Viewing Earth’s surface as a soft-matter landscape

Abstract

The Earth's surface is composed of a staggering diversity of particulate-fluid mixtures: dry to wet, dilute to dense, colloidal to granular, and attractive to repulsive particles. This material variety is matched by the range of relevant stresses and strain rates, from laminar to turbulent flows, and steady to intermittent forcing, leading to anything from rapid and catastrophic landslides to the slow relaxation of soil and rocks over geologic timescales. Geophysical flows sculpt landscapes, but also threaten human lives and infrastructure. From a physics point of view, virtually all Earth and planetary landscapes are composed of soft matter, in the sense they are both deformable and sensitive to collective effects. Geophysical materials, however, often involve compositions and flow geometries that have not yet been examined in physics. In this review we explore how a soft-matter perspective has helped to illuminate, and even predict, the rich dynamics of Earth materials and their associated landscapes. We also highlight some novel phenomena of geophysical flows that challenge, and will hopefully inspire, more fundamental work in soft matter.

Measuring landscape evolution from inception to senescence

“…The conditions at the CSM parallel many physical based models of idealized systems. It validates landscape evolution inferences from physical modelling 31 and supports the coupling between granular material physics and landscape change 45 .…”

Depth-dependent soil mixing persists across climate zones

“…Combined with termites and earthworms, it is not unreasonable that significant amounts of soil mixing could occur by this biotic, but not climate-sensitive, process. Although the focus on soil mixing is largely on bioturbation (2), a significant amount of mixing occurs by abiotic processes (3), and it is important to note that shear in a granular material always induces mixing (12,15,70). As our data cannot distinguish between specific processes, only the result of mixing, the relative efficacy of various soil mixing processes arises as an outstanding question.…”

Tuning sedimentation through surface charge and particle shape

“…(Manley et al, 2005), calcium carbonate of diameter 70 nm (Allain et al, 1995), PMMA spheres of diameter 0.37 µm (Starrs et al, 2002b), and kaolinite Attractive kaolinite suspensions appear to immediately form large aggregated clusters even at fairly low concentrations (φ 0 < 2%), which display characteristics of a soft solid. Kaolinite suspensions exhibit similar dynamics to colloidal gels formed by distinctly different surface interactions (Starrs et al, 2002a), suggesting that recent progress in the formation and rheology of idealized gels may be transferable to natural mud suspensions (Jerolmack & Daniels, 2019). The evolution of the sedimenting interface in clay gel indicates two distinct regimes of transitional collapse or consolidation, associated with dissipation of the excess interstitial fluid pressure followed by a creep regime associated with mechanical densification of the deposit.…”

“…Under continued sedimentation in a quiescent fluid, this "house of cards" structure eventually collapses to produce a consolidated mud deposit (Toorman & Berlamont, 1991;Dankers & Winterwerp, 2007;Bartlett et al, 2012;Teece et al, 2014). In the presence of shear, however, this "fluid mud" suspension may persist indefinitely in a quasi-stable state until some perturbation causes it to suddenly yield and flow (Traykovski et al, 2000;Heymann et al, 2002;McAnally et al, 2007;Mueller et al, 2010;Talling et al, 2012;Nie et al, 2020;Jerolmack & Daniels, 2019). It is hypothesized that fluid muds are a kind of particulate gel (Coussot, 2017;Dankers & Winterwerp, 2007;Talling et al, 2012;Ali & Bandyopadhyay, 2016), in which percolated clusters of aggregates form a soft, metastable solid at much lower values of φ than is observed for repulsive particles.…”