  Optical excitation of colloids can be harnessed to realize soft matter
systems that are out of equilibrium. In this paper, we present our experimental
studies on the dynamics of silica colloids in the vicinity of a silver nanowire
propagating surface plasmon polaritons (SPPs). Due to the optothermal
interaction, the colloids are directionally pulled towards the excitation point
of the nanowire. Having reached this point, they are spatio-temporally trapped
around the excitation location. By increasing the concentration of colloids in
the system, we observe multi-particle assembly around the nanowire. This
process is thermophoretically driven and assisted by SPPs. Furthermore, we find
such an assembly to be sensitive to the excitation polarization at input of the
nanowire. Numerically-simulated temperature distribution around an illuminated
nanowire corroborates sensitivity to the excitation polarization. Our study
will find relevance in exploration of SPPs-assisted optothermal pulling,
trapping and assembly of colloids, and can serve as test-beds of plasmon-driven
active matter.
