Short granular chain under vibration: Spontaneous switching of states

We study experimentally a short chain of N(≤8) loosely connected spheres bouncing against a horizontal surface that vibrates sinusoidally at intensity Γ. Distinct states are identified: a base state of uniform bouncing in-sync with the substrate prevails at low values of Γ, whereas increasing Γ can induce transitions to two excited states with appreciable storage of energy around one or both ends of the chain. We find that, in a transitional window of Γ, the chain can even switch spontaneously among states, resolving the mystery why different modes of motion can be initiated at the same position in our previous work along a gradient of vibration [Phys. Rev. Lett. 112, 058001 (2014)]. Preliminary interpretations on the parametric dependences and the optimal frequency window for seeing these transitions are offered, based on the microscopic and statistical evidence in our experiments up to date.