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Chair: Peter BeltramoAbstractColloidal particles trapped at fluid interfaces have numerous applications, ranging from emulsion stability to fabricating two-dimensional (2D) metamaterials. Their microstructural organization dictates resultant material properties, such as increased yield stress of kinetically arrested aggregates or the emergence of optical metasurface effects from ordered assemblies. These microstructures may be engineered by manipulating particle surface topography or shape anisotropy to control how individual particles pin at the interface and subsequent interparticle interactions. This thesis investigates how particle surface roughness affects the interfacial pinning and interactions of shape-isotropic spheres and shape-anisotropic ellipsoids, and examines how these effects govern collective monolayer behavior, including interfacial assembly and mechanics. By rationally designing spherical and ellipsoidal polymer colloids with controlled roughness and topography (convex or concave), we demonstrate that the effect of roughness on interfacial pinning and deformation depends on particle shape. For spheres, surface roughness enhances quadrupolar interfacial deformation and thereby capillary attraction, as theoretically predicted. Surprisingly, for ellipsoids, roughness suppresses quadrupolar deformation and thus weakens capillary attraction between rough ellipsoids relative to their smooth analogues. To interrogate how these single-particle and pairwise behaviors influence multiparticle assembly, we slowly compacted a monolayer of particles at an air-water interface using a Langmuir trough and simultaneously measured the monolayer surface pressure and optically imaged the resultant microstructures. With increasing roughness, surface pressure increases for spheres and decreases for ellipsoids, consistent with our previously measured trends in capillary interaction between particles. However, optical microscopy reveals that the specific topography, in addition to shape and roughness, dictates monolayer microstructure. Notably, while convex and concave ellipsoids both show attenuated capillary attraction, only concave rough ellipsoids form orientationally ordered monolayers at close packing, as opposed to the disordered multiparticle aggregates formed by smooth and convex ellipsoids. Surface topography therefore influences particle organization beyond simply reducing capillarity. Finally, we experimentally determine the aspect ratio (AR)-area fraction assembly phase diagram of concave rough ellipsoids. As area fraction increases, monolayers undergo a disorder-order transition from isotropic to plastic crystal phases at low AR and an isotropic to nematic transition at high AR, agreeing qualitatively with theoretical predictions for hard ellipses. These assemblies can be successfully transferred onto solid substrates via Langmuir-Blodgett deposition, demonstrating the potential of using rough ellipsoids to create 2D ordered anisotropic patterns through interfacial templating. Ultimately, this thesis comprehensively establishes using rough particles to engineer fluid interfaces by combining particle synthesis, quantitative individual particle pinning and interaction measurements, and evaluation of monolayer mechanics and assembly. These findings provide a route to complex, ordered 2D anisotropic microstructures through particle surface engineering and assembly at fluid interfaces and lay the groundwork for creating scalable anisotropic metasurfaces.
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