Chirality transitions in a system of active flat spinners

Diffusive regimes in a two-dimensional chiral fluid

“…Most importantly, the actual control parameter of chiral diffusion is chiral flow vorticity ω, which is supported by a strong experimental evidence in the universal curves of the diffusion coefficients vs. ω in Figure 3. It actually makes sense that diffusion does not depend on particle (chiral) activity alone, since one could expect that the underlying chiral fluid flow has a strong impact on diffusion processes, and contrary to what is suggested in a previous work, fluid chirality is not controlled by particle chiral activity [12]. Furthermore, we have clearly seen that when represented vs. reduced particle activity, diffusion coefficients still vary and transition between the aforementioned three diffusive regimes we detected (see Figure 9 in the Supplementary Material file, for more detail).…”

“…More importantly, as we will see, the same twodimensional chiral fluid displays three distinct types of diffusive behavior: quasi-normal diffusion in the diagonal elements of the diffusion tensor, with vanishing odd diffusion coefficient, a strikingly strong superdiffusion in the diagonal elements with large odd diffusion, and subdiffusion with large odd diffusion. Furthermore, each of these three distinctive diffusive regimes appears to be closely related to the emergence of the respective chiral flow phases that have been recently found to appear as structural changes in the statistical correlations, at the particle dynamics level, take place [12].…”

“…We performed experiments with a prototypical chiral fluid system composed of a set of identical disks-shaped particles, each provided with a set of 14 evenly spaced, equally tilted blades, which yields an intrinsic chirality to the particles and hence to their stationary base flows [12]. The disks are driven by turbulent air wakes (for a more complete description of the driving mechanism see [12]) produced to a homogeneous air upflow past the particles. This upflow is also responsible for the continuous particle spin, as it goes through the blades.…”

“…As explained in a previous work [12], the set of negatively spinning particles presents 3 different types of flow chirality, according to the relevant parameters in the system. These phases can be characterized by the sign of the global vorticity ω (i.e., the sign of the stationary vorticity averaged over all points in the system).…”

“…In Figure 1 we present, in different forms, experimental measurements of particle time-relative displacements, with the aim of illustrating the qualitatively different behaviors we found. In particular we present series of measurements (here, at a packing fraction φ ≡ N (σ/L) 2 = 0.45, where N is the total number of particles in the system) and varying ω (if the driving air flow is increased, the 3 distinctive phases gradually and consecutively appear [12]). First, Figure 1 (a) presents the ensemble mean squared displacement ∆r(t) 2 ≡ ∆x(t) 2 + ∆y(t) 2 , where ∆x(t) ≡ (1/N (t)) {t0} [x(t + t 0 ) − x(t 0 )] has been averaged, for each t, through all of the available N (t) initial states t 0 for each experiment.…”