Microgels are colloidal gel particles that consist of chemically cross-linked three-dimensional polymer networks. Microgels have both the properties of a gel and a colloid. Microgel suspensions that range within the nano- to micrometer scale show unique colloidal behavior. As opposed to hard-sphere glasses that cannot exceed the close-packing fraction, microgels can be packed at a much higher volume fraction due to their deswelling and deformability. This characteristic would be of high significance not only to fundamental physicists but also engineers who are working towards many applications, such as: paints, pharmaceutical formulations and cosmetic products. Consequently, the viscoelasticity, yielding behavior, deformation and dynamics of highly concentrated microgel suspensions have been investigated using various techniques, such as: microscopy, and small-angle X-ray and neutron scattering.
Dynamic light scattering (DLS) is a technique that allows researchers to investigate nondestructively the dynamics (Brownian motion) of colloidal dispersions of 1 nm–5 mm in size, regardless of hard particles or swellable microgels. Unfortunately, to date, very few DLS studies have been reported on the physical properties of highly concentrated microgel suspensions, such as a microgel paste.
Therefore, the physical meaning of highly concentrated microgel suspensions dynamics remains a matter of debate. In this view, a group of researchers from the Institute for Solid State Physics at The University of Tokyo: Professor Mitsuhiro Shibayama and Dr. Takuma Kureha, in collaboration with Ms. Haruka Minato, Professor Daisuke Suzuki at Shinshu University, and Professor Kenji Urayama at the Kyoto Institute of Technology investigated the dynamics of colloidal gel particle suspensions by dynamic light scattering over a wide concentration range. Their work is currently published in the research journal, Soft Matter.
Ideally, microgels are composed of poly (N-isopropyl acrylamide) (pNIPAM), which is the most commonly used model for microgels. The researchers analyzed pNIPAM microgel suspensions in a wide concentration range by DLS measurements in the same manner as the case of hard colloidal glasses. The team analyzed the microgel paste in the same way as the nonergodic media. In fact, the fast mode enabled them characterize the local dynamics of the microgels in the paste, i.e., cooperative diffusion of polymer network chains.
The authors reported that in the dilute concentration region, translational diffusion of microgels was noted to be similar to conventional colloidal particles. Moreover, the team observed that at the intermediate concentration, where the volume fraction of the microgels was higher than the random close packing fraction for hard colloidal particle (φcp ≈ 0.64), their dynamics changed from the translational to soft jamming mode, i.e., from a single mode to double mode relaxations accompanied by nonergodicity.
In summary, the researchers investigated the dynamics of microgel suspensions, where the microgels were small enough to suppress multiple scattering, by DLS as a function of concentration. Overall, in an interview with Advances in Engineering, Professor Mitsuhiro Shibayama, the lead author praised DLS for being a powerful tool to investigate the dynamics from a dilute colloidal suspension to microgel paste. Moreover, he pointed out that their finding had potential to offer novel insights into the complex structural behavior in highly dense states from the viewpoint of light scattering, offering the possibility to develop a more profound physical picture of colloidal systems. Altogether, the study demonstrated DLS as a powerful tool to investigate the dynamics from a dilute colloidal suspension to microgel paste.
Takuma Kureha, Haruka Minato, Daisuke Suzuki, Kenji Urayama, Mitsuhiro Shibayama. Concentration dependence of the dynamics of microgel suspensions investigated by dynamic light scattering. Soft Matter, 2019, volume 15, page 5390.Go To Soft Matter