Rheo-Optical Study on the Viscoelastic Relaxation Modes of a Microgel Particle Suspension around the Liquid–Solid Transition Regime

Abstract

Dynamic viscoelasticity and dynamic birefringence of a microgel system were investigated around the liquid–solid transition concentration to clarify the molecular origin of the viscoelastic response of the microgel system. The complex modulus showed viscoelastic liquid-like behavior at concentrations, c, below a threshold c jamming for random close packing of the microgels, whereas viscoelastic solid-like behavior at c > c jamming. The imaginary part of the complex strain-optical coefficient changed its sign with increasing angular frequency ω in a liquid-like regime, suggesting that the stress and birefringence involved three relaxation mechanisms. Utilizing the stress-optical rule, SOR, for each relaxation process, the complex shear modulus at c < c jamming was separated into the orientational stress of network strands in temporary contacted microgels, deformation of particle, and the redistribution process, from high frequencies in this order. On the other hand, in a solid-like regime at c > c jamming, the ordinary SOR held well with a single stress-optical coefficient, C, implying that only one stress origin is dominant, which can be attributed to the orientational stress of densely packed microgels in permanent contact. The strain-optical coefficients evaluated using the Onuki–Doi theory reproduced the measurements qualitatively and supported these assignments.

PMMA Nanocomposites Based on PMMA-Grafted α-Zirconium Phosphate Nanoplatelets

“…The entanglement relaxation time (τ e ), which represents the required time to achieve a configurational rearrangement for a molecular chain, can be determined from the reverse of the cross-over frequency of G ′ and G ″ curves (Figure ). It is noted that if the ZrP is aggregated in PMMA, the τ e becomes faster than that of neat PMMA, which is consistent with previous findings, ,,, that is, lack of strong interaction at the interface between the grafted polymer layer and matrix would lead to a lubrication effect near the interface, thus reducing τ e . For well-dispersed systems, it is found that with increasing grafted polymer brush length, which would then increase entanglement density, τ e would be delayed.…”

Microstructural and Rheological Transitions in Bacterial Biofilms

“…In doing so, our experiments have confirmed the presence of packing fraction‐dependent increases in elasticity, which is a well‐known feature of polymer‐colloidal systems. [ 30 , 31 , 32 , 33 ] The origin of this relationship could be attributed to the increasing spatial density of bacteria in comparison to the ECM, which have elastic moduli on the order of 0.1 − 1 MPa and 40 Pa, respectively. [ 55 , 56 ] We anticipate that this behavior will be applicable across biofilms of different species.…”

“…[ 28 , 29 ] Within such systems, factors like the particle packing fraction and polymer concentrations have been adjusted to control the viscoelasticity. [ 30 , 31 , 32 , 33 ] In contrast to abiotic systems, biofilms introduce a biological pathway for controlling structure and rheology through genetic manipulation. [ 11 , 13 , 25 , 34 ] This route has been used to study the influence of ECM composition on biofilm structure or rheology.…”

Experimental Studies of Flow-Induced Birefringence in Complex Fluids

“…Recently, mode separation (e.g., orientation mode, glass mode) of various polymer fluids has been reported by means of measuring FIB and viscoelasticity simultaneously (the rheo-optical method) [15][16][17][18] . Furthermore, Shangwei et al 19) examined the dynamics of microgel particle dispersions using the rheo-optical method and reported the method as being appropriate for researching microgel particles on a microscopic level.…”