Non-Equilibrium Dynamics and Non-Gaussian Fluctuations of an Optical Matter System Manifesting Pseudorotation

Gaussian
fluctuations are intrinsic to systems in thermal
equilibrium
and are also a tenet of near-equilibrium systems related by linear
response. We recently introduced a Gaussian (fluctuation) approximation
to demonstrate that the entropy production rate and power dissipation
are equal to each other in multiparticle overdamped nonconservative
nonequilibrium systems. The fluctuations of the nanoparticle constituents
of the optical matter (OM) systems studied, characterized through
their collective modes of motion, satisfied the Gaussian approximation.
Here, we report a type of collective mode and motion in a different
OM system that manifests strong non-Gaussian behavior. We show through
experiments and simulations that the collective motion is a pseudorotation
of the overdamped and nonconservative 8-silver-nanoparticle OM structure
in water. The OM system has D2 point group symmetry (in
2-dimensional space) and exists in a nonequilibrium steady state (NESS)
at various temperatures and solution ionic strengths. We developed
a weighted principal component analysis (w-PCA) and state-free nonreversible
VAMPnet (Variational Approach to Markov Process solved via neural
network) method to identify the collective modes of the nanoparticle
motion and the time scales of their dynamics, including pseudorotation.
We show that the confinement exerted by the outer four particles on
the inner four particles has a significant temperature-dependent impact
on the pseudorotation dynamics. We attribute the counterintuitive
change of the dynamics with increasing temperaturechanging
from monomodal Gaussian-like to bimodal with the same meanto
the implicit nature of the interparticle interactions and resultant
forces. The nonconservative force field determined at each time step
of our simulations is an intrinsic characteristic of these nonequilibrium
many-body interacting OM systems. We anticipate that our w-PCA+VAMPnet
method will be useful in studies of collective motions of complex
overdamped and nonconservative systems, and of particle dynamics in
other systems such as cluster liquids (e.g., liquid sulfur).