Non-equilibrium Colloidal Phenomena in Magnetic Fields and Photoillumination: From Controlling Living Microbots to Understanding Microplastics

Colloids are a ubiquitous class of materials composed of microscopic particles suspended in a continuous phase which are found in everyday products and in nature. Colloids are also useful models for studying the spontaneous arrangement of matter from individual building blocks to mesophases. Standard treatment of colloid science is based on the assumption of equilibrium conditions, as defined in traditional thermodynamics. However, novel assembly mechanisms and motility are unlocked by pushing colloids away from equilibrium using external energy. In addition, many colloids in nature and in industrial applications exchange energy and mass with the surrounding environment thus behaving in a far from equilibrium manner. There is a push for progressing our understanding of the dynamic properties and functions of colloids in non-equilibrium conditions. This Ph.D. dissertation presents strategies to study the out-of-equilibrium behaviors of colloids under two distinct sets of conditions. In the first set, we investigate methods to magnetize both inanimate particles and living organisms dispersed in magnetic nanoparticle dispersions known as ferrofluids. We show the ability to assemble anisotropic clusters known as supraparticles, and to reconfigure the phase of colloidal materials by tuning the balance of attraction and repulsion among a collective of particles. By immersing C. elegans nematode worms in ferrofluid, we display the ability to control their motion and spatial organization with magnetic fields. In the second set, we study the effects of weathering and drying of colloidal suspension on the dynamic changes to the properties and structure of particles. We test the degradation of microplastics exposed to simulated photoillumination. Sunlight exposure rapidly increases the dispersibility of microplastics in water, and it also affects their ability to adsorb molecular pollutants. Concurrently, we study particles that are continuously reconfiguring within a drying vii droplet and forming a shell at the air-water interface. We find that shells formed with spherical nanoparticles regularly buckle and form broken donut shapes whereas shells formed with high aspect ratio rod-like particles maintain stable spherical shapes. The findings expand our fundamental understanding of non-equilibrium colloid science and inform the engineering of active and driven materials.