CAP Lecture - Balls and Strings

Tuesday, March 22, 2016

Dr. Colin Denniston
University of Western Ontario

Date:  Tuesday, March 22, 2016
Time:  2:30pm
Location:  TB 360

Balls and Strings

Designer materials are materials manufactured to display certain properties rather than randomly tested for the existence of these properties.  For example, many researchers are attempting to produce photonic crystals (materials with periodicity on the nm to mm scale to match the wavelength of light) to replace electronics with faster photonic devices. One promising area of research begins with colloidal suspensions, sub-micron sized particles suspended in a fluid, and uses hydrodynamics to facilitate the self-assembly into crystal structures with periodicity on the scale of nano-meters to micrometers.  Most colloidal suspensions, such as milk or paint, are made up of spherically symmetric particles.  While spherical colloids in isotropic fluids have many potential uses, they typically all result in very similar structures, and in particular if you dry them out lead to fcc or bcc lattice structures.  Particles with anisotropic interactions have a much higher potential as the synthetic building blocks for self-assembled materials with desirable properties, such as a photonic band-gap, at the nano- or micro-scale.  A spherical object, such as a colloidal particle, in an anisotropic fluid, like a liquid crystal, behaves very differently from a colloidal particle in a simple fluid.  Boundary conditions at the surface of the sphere typically cause the director field, the direction along which the liquid crystal molecules are aligned, to sit parallel or perpendicular to the surface.  This makes the colloidal particle in a liquid crystal look like it has poles (parallel alignment) or look like a hedgehog (perpendicular alignment).  Such alignments of the liquid crystal field are energetically unfavourable, and to avoid them, topological defects form either on or close to the surface of the sphere, giving rise to exotic patterns of string defects around the sphere. As a result, a spherical particle in a cholesteric liquid crystal can generate a tetravalent bonding structure and resulting in the particles self-assembling into double-bonded chains or more exotic lattice structures.