(438h) Cloaking Anisotropic Capillary Interactions Using Nanoscale Surface Topography

Colloidal assembly at immiscible fluid interfaces is an effective “bottom-up” method of creating interesting two-dimensional (2D) micro/nano-structures because large pinning energies force the constituent pieces into alignment along the plane of the interface. However, the capillary forces between physically-anisotropic micron-scale particles pinned to fluid interfaces overwhelm all other naturally-present forces and lead to strong, orientationally-specific, interparticle attractions. Reducing the magnitude of the interparticle capillary energy as well as its dependence on in-plane particle orientation is key to the controlling the resultant assemblies and creating more complicated 2D structures. The work presented here studies the how the introduction of nanoscopic chemical and physical heterogeneity to the surface of polymer microellipsoids alters the interparticle interactions when they are pinned at an aqueous-air interface. We synthesize novel anisotropic ellipsoids with controllable rough and porous surface topography via seeded emulsion polymerization (SEP) and subsequent mechanical stretching. We use a combination of Mirau interferometry and video microscopy to show that porous micron-sized ellipsoids at an aqueous-air interface behave in fundamentally different ways than their smooth counterparts. Specifically, the capillary interaction energy is reduced by over an order of magnitude, making porous ellipsoids ideal building blocks for interfacial assembly. Mirau interferometry is used to measure the interfacial deformation surrounding a single interfacially pinned particle with ±1 nm resolution along the axis perpendicular to the fluid interface. Particles with a nanoscale porous network show no quadrupolar deformation of the fluid interface, a trademark of smooth, homogeneous, colloidal ellipsoids due to Young’s law. Eliminating the quadrupolar deformation via surface engineering is direct evidence of decreasing interparticle attraction, since the magnitude of the capillary interaction energy is directly proportional to the magnitude of the quadrupolar deformation squared. This observation is confirmed using video microscopy monitoring of the trajectories of two particles as they approach towards one another. In agreement with the interferometry measurements, deducing the capillary interaction energy via this method likewise supports over an order of magnitude decrease in the capillary attraction for porous ellipsoids. Moreover, these measurements provide direct evidence of a shorter-range attraction between porous ellipsoids relative to homogeneous, smooth ellipsoids. The interactions of particles with a nanoscale porous network also appear to have no orientational specificity, in stark contrast to the homogeneous particles which have increasingly constrained rotational degrees of freedom upon approach as well as specific final arrangements. Taken together, these results indicate that incorporating nanoscale surface topography into anisotropic particles is an effective avenue to control interparticle interactions, resulting in particles with similar overall size and aspect ratio behaving in stark contrast depending on their surface topography. As a result, such particles are promising candidates as building blocks for assemblies of anisotropic particles with long-range order.