We experimentally study a monolayer of vibrated disks with a built-in polar asymmetry which enables them to move quasibalistically on a large persistence length. Alignment occurs during collisions as a result of self-propulsion and hard core repulsion. Varying the amplitude of the vibration, we observe the onset of large-scale collective motion and the existence of giant number fluctuations with a scaling exponent in agreement with the predicted theoretical value.

We study the spontaneous motion, binary collisions, and collective dynamics of "polar disks", i.e. purpose-built particles which, when vibrated between two horizontal plates, move coherently along a direction strongly correlated to their intrinsic polarity. The motion of our particles, although nominally three-dimensional and complicated, is well accounted for by a two-dimensional persistent random walk. Their binary collisions are spatiotemporally extended events during which multiple actual collisions happen, yielding a weak average effective alignment. We show that this wellcontrolled, "dry active matter" system can display collective motion with orientationally-ordered regions of the order of the system size. We provide evidence of strong number density in the most ordered regimes observed. These results are discussed in the light of the limitations of our system, notably those due to the inevitable presence of walls.

Vibrated polar disks have been used experimentally to investigate collective motion of driven particles, where fully-ordered asymptotic regimes could not be reached. Here we present a model reproducing quantitatively the single, binary and collective properties of this granular system. Using system sizes not accessible in the laboratory, we show in silico that true long-range order is possible in the experimental system. Exploring the model's parameter space, we find a phase diagram qualitatively different from that of dilute or point-like particle systems.PACS numbers: 05.70. Ln, 64.60.Cn, 05.65.+b Collective motion in driven or self-propelled particle systems is a topic of recent interdisciplinary interest [3][4][5][6]. Within physics, following the works of Vicsek et al. [7,8] and Toner and Tu [9][10][11], most progress was achieved by studying microscopic models [7,8,[12][13][14][15][16][17][18][19][20][21][22][23][24] and their continuous descriptions [9][10][11][25][26][27][28][29][30][31][32][33][34][35][36][37][38]. For the simplest situation in which the surrounding fluid can be neglected ("dry flocking") and the sole interaction is some local effective alignment, a picture of basic universality classes has emerged, which connects models similar to the Vicsek model [7] to continuous theories of the Toner-Tu type [9][10][11][25][26][27][28][29][30][31]. Among the landmark results are the possibility of true long-range orientational order in two dimensions, the generic presence of strong, long-range correlations [9][10][11]25] and/or spontaneously segregated dense and highly ordered nonlinear structures in moving, ordered, fluctuating phases [29,31].These numerical and theoretical results still largely lack experimental confirmation. This is mostly due to the fact that decisive experimental tests must be performed on large numbers of objects under controlled conditions. The advent of experiments using purified proteins (motors, filaments, etc.) offers a promising playground [39][40][41][42][43], but another line of attack, for dry flocking, is to build on the experience of the granular physics community, and to shake man-made objects [1,2,[44][45][46][47][48]. Recently, some of us have designed and studied the collective motion of vibrated polar disks, i.e. millimeter-size objects with a built-in oriented axis and a circular top metallic part rendering the particles isotropic with respect to collisions ( Fig. 1a,b; [1, 2]). Large-scale collective streams and anomalous, "giant" number fluctuations were reported in collections of approximately a thousand disks moving on a carefully vibrated plate. Unfortunately, in this experiment -as in others involving man-made objects [44][45][46][47][48] -the number of particles used was still too small to reach asymptotic results. Moreover, the most ordered regimes that could be explored were probably close to the onset of collective motion, making it impossible to disentangle the properties of the ordered moving phase from those of the order-disorder transition.In ...