  Microbullet particles, cylinders with one blunt and one spherical end, offer
a novel platform to study the effects of anisotropy and curvature on colloidal
assembly in complex fluids. Here, we disperse microbullets in
4-cyano-4'-pentylbiphenyl (5CB) nematic liquid crystal (NLC) cells and form
oriented elastic dipoles with a nematic point defect located near the curved
end. This feature allows us to study particle interactions as a function of
dipole alignment. By careful control of the surface anchoring at the particle
surface and the confining boundaries, we study the interactions and assembly of
microbullets under various conditions. When microbullets with homeotropic
surface anchoring are dispersed in a planar cell, parallel dipoles form linear
chains parallel to the director, similar to the observations of spherical
particles in a planar cell, while antiparallel dipoles orient side-to-side. In
a homeotropic cell, however, particles rotate to orient their long axis
parallel to the director. When so aligned, parallel dipoles repel and form 2D
ordered assemblies with hexagonal symmetry that ripen over time owing to
attraction between antiparallel neighbors. Further, we show that the anchoring
conditions inside the cell can be altered by application of an electrical
field, allowing us to flip microbullets to orient parallel to the director, an
effect driven by an elastic torque. Finally, we detail the mechanisms that
control the formation of 1D chains and hexagonal lattices with respect to the
elasticity of the NLC.
