Cloaking Anisotropic Capillary Interactions Through Tunable Nanoscale Surface Topography

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Novel materials, such as solar photovoltaics, antireflective coatings, synthetic membranes, and biosensors, may be engineered for improved performance through the nano- and micro-scale ordering of small particles. Next generation versions of these technologies require ordered two-dimensional structures that have direction-dependent organization. In this project, the research team will investigate a new approach to creating such ordered assemblies of particles using stretched polymer spheres, termed “ellipsoids”. Such particles irreversibly pin to air-water interfaces to conveniently create a two-dimensional layer; however, forces between the particles cause disorganized assemblies to form. This hurdle will be overcome in this project by engineering the interactions between the particles through novel particle synthesis techniques that give the particle surface a controlled degree of roughness. The project will focus on how particle roughness can be designed to dictate the forces between the particles and lead to their ultimate ordered assembly at the air-water interface. By doing this fundamental science, the foundational paradigm to develop two-dimensional materials applicable to a variety of fields, including plasmonics, solar photovoltaics, coatings, membranes, and biosensors, will be established. In addition, several educational and outreach activities are integrated into the project. Undergraduate students from local community colleges will be exposed to opportunities in STEM by recruitment for summer research experiences. The PI will also develop active learning workshops for middle and high school students aimed at increasing interest in STEM fields by exposing students to the exciting real-world applications of particles at interfaces. The project will investigate how nanoscale surface topography (roughness or porosity) dictates the capillary interactions and assembly of anisotropic polymer ellipsoidal particles at fluid interfaces. This work is motivated by the plethora of novel materials (e.g., antireflective coatings, synthetic membranes, hierarchical surfaces, biomimetic materials) enabled from the two-dimensional ordering of anisotropic colloids. However, strong capillary attraction between particles at fluid interfaces dooms anisotropic particle assembly. The central hypothesis is that the capillary forces can be tuned by altering the curvature of the fluid interface surrounding pinned microparticles through the rational design of particles with controlled porosity or roughness. The research team seeks to apply a novel synthetic approach to create polymer ellipsoids with tunable roughness and porosity, quantitatively characterize the capillary interactions between such particles, and ultimately control the microstructural organization of particles whose detrimental capillary interactions have effectively been “cloaked” via their surface topography. First, seeded emulsion polymerization will be used to create biphasic, chemically patchy polymer colloids which can be transformed into rough and porous ellipsoids. Second, the capillary interaction energy between particles will be determined via a combination of monitoring two-particle approach profiles and using Mirau interferometry to measure undulations in particle-liquid contact line with nanometer scale precision. Finally, ordered assemblies will be created using particles with promising interaction energies. By linking the nanoscale particle surface characteristics with interfacial interactions, we will be able to identify design principles for nanoscale surface topography that minimize (i.e., “cloak”) strong capillary interactions to unlock ordered 2D microstructures comprised of anisotropic particles. These well-defined and otherwise inaccessible monolayer assemblies are directly relevant to applications in the fields of energy harvesting, photonics, particle-stabilized emulsions, biological interfaces, membranes, and/or hierarchical materials. The project will include the development of active learning workshops focused on interfacial materials and self-assembly concepts for K-12 educators and school-aged children, as well as expanding access to engineering research opportunities for underrepresented groups through coordination with local community colleges.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.