Non-Gaussian, transiently anomalous and ergodic self-diffusion of flexible dumbbells in crowded two-dimensional environments: coupled translational and rotational motions

Abstract

We employ Langevin-dynamics simulations to unveil non-Brownian and non-Gaussian center-of-mass self-diffusion of massive flexible dumbbell-shaped particles in crowded two-dimensional solutions. We also study the intra-dumbbell dynamics due to the relative motion of the two constituent elastically-coupled disks. Our main focus is on effects of the crowding fraction φ and the particle structure on the diffusion characteristics. We evaluate the time-averaged mean-squared displacement (TAMSD), the displacement probability-density function (PDF) and the displacement autocorrelation function (ACF) of the dimers. For the TAMSD at highly crowded conditions of dumbbells, e.g., we observe a transition from the short-time ballistic behavior, via an intermediate subdiffusive regime, to long-time Brownian-like spreading dynamics. The crowded system of dimers exhibits two distinct diffusion regimes distinguished by the scaling exponent of the TAMSD, the dependence of the diffusivity on φ, and the features of the displacement-ACF. We attribute these regimes to a crowding-induced transition from a viscous to a viscoelastic diffusion medium upon growing φ. We also analyze the relative motion in the dimers, finding that larger φ suppress their vibrations and yield strongly non-Gaussian PDFs of rotational displacements. For the diffusion coefficients D(φ) of translational and rotational motion of the dumbbells an exponential decay with φ for weak and a power-law D(φ) ∝ (φ - φ*)2.4 for strong crowding is found. A comparison of simulation results with theoretical predictions for D(φ) is discussed and some relevant experimental systems are overviewed.

In Silico Studies of Active Probe Dynamics in Crowded Media

“…In general, the motion of passive particles is governed by equilibrium thermal fluctuations owing to random collisions with the surrounding fluid molecules, 8 while the active particles are driven by the interplay between random fluctuations and self-propulsion that drives them out of equilibrium. 1 To describe the motion of active particles, researchers have proposed different models, such as active Brownian particles, 1,9 active Ornstein−Uhlenbeck particles, 10,11 and run and tumble particles.…”

“…. 8,12,15 At very short time, the P(Δx; τ) of the symmetric dumbbell curves merge for smaller F a , and the distributions become broader for larger values of F a (Figure 8). As time progresses, the dumbbell explores more space inside the gel, and P(Δx;τ) becomes broader with increasing activity.…”

“…, the cumulative angle θ is calculated from the simulation; therefore, the angle difference is no longer bounded by 2π. 8 A chemically symmetric dumbbell with both halves sticky (ϵ = 1) to the monomers of the polymer gel is considered and investigated for the effect of self-propulsion on its transport inside the polymer gel. The self-propulsion always results in faster dynamics with increasing F a , as shown in Figure 6.…”

“…Recently, Klett et al studied the self-diffusion of flexible passive dumbbells in crowded twodimensional solutions. 8 They focused on the effects of the crowding fraction and of the particle structure on the diffusion characteristics. They reported that the crowded system of dimers exhibits two distinct diffusion regimes attributed to the crowding-induced transition from a purely viscous to a viscoelastic behavior of the diffusion environment upon increasing crowding fraction.…”

“…They reported that the crowded system of dimers exhibits two distinct diffusion regimes attributed to the crowding-induced transition from a purely viscous to a viscoelastic behavior of the diffusion environment upon increasing crowding fraction. 8 We consider a rigid dumbbell in the polymer network, and hence our model differs from their system in terms of the dumbbell properties and density of crowders.…”