Brandeis in Aspen II: Physics of granular materials

This post is a companion to Brandeis in Aspen I, and describes a workshop attended by Bulbul Chakraborty and Aparna Baskaran at the Aspen Center for Physics. The format of Aspen workshops is different from the usual academic workshop.  Each day has just one or two talks, which are primarily self-organized on a volunteer basis among the participants.  The format is designed to encourage  physicists working in a particular area to share research findings and enable cross-pollination of ideas in an informal and loosely structured setting.

The workshop attended by Chakraborty and Baskaran was entitled “Fluctuation and Response in granular materials”. Granular materials are ubiquitous in nature and industry. Examples range from sand and other geological materials, food and consumer products, and pebble beds in nuclear reactors. Understanding and controlling the properties of granular materials impacts such diverse processes as oil recovery, nuclear pebble bed reactors, printing and copying, and pharmaceutical processing. Granular media pose difficult and unique scientific challenges that distinguish them from atomic, nano-scale, and colloidal materials. Being intrinsically out of thermal equilibrium, assemblies of grains readily become trapped in metastable states, are extremely sensitive to preparation conditions, and can have strongly time-dependent properties.  Relaxing the constraints of thermal equilibrium, however, offers an advantage by opening up possibilities for creating novel static and dynamic phases that have distinctive functional properties.

At Aspen, the one-on-one and small sub group interactions among the participants covered a wide range of topics that are at the forefront of materials research, however, the program as a whole primarily focused on two questions. The first question was: What do we understand about jamming of granular materials? Jamming is what occurs in everyday life when we are trying to get coffee beans out of a hopper and they suddenly stop flowing. We fix this by tapping on the hopper. But this same phenomenon when it happens in giant grain silos causes them to collapse. So, one of the challenges is to be able to predict jamming events. The role of the physicist here is to design and carry out experiments in minimal model systems and develop theoretical frameworks that lead to predictive models of observed phenomena. Statistical Mechanics provides a powerful theoretical tool to address this question and our own Professor Chakraborty is one of the leading experts in the theory of jamming. The participants at the workshop had several robust discussions on the current understanding of this phenomenon and theoretical and experimental challenges that remain to be addressed.

The second question that the workshop focused on was : How does a dense granular material behave when sheared? Granular materials are called rheological fluids in that they exhibit shear-thinning and shear thickening behavior. In everyday life, we are all familiar with shear thinning. When we squeeze a tube of toothpaste, we are shearing it and it flows onto our brush. But once on the brush it stays put. This behavior is called shear thinning. Understanding rheology of granular materials is important for diverse applications ranging from pharmaceutical processes to being able to print well. The participants discussed in detail the physics of sheared granular materials and shared insight obtained from theory, simulations and experiments.

All participants departed the workshop invigorated by the robust exchange of ideas, ready to address the challenges presented by these complex materials.

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