Large Impacts of Small Particles: The Effects of Active Particles on Colloidal Gels and Crystals and of Inulin Microparticles on Gut Retention in Mice

Abstract

We investigate how colloidal particles affect and interact with their environments. Two colloidal particles are studied: active Janus particles and electrosprayed polyvinyl alcohol (PVA) / inulin microparticles. Inspired by the motion seen in nature, active colloids are self-propulsive and inherently out of equilibrium. We study how active colloids affect nonlinear rheological properties of colloidal gels and interact with defect-rich colloidal crystals. Inulin is a prebiotic fiber commonly used to modify rheological properties of foods and, more recently, to work synergistically with cancer immunotherapies. We study how electrospraying inulin into microparticle form affects its rheological properties and gut retention. In chapter two, we explore how a small fraction of active particles (active-to-passive particle ratio of ~1:1200) affects the yield stress behavior of colloidal gels. The active particle system used is a platinum Janus colloid whose active motion results from the asymmetric decomposition reaction of hydrogen peroxide. Colloidal gels are prepared by the addition of a divalent salt to a suspension of polystyrene microparticles and Janus particles in water and hydrogen peroxide. We find that active particles cause a significant yield stress reduction (up to a factor of three); however, no change in yield strain is observed. We explain how the small number of active particles can have this rheological effect through modeling. We use a combined theory of how activity changes the spring constant of interparticle bonds in the gel with an argument that the number of active clusters – rather than number of active particles – controls the rheological impact of the activity. The modeling of this theory agrees well with experimental data. In chapter three, we explore how a small fraction of active particles (active-to-passive particle ratio of ~1:720) interact with and affect defect-rich colloidal crystal monolayers. This study utilizes the same platinum Janus particle as chapter two. The crystal monolayer is made of passive, Brownian particles with diameters four times that of the Janus particles. These larger particles are assembled into a crystal monolayer by an alternating current electric field. We find that active particles cause a reduction in void number (by up to ~50%) but an increase in void size and anisotropy (by up to 190% and 40%, respectively). We examine how these changes may relate to differences in the microdynamics of active particles by region. We specifically compare microdynamics in void, void-adjacent (within three particle diameters of a void), and interstitial regions. At the highest active energy, dynamics are increased in void-adjacent regions, about the same in interstitial regions, and decreased in void regions compared to the ensemble dynamics. In chapter four, we investigate the relationship between the rheological properties and gut retention of PVA-inulin microparticles. Inulin is a fructan polysaccharide with prebiotic properties which is found in many vegetables. PVA is a biocompatible polymer that can exhibit mucoadhesive properties. Microparticles are created via the electrospraying method which utilizes large electrical potential differences to create nano- and micro-sized features. In suspension, we determine that microparticle form causes shear thinning behavior. Furthermore, the release and dissolution of inulin causes an increase in sample modulus with time. When administered to mice, the PVA-inulin microparticle suspensions exhibit signs of greater gut retention than as-received inulin and mixtures of equivalent concentrations. We hypothesize that gut retention depends on established mucoadhesive properties of PVA and rheological properties of the samples.