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).