Dynamics of Colloids at Equilibrium and Thermal Non-equilibrium: From Microrheology to Environmental Sensing

Colloidal dispersions display diverse dynamics depending on whether they are at equilibrium or driven out of equilibrium. At equilibrium, colloids undergo Brownian motion and govern processes such as self-assembly and phase transition. When exposed to thermal gradients, colloids exhibit driven motion that is useful in separation, enrichment, and biochemical sensing applications. This motion is sensitive to surface chemistry, system composition, and background temperature. In the first part of this thesis, colloid dynamics at equilibrium are exploited for microrheological characteri-zation of polymer solutions in different polymer concentration regimes. For this end, generalized theoretical equations are developed for the mean squared displacement and specified for different rheological models. We demonstrate that the polymer concentration regimes can be distinguished using the fractional rheological parameters. We further propose simple approximations for the critical overlap concentration and the shear viscosity of viscoelastic liquidlike solutions. At thermal non-equilibrium, we examined the thermophoretic transport of colloid particles in different liq-uid media. In non-polar polymer solutions, we extracted the van der Waals (vdW) interactions from ther-mophoresis measurements. This was achieved by developing a theoretical framework for colloid thermophore-sis in polymer solutions. The theory reveals the influence of vdW interactions and polymer concentration on colloidal thermophoresis. A non-monotonic dependence of colloid thermophoresis on polymer concentration was observed and attributed to the opposing effects of increased polymer concentration and higher solution viscosity on polymer distribution. In aqueous solutions, the sensitivity of thermophoretic motion to surface chemistry was exploited to detect per- and polyfluoroalkyl substances (PFAS) adsorbed on various model microplastics. It was observed that the adsorption of PFAS molecules on particles produces distinct thermophoretic responses according to the chain length and head group of PFAS. Through the application of the mode-coupling model (MCM) for thermophoresis, we correlated the number of adsorbed PFAS molecules with the number of water molecules in the hydration shell around the colloid. This work addresses how colloidal dynamics can be utilized at equilibrium for soft matter characterization and thermal non-equilibrium for measuring intermolecular interactions and sensing toxic chemicals. The findings highlight strategies for the development of colloid-based microfluidic PFAS sensors and controlling particle motion in polymer solutions for colloidal printing and particle detection applications.