Microrheology of thermoresponsive poly(N-isopropylacrylamide) microgel dispersions near a substrate surface

Unveiling the rheological behavior of poly(styrene-co-N-isopropylacrylamide) microgel aqueous suspensions

Poly(N-isopropylacrylamide)-based microgels are soft and deformable colloids, which can swell and shrink in response to external stimuli such as temperature or pH, making them suitable for various industries. Herein, poly(styrene-co-N-isopropylacrylamide) microgels with varying softness were synthesized through the doping of different proportions of styrene into their network structure. Rheological tests revealed that the deformability, surface charge, and wettability of microgels were precisely regulated through adjustments to styrene doping levels. As the molar ratio of styrene increased, the range of the linear viscoelastic region regarding stain did not exhibit a consistent change. The microgel suspensions exhibited a pronounced viscous-like behavior, as indicated by their frequency dependence. Additionally, the concentration or volume fraction of microgels significantly influenced the phase distribution in the phase diagram. At lower concentrations, the microgel suspensions typically underwent a colloidal gel-to-liquid-to-glass transition, with the liquid-like regime expanding as the styrene doping level increased. At higher microgel concentrations, the volume phase transition shifted to a direct colloidal gel-to-glass transition, particularly at high styrene levels. This study also revealed a strong correlation between the power-law viscoelastic exponents and the loss tangent during the entire phase transition, offering valuable insights for structure and rheological properties of poly(N-isopropylacrylamide)-based microgels.

Total internal reflection microscopy: a powerful tool for exploring interactions and dynamics near interfaces

The occurrence of many micro/macrophenomena is closely related to interactions and dynamics near interfaces. Hence, developing powerful tools for characterizing near-interface interactions and dynamics has attached great importance among researchers. In this review, we introduce a noninvasive and ultrasensitive technique called total internal reflection microscopy (TIRM). The principles of TIRM are introduced first, demonstrating the characteristics of this technique. Then, typical measurements with TIRM and the recent development of the technique are reviewed in detail. At the end of the review, we highlight the great progress of TIRM during the past several decades and show its potential to be more influential in measuring interactions and dynamics near interfaces in various research fields.

Unveiling the structural relaxation of microgel suspensions at hydrophilic and hydrophobic interfaces

HYPOTHESIS

Poly(N-isopropylacrylamide) (PNIPAM) microgel particles show considerable hydrophilicity below the lower critical solution temperature (LCST) while they become hydrophobic above LCST. We hypothesize that interfacial wettability could tune particle-surface interaction and subsequent structural relaxation of microgel suspensions at interfaces during the volume phase transition.

EXPERIMENTS

The evanescent-wave scattering images of microgels at hydrophilic and hydrophobic interfaces are analyzed by a density-fluctuation autocorrelation function (δACF) over a wide range of particle volume fraction ϕ. The structural relaxation is characterized by the decay behavior of δACF. The scattering images in bulk are also processed as a comparison.

FINDINGS

A two-step relaxation decay is observed at both hydrophilic and hydrophobic interfaces. Relative to fast decay, the rate of structural relaxation in slow decay is reduced by a factor of ∼ 500 and ∼ 50 at hydrophilic and hydrophobic interfaces, respectively. The relaxation times obey divergent power-law dependences on intermediate regime of observing length scales at the two interfaces. Besides, the distribution of fluctuation for relaxation time at different local regions reveals that the structural relaxation is much more homogenous at hydrophilic interfaces than that at hydrophobic interfaces, especially at high ϕ.

Microfluidic Fabrication and Applications of Microgels and Hybrid Microgels

Smart microgels have gained much attention because of their wide range of applications in the field of biomedical, environmental, nanotechnological and catalysis sciences. Most of the applications of microgels are strongly affected by their morphology, size and size distribution. Various methodologies have been adopted to obtain polymer microgel particles. Droplet microfluidic techniques have been widely reported for the fabrication of highly monodisperse microgel particles to be used for various applications. Monodisperse microgel particles of required size and morphology can be achieved via droplet microfluidic techniques by simple polymerization of monomers in the presence of suitable crosslinker or by gelation of high molecular weight polymers. This report gives recent research progress in fabrication, characterization, properties and applications of microgel particles synthesized by microfluidic methods.