Non-invasive rheological testing overcomes a critical setback

It is essential to discern stiffness and viscosity when engineering complex material products like paint and fine-tuning the sensory appeal of food. To avoid disturbing the soft products while investigating their rheological properties, researchers use diffusing wave spectroscopy (DWS).

However, DWS can only be performed on small samples and scaling the data poses a challenge. The dynamics measured by DWS are interpreted using Generalized Stokes-Einstein Relation (GSER), but the resulting measurements do not quantitatively agree with bulk measurements. Li et al. use two-point GSER to show quantitative agreement between microrheology measurements of DWS and bulk rheology.

“Two-point GSER considers how each particle moves the neighboring particles in a crowded environment,” said author Eric Furst. “Unlike the original GSER, which only accounts for how individual particles move.”

The study investigated two colloidal suspensions, silica-water, and silica-PEG, using DWS. The mean-squared displacement moduli were first calculated using the ‘one-point’ GSER, which overestimated the moduli measured by bulk rheometry. Then, the two-point GSER was adopted and the resulting microrheology experimental data matched with the bulk rheometric calculation.

DWS can now be used to efficiently study industrial or emerging materials non-invasively.

“We want to study particle suspensions of different shapes and sizes to see if the agreement we present between microrheology and macrorheology holds,” said Furst. “We are keen on exploring the application of the principles in this work to other techniques such as single scattering (DLS) and differential dynamic microscopy experiments.”

Source: “Two-point microrheology and diffusing wave spectroscopy,” by Qi Li, Kimberly A. Dennis, Yu-Fan Lee, and Eric M. Furst, Journal of Rheology (2023). The article can be accessed at https://doi.org/10.1122/8.0000664 .