Phase transition in an out-of-equilibrium monolayer of dipolar vibrated grains.

We report an experimental study on the transition between a disordered liquidlike state and an ordered solidlike one, in a collection of magnetically interacting macroscopic grains. A monolayer of magnetized particles is vibrated vertically at a moderate density. At high excitation a disordered, liquidlike state is observed. When the driving dimensionless acceleration Γ is quasistatically reduced, clusters of ordered grains grow below a critical value, Γ(c). These clusters have a well-defined hexagonal and compact structure. If the driving is subsequently increased, these clusters remain stable up to a higher critical value, Γ(l). Thus, the solid-liquid transition exhibits a hysteresis cycle. However, the lower onset Γ(c) is not well defined as it depends strongly on the acceleration ramp speed and also on the magnetic interaction strength. Metastability is observed when the driving is rapidly quenched from high acceleration, Γ>Γ(l), to a low final excitation, Γ(q). After this quench, solid clusters nucleate after a time lag, τ(o), either immediately (τ(o)=0) or after some time lag (τ(o)>0) that can vary from seconds up to several hundreds of seconds. The immediate growth occurs below a particular acceleration value, Γ(s) (~/<Γ(c)). In all cases, for t≥τ(o) a solid cluster's temporal growth can be phenomenologically described by a stretched exponential law. The evolution of the parameters of this law as a function of Γ(q) is presented and the values of fitted parameters are discussed.

High-speed tracking of rupture and clustering in freely falling granular streams.

Thin streams of liquid commonly break up into characteristic droplet patterns owing to the surface-tension-driven Plateau-Rayleigh instability. Very similar patterns are observed when initially uniform streams of dry granular material break up into clusters of grains, even though flows of macroscopic particles are considered to lack surface tension. Recent studies on freely falling granular streams tracked fluctuations in the stream profile, but the clustering mechanism remained unresolved because the full evolution of the instability could not be observed. Here we demonstrate that the cluster formation is driven by minute, nanoNewton cohesive forces that arise from a combination of van der Waals interactions and capillary bridges between nanometre-scale surface asperities. Our experiments involve high-speed video imaging of the granular stream in the co-moving frame, control over the properties of the grain surfaces and the use of atomic force microscopy to measure grain-grain interactions. The cohesive forces that we measure correspond to an equivalent surface tension five orders of magnitude below that of ordinary liquids. We find that the shapes of these weakly cohesive, non-thermal clusters of macroscopic particles closely resemble droplets resulting from thermally induced rupture of liquid nanojets.