Non-linear resonance-like processes in confined driven granular alignments and energy harvesting

Energy extraction from one source and perhaps in tandem from multiple natural sources such as wind, ocean waves and perhaps geothermal resources with devices that are not susceptible to wear and tear remains an outstanding challenge. Here we propose a simple, strongly non-linear system which is potentially capable of being tuned to just about any driving frequency and amplitude. The system consists of an alignment of N elastic spheres that are barely in contact and are of fixed or variable radius and held between two ends. When a persistent driving force is imparted at an edge grain, the system can end up in an oversqueezed state. Since a mechanical system must respond to unbalanced forces, such an oversqueezed system must eventually dilate. Persistent presence of the driving force can then end up forcing the system to vacillate between the oversqueezed and dilated states. The frequencies associated with such breathing processes are non-universal, depending on the system parameters and the details of the driving force itself. This is why confined driven granular alignments can be made to respond to a wide range of frequencies and hence can turn out to be potentially useful tools for energy extraction. Here we briefly summarize our earlier work where we showed how these systems respond to a fixed driving force and then provide new results on the system response when time-dependent driving forces are involved. Needless to say, these systems may, in due course, be realized as both mechanical systems and as electrical circuits.