Soft-microgels: crossover from translational to jamming dynamics

Significance 

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.

Soft-microgels: crossover from translational to jamming dynamics - Advances in Engineering

About the author

Takuma Kureha obtained his PhD from Shinshu University (2018) for JSPS-sponsored postgraduate studies (DC1, 2015-2018) on the synthesis of functionalized nanogels, which he conducted under the tutelage of Prof. Dr. Daisuke Suzuki. He is currently carrying out postdoctoral fellow under the supervision of Prof. Dr. Mitsuhiro Shibayama at Institute of Solid State Physics, The University of Tokyo, for which he has been awarded a JSPS Research Fellow (PD). His current research is concerned with the characterization of hydrogels and nanogels by scattering methods in order to control their function.

About the author

Mitsuhiro Shibayama is Professor and Director of Neutron Science Laboratory, Institute of Solid State Physics, The University of Tokyo. He graduated from Kyoto University in 1982 with Dr. of Engineering in 1983. He served as Assistant Professor (1984), Associate Professor (1988) and Professor at Kyoto Institute of Technology (1997). Then, he moved to Univ. Tokyo in 2000. His recent awards include the Commendation for Science and Technology by the Minister of Education, Culture, Sports, Science and Technology (2019, Japan), and Colloid and Polymer Science Lecture Award (2017, Germany).

Reference

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

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