Counterintuitive finding shows that fillers can change composite materials strength and stiffness non-monotonically

Solid fillers are commonly used to reinforce composite materials. The technique is well-established in polymeric nanocomposites, but less understood in the emerging field of filled colloidal gels such as those used to develop new battery electrolytes. Jiang et al. discovered an abnormal rheological behavior that has not been previously reported and used simulations to investigate its underlying physics.

Counterintuitively, the team’s experiments showed that grain fillers added into colloidal gels non-monotonically reinforced their mechanical properties. The fillers increased the yield strength of the gel, whereas the stiffness increased up to a certain filler fraction and then decreased. Simulations revealed that this softening effect resulted from structural changes in the gel matrix when enough fillers were introduced, which increased interphase pressure without changing yield resistance.

The team fabricated a transparent model system for colloidal gels filled with silica particles of varying wetting properties, and measured attributes such as yield strength via rheological measurements. They then used Langevin dynamics to reproduce their experiments and measured rheological parameters.

“The work opens new avenues for characterizing, designing, and tuning the mechanics of soft materials,” author Yujie Jiang said. “Our simulations led us to propose a ‘filler-removal’ protocol in which fillers are removed after gelation to independently control strength and stiffness of the gel matrix.”

Based on these simulations, the authors proposed possible implementations of the protocol for future experiments, which they plan to conduct on gels used in industry.

“Our system closely resembles that of lithium battery slurries, making it an ideal model system for studying such real industrial products,” author Ryohei Seto said. “We may shift our focus to its electrochemical properties, exploring how variations in composition and processing methods influence conductivity.”

Source: “Filled colloidal gel rheology: Strengthening, stiffening, and tunability,” by Yujie Jiang, Yang Cui, Yankai Li, Zhiwei Liu, Christopher Ness, and Ryohei Seto, Journal of Rheology (2024). The article can be accessed at https://doi.org/10.1122/8.0000878 .