flow like liquids but contain solid particles, too. When scientists examine these substances under a microscope, they see that the solid particles within gels form a network, like the structure of a building. To make the substance flow so that you can squeeze it or spread it thin, you need to break that structure. When this requires a lot of force, the substance is stiff and has a high elastic modulus. When less force is required, the substance flows easily and has a lower elastic modulus.

The research group led by Furst studied a gel made of particles of poly(methylmethacrylate) (PMMA) latex, commonly known as acrylic, dispersed in a mixture of two colorless liquids – cyclohexane and cyclohexyl bromide. They found that this gel was composed of glassy clusters of particles connected to each other with weak areas in between. To understand how these glassy clusters contributed to the gel’s properties, the team wanted to determine the boundaries where each cluster began and ended.

“This is like Facebook,” said Furst. “We were trying to figure out – who is connected locally to whom?”

Collaborator James Swan, assistant professor of chemical engineering at MIT, conducted simulations to explore the physics behind the clusters. He then applied graph theory, the mathematical study of graphs, to the simulation data to figure out which clusters connected to each other, as well as to identify the edges of each group and to color-code the clusters. It was like defining the boundaries of intermingling friend groups.

Next, the researchers compared the simulation results to physical studies of the gels, and confirmed that the connections and distributions matched the predictions. They determined that the way these locally glassy clusters pack together determines the material’s elastic modulus, with the interconnected clusters acting as rigid, load-bearing units within the gel.

“Until now, no one had seen and described how these clusters packed and how they affected elasticity,” said Furst. “We brought the puzzle together.”

This paper was years in the making as the investigators followed up on lingering questions that bothered them and prompted them to keep working. “This discovery was the result of the teamwork of the principal investigators, the experimental skills of our students, and the passion and tenacity we all brought as we worked through this problem,” said Furst.