CBE Doctoral Dissertation Defense: “Shape Changing Droplets under Confinement and Particle Coverage” (Emery Hsu)

Abstract:

Crystalline monolayers at fluid interfaces possess unique elastic properties that have important implications in biology and nanotechnology and the synergistic incorporation of colloidal particles enables novel strategies for the design of responsive soft matter. A good command of their morphologies depends on the precise understanding of structural properties and the interplay between adsorbed particles and interfacial crystals. This work investigates the elastic characteristics of the crystalline monolayers, discusses the mechanisms governing the interfacial orientation of anisotropic particles and probes into the influence of adsorbed particles on temperature-controlled emulsion shape transformations, providing a framework for the development of smart emulsion systems with tunable properties.

Leveraging the shape transitions of oil-in-water emulsion droplets, upon which droplets adopt cylindrical shapes and elongate, we investigate the elastic characteristics of the crystalline monolayers covering their interfaces. To unravel the conditions determining Euler buckling and Brazier kink formation in these cylindrical interfacial crystals, we confine the elongating cylindrical droplets within microfluidic wells. Our experiments unveil a nonclassical relation between the Young’s modulus and the bending modulus of these two-dimensional crystals. Intriguingly, this relation varies with the radius of the cylindrical crystal, presenting a nonclassical mechanism for tuning elasticity of two-dimensional materials in nanotechnology applications.

Since many practical emulsions are synergistically costabilized by both surfactants and colloidal particles, we explore the influence of adsorbed particles and their shape anisotropy on emulsion shape transformations. To understand how parallel or perpendicular interfacial orientation of rod-like anisotropic particles affects interfacial crystals and resulting shape transformation, we also address the challenge of controlling the orientation of rod-like anisotropic particles on emulsion droplets. While thermodynamic models predict that rod-shaped anisotropic particles typically favor a parallel orientation on the oil-water interface to minimize interfacial energy, perpendicular orientation is highly desirable for achieving spiky morphologies with unique functionality. We demonstrate that the interfacial orientation of end-functionalized silica nanorods can be systematically controlled through post-synthesis silanization. By varying the silanization time, the rods exhibit a reentrant transition from interface-parallel to interface-normal and back to predominantly parallel configurations when adsorbed on oil-in-water emulsion droplets. Surface characterization shows that silanization progressively modifies both the hydrophobicity and charge of the rods while amplifying the intrinsic chemical asymmetry arising from PVP residues localized at one rod end, thereby changing the interfacial configuration of silica nanorods.

We incorporate such silica rods into shape changing emulsions and show that shape transformations remain robust despite high interfacial concentrations of spherical or rod-like particles, and emulsions with particles change shapes at lower temperatures as opposed to surfactant only emulsions. Surprisingly, the high aspect ratio of surface-adsorbed rods does not influence the morphologies available to faceting emulsion droplets and we are able to obtain similar shapes with spherical or rod-like silica particles. The temperature range in which rod-covered droplets start faceting also overlaps with that of droplets covered with spheres, indicating that silica spheres and silica rods have comparable effect on emulsion shape transformations. The mechanisms underlying emulsion shape transformation and how subtle changes in surface chemistry regulate the configuration of anisotropic colloids at fluid interfaces have broad applications for designing Pickering emulsions with adaptable surface morphology.