  Rational control over the periodic arrangement of particles by means of
external stimuli is a technologically important aspect of colloidal science
with important physical underpinnings. Here, a robust structural control of
particle assemblies in a nematic liquid crystal (NLC) is demonstrated by
dissolving trace amounts of light-responsive azo-dendrimer molecules which
spontaneously get adsorbed on the particle surface. The azo-dendrimer molecules
in the presence of external UV irradiation undergo conformational change
(trans-cis); as a result, they transmit the mechanical torque to surrounding LC
molecules and alter the near-field director orientation. The director
re-orientation at the surface of the particles causes topological defect
transformation which involves elastic dipoles, quadrupoles and hexadecapoles.
The defect transformation can be emulated in colloidal assemblies towards
different purposes such as rotation of chains and restructuring of 2D colloidal
crystals. In this study, various topological aspects of light-activated defect
transformation and its application in the collective manipulation of colloidal
assemblies are presented.
