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Complex dynamics of driven granular solids: Physics and applications

Final Report Summary - COMGRANSOL (Complex dynamics of driven granular solids: Physics and applications)

Granular materials play an important role in many industrial and natural processes. Researchers are combining theoretical, computational and experimental techniques to both investigate the fundamental behaviour of these materials and design new acoustic devices.

The development of new materials often leads on to further technical innovations, as scientists seek to harness their properties to develop new technologies and devices. Granular crystals can act as important building blocks in materials development, while they also hold intrinsic scientific interest, both of which are key motivating factors behind the work of the Comgransol project. The main objectives of the project are on the one hand to achieve a better understanding of the physics of driven granular media. Then secondly, to design granular-based acoustic devices for the control of acoustic propagation. These granular materials are effectively collections of macroscopic grains; analysis of these grains and their behaviour within wider structures can lead to new insights in physics and materials science. Understanding the physics of contact forces in these materials enables us to develop a deeper understanding of wave physics in granular solids, and also of the physics of granular materials in general.

Granular solids
This area forms a central part of my project's overall agenda combining theoretical, computational and experimental techniques to gain new insights into the physics of granular materials. Experiments are being performed on metallic grains, such as stainless steel; there are two main reasons behind the decision to use this type of material. We are using external magnetic fields to magnetize the grains and thus design particular granular structures, so it is necessary to use magnetisable grains. On the other hand, we've found metallic grains with a very smooth surface, which is very important for us. The interaction of grains happens due to contact forces. Thus, the high quality of smoothness of their surface means we don't have to use more complicated processes. We are also looking at the behaviour of different granular structures. We investigate waveguides in different network configurations, such as branched waveguides, L-shaped waveguides, or two- dimensional crystals in different geometries, like square or honeycomb crystal geometries.

The great advantage of using these granular solids, in comparison to other solid structures, is the strong nonlinear response. This nonlinearity is derived from the geometry of these solids, from contact deformations of the particles. Our goal is to take advantage of this strong nonlinear response and combine it with the proper geometry of the structure – using different network configurations – to achieve an advanced level of control over wave propagation. Although adding nonlinearity makes the wave physics much more complicated, it also opens up the possibility of enabling more precise control of waves. There are a plethora of nonlinear phenomena like harmonic generation, soliton propagation and bifurcations, which can be used to help control waves. So having a medium with a strong nonlinear response can strongly enrich the control of waves.

We are studying both homogenous and heterogenous granular crystals as part of this project. From these foundations, we hope to learn more about the behaviour of different complex granular structures, like network of waveguides or two dimensional crystals, which could in future inform the design and development of advanced acoustic structures. For the moment, we are basing our studies on granular structures made of spherical particles in contact, but developing new, even more
complicated structures – using advanced 3D printing techniques – with preselected interaction forces could lead to significant advances in terms of the development of certain mechanical devices.

There is also a purely theoretical dimension to the project’s research. We are investigating questions of fundamental interest, such as how granular nonlinearities influence the disorder induced
localization, or how acoustic waves propagate in complex solid structures like polydispersed granular solids. In this case, we start with simplified models to answer the questions
we set ourselves, and then we progressively add different features that make the problem more and more difficult, but also more and more realistic. Hertzian contact law is sufficient to capture part of
the dynamics of these granular structures, yet this approach will not be able to fully answer more complex questions, so in future it will be necessary to consider additional nonlinear contact laws. More complicated contact laws should eventually be studied, especially when one studies the propagation of waves which are not longitudinal.

Perspectives
The results of this experimental research could hold wider relevance in terms of the project's computational work. More detailed computational methods could in future enable researchers to predict the properties of a metamaterial with a specific granular structure. Simplified models are used at the moment to capture the main physical wave phenomena in different types of granular structures. Other granular features though, like misalignments of the particles in waveguides, or rearrangements of particles under strong excitations, should also taken into consideration. The latter could lead either to mechanical failure of the structures, or could even be used for more advantageous wave control devices. One major application I envision for this research is in the design of advanced elastic devices for the control or elastic wave propagation. For example elastic waves and waveguides have been used widely in RF telecommunications. To move in this direction, we will need to scale down these granular structures to the microscopic regime.

This forms part of my future agenda, with plans to investigate microscopic granular structures consisting of microspheres, work which could be relevant to RF telecommunications applications. Research in the fundamental physics and emerging applications of granular crystals is flourishing, and I intend to continue his work in this area. I plan to extend these studies to other mechanical structures with complicated configurations that could be built using the advanced 3D techniques that are currently available.