(579d) Vertically Aligning Ellipsoids with Electric Fields

Authors: 
Beltramo, P. J., ETH Zürich
Trevenen, S., UMass Amherst
Klier, J., University of Massachusetts Amherst
There are many examples in biology of nanoscale morphologies imparting unique macroscopic material properties, such as the photonic crystal found in butterfly wings, the drag-reducing riblets of shark skin, and the hydrophobic papillae of the lotus leaf. In particular, the 2D microstructural ordering of a moth’s eye surface is responsible for providing broadband antireflectance that exceeds the current state-of-the-art in the manufacture of industrial antireflective coatings. Externally directed assembly of colloidal particles offers a bottom-up approach to such novel materials, since broadband antireflectance is predicted from a monolayer of vertically aligned ellipsoids. Towards that end, we present an experimental approach to align micrometer sized polystyrene ellipsoid particles pinned at an immiscible liquid interface using an AC electric field. We first examine the behavior of ellipsoids in bulk aqueous solutions by considering the effect of applied field characteristics (magnitude and frequency) on the resulting particle alignment. Next, alignment of particles in the bulk is extended to particles pinned at an immiscible liquid interface to create ordered monolayer structures with unique morphology. The approach and results are understood by considering the interplay between the torque induced by the interface on the particle and that imposed by the external field. Together, the principles outlined through this approach inform the creation of ordered monolayer metamaterials through electric field-driven assembly of interfacially pinned microparticles.