Confined sheared flows of hard and soft granular materials: Some challenges in tribology and fault mechanics

This paper belongs to a Focus Series dedicated to Challenges in Granular Matter. As such, it does not report any new finding, but rather offers an overview of the state of the art and of current and future challenges for the granular physics community. The focus is placed on the case of confined sheared flows of granular samples with particular complexities, such as soft, angular, or cemented grains, with applications in tribology and fault mechanics. We first explain why granular science may help to improve our understanding of dry friction in these two applications, and illustrate some recently unveiled aspects regarding the complexity of the problem. We then describe a few avenues for future research on the topic, focusing on the dialogue between simulations and experiments, on scale-related challenges, and on some pivotal computational aspects.