Soft matter physics of the ground beneath our feet.

The soft part of the Earth's surface - the ground beneath our feet - constitutes the basis for life and natural resources, yet a general physical understanding of the ground is still lacking. In this critical time of climate change, cross-pollination of scientific approaches is urgently needed to better understand the behavior of our planet's surface. The major topics in current research in this area cross different disciplines, spanning geosciences, and various aspects of engineering, material sciences, physics, chemistry, and biology. Among these, soft matter physics has emerged as a fundamental nexus connecting and underpinning many research questions. This perspective article is a multi-voice effort to bring together different views and approaches, questions and insights, from researchers that work in this emerging area, the soft matter physics of the ground beneath our feet. In particular, we identify four major challenges concerned with the dynamics in and of the ground: (I) modeling from the grain scale, (II) near-criticality, (III) bridging scales, and (IV) life. For each challenge, we present a selection of topics by individual authors, providing specific context, recent advances, and open questions. Through this, we seek to provide an overview of the opportunities for the broad Soft Matter community to contribute to the fundamental understanding of the physics of the ground, strive towards a common language, and encourage new collaborations across the broad spectrum of scientists interested in the matter of the Earth's surface.

Microscopic origin of granular fluidity: An experimental investigation.

Granular fluidity has been central to the development of nonlocal constitutive equations, which are necessary for characterizing nonlocal effects observed in experimental granular flow data. However, validation of these equations has been largely computational due to challenges in laboratory experiments. Specifically, the origin of the fluidity on a microscopic, single-particle level is still unproven. In this work, we present an experimental validation of a microscopic definition of granular fluidity, and show the importance of basal boundary conditions to the validity of the theory.

Stability of the Interaction between Two Sand Dunes in an Idealized Laboratory Experiment.

Sand dunes, which arise spontaneously due to the dynamical interplay between a sedimentary interface and a fluid flow, are one of the most famous examples of emergence in a geological system. The large scale organization of a dune field is believed to be controlled by pairwise (either remote or direct) dune-dune interactions. Recent studies have shown that remote long-range feedback is closely related to the turbulent wake structure forming downstream of a dune. Here, we study the stability of an idealized two-dune system arising as a consequence of such remote, wake-induced interactions. The system is realized in a subaqueous quasi-2D laboratory experiment and the results are compared with a qualitative dynamical systems model. Despite its simplicity, the system exhibits rich dynamical behavior. In particular, we show that, depending on the parameter regime, the dune-dune feedback can either stabilize or destabilize the symmetric dune configuration, and we demonstrate the existence of an asymmetric attracting state coupling dunes of different sizes.

Wake Induced Long Range Repulsion of Aqueous Dunes.

Sand dunes rarely occur in isolation, but usually form vast dune fields. The large scale dynamics of these fields is hitherto poorly understood, not least due to the lack of longtime observations. Theoretical models usually abstract dunes in a field as self-propelled autonomous agents, exchanging mass, either remotely or as a consequence of collisions. In contrast to the spirit of these models, here we present experimental evidence that aqueous dunes interact over large distances without the necessity of exchanging mass. Interactions are mediated by turbulent structures forming in the wake of a dune, and lead to dune-dune repulsion, which can prevent collisions. We conjecture that a similar mechanism may be present in wind driven dunes, potentially explaining the observed robust stability of dune fields in different environments.