When we illuminate gold nanofluids over indium-tin-oxide (ITO)-coated substrates,
nanoparticle chains self-assemble via optical binding forces. We speculate that charge
transfer between gold and ITO pins nanoparticles to the substrate and reduces the
lateral Brownian motion as they attach to the substrate. We correspondingly model
the self-assembly with additional stochastic or random forces. Simulations show a
nonequilibrium-phase transition: when the stochastic force is small, nanoparticle
chains align perpendicular to the light polarization and nanoparticles settle at shallow
but stable nodes; when the stochastic force is large, however, the nanoparticle chains
align parallel to the light polarization and nanoparticles settle at saddlepoints
where the optical binding force is largely zero. Since the presence and strength of
Brownian forces influence which state is formed, we reconsider the role that surfaces
have-not only in relation to charge transfer but also heat transfer.
