(533e) Controlling Self-Assembly and Interfacial Mechanics of Polymer Spheres and Ellipsoids at Fluid Interfaces Via Particle Surface Roughness

Colloidal particles stabilize interfacial systems by becoming trapped at fluid interfaces through minimizing the energetically costly contact area between two immiscible fluid phases, with a desorption energy thousands of times higher than the thermal energy. Once at an interface, particle monolayers self-assemble into various structures dictated by the interfacial pinning and interparticle interactions induced by colloid surface chemistry, topography, and any shape asymmetry. These attributes of fluid interfaces straddled by particles can be exploited in numerous soft matter applications, ranging from emulsion stability to structured two-dimensional (2D) metamaterials. Recently, colloidal particles with rough surfaces and ellipsoidal shapes are receiving increasing attention owing to their unique material properties emerging from the interfacial quadrupole that forms and leads to strong interparticle capillary attraction. While rough spherical particles can be used in creating universal Pickering emulsions and phase inversible emulsions due to their long-lived metastable pinning induced contact angle hysteresis, smooth ellipsoidal particles can be exploited to stabilize foams and emulsions to a higher degree with less particles due to the capillary attraction mediated disordered multiparticle aggregates.

When both roughness and shape anisotropy are combined, an interesting behavior emerges. In our recent work, we have demonstrated that while roughness strengthens the capillary attraction in spheres as expected by theory, in stark contrast, it weakens the same in ellipsoids. In situ measurements of the particle pinning have attributed this to the surface roughness cloaking the macroscopic particle shape in conjunction with the particles becoming more hydrophilic and having decreased apparent contact angle with increasing roughness. These single-particle results have motivated us to investigate how particle surface roughness and shape anisotropy couple to impact multiparticle monolayer assembly behavior and stability at interfaces.

To that end, we synthesized spherical and ellipsoidal polystyrene particles with controlled roughnessspecifically, convex rough particles which possess chemically heterogeneous patches protruding from the particle surface, and complementary concave rough particles having chemically homogeneous dimples on the particle surface. Using a Langmuir trough, we measured the surface pressure as a function of particle areal density while simultaneously recording the microstructure via optical microscopy at an air-water interface. We observed a direct dependence of the interfacial behavior on the surface roughness magnitude and topography (convex/concave) and the shape heterogeneity of the particles. With increasing roughness, the measured surface pressure increases for spheres, in accordance with stronger capillary interactions, and decreases in ellipsoids, likewise confirming our single particle experiments showing weakened capillary interactions. For spherical particle monolayers, rough particles impair the stability of the interface against deformation as demonstrated by their lower compressive modulus compared to smooth analogues, contrasting the theoretically predicted enhancement of particle monolayers’ elasticity due to surface roughness induced capillary interaction. Between the two types of surface geometry, convex particles withstand higher interfacial deformation, which can be ascribed to tangential contact forces as well as surface asperity interlocking that hinders the relative motion of the contacting particles. This phenomenon also dictates the way monolayers collapse: convex rough particles via folding and concave rough particles via subduction. Surface roughness accelerates the onset of jamming in spheres and yields a monolayer that lacks long-range order. However, the stability of ellipsoidal monolayers shows a complex dependency on surface roughness and geometry. Surface roughness delays the onset of jamming in ellipsoids, especially for concave rough ellipsoids which form a complete monolayer at an area fraction of 0.79 compared to kinetically arrested disordered heterogeneous structures from smooth and convex rough counterparts that jam at an area fraction of 0.68. Moreover, monolayers of concave ellipsoids form directionally ordered structures. We characterize the assembly by defining a global nematic order parameter to show how the particles rearrange orientationally as the monolayer becomes more close-packed. Findings of this study open up opportunities to manipulate fluid-fluid interface stability in emulsions and foams and realize complex 2D ordered microstructures from anisotropic particles by leveraging particle surface and shape engineering.