What keeps sandcastles standing?

Hornbaker, D. J.; Albert, Réka; Albert, István; Barabási, Albert László; Schiffer, P.

doi:10.1038/42831

Cited by 294 publications

(235 citation statements)

References 4 publications

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“…The meniscus formed between clay particles binds them together with capillary forces, increasing the unconfined yield strength of the granular assembly. The same physics is involved when wet sand is used to build sand castles, which would not be possible to make with dry sand [6][7][8][9][10].…”

Section: Introductionmentioning

confidence: 99%

Capillary adhesion at the nanometer scale

Cheng

Robbins

2014

Phys. Rev. E

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Molecular dynamics simulations are used to study the capillary adhesion from a nonvolatile liquid meniscus between a spherical tip and a flat substrate. The atomic structure of the tip, the tip radius, the contact angles of the liquid on the two surfaces, and the volume of the liquid bridge are varied. The capillary force between the tip and substrate is calculated as a function of their separation h. The force agrees with continuum predictions based on macroscopic theory for h down to ∼5 to 10 nm. At smaller h, the force tends to be less attractive than predicted and has strong oscillations. This oscillatory component of the capillary force is completely missed in the macroscopic theory, which only includes contributions from the surface tension around the circumference of the meniscus and the pressure difference over the cross section of the meniscus. The oscillation is found to be due to molecular layering of the liquid confined in the narrow gap between the tip and substrate. This effect is most pronounced for large tip radii and/or smooth surfaces. The other two components considered by the macroscopic theory are also identified. The surface tension term, as well as the meniscus shape, is accurately described by the macroscopic theory for h down to ∼1 nm, but the capillary pressure term is always more positive than the corresponding continuum result. This shift in the capillary pressure reduces the average adhesion by a factor as large as 2 from its continuum value and is found to be due to an anisotropy in the pressure tensor. The component in the plane of the substrate is consistent with the capillary pressure predicted by the macroscopic theory (i.e., the Young-Laplace equation), but the normal pressure that determines the capillary force is always more positive than the continuum counterpart.

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Section: Introductionmentioning

confidence: 99%

Capillary adhesion at the nanometer scale

Cheng

Robbins

2014

Phys. Rev. E

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“…Some light has been shed on this subject recently through experimental and theoretical studies which have shown that small quantities of liquid added to a sandpile comprised of rough spherical grains can cause sufficient intergrain adhesion so that the angle of repose after failure [3] and also the maximum static angle of stability of the sandpile before failure [4], known as the critical angle, θ c , greatly increase. A continuum theory that links stress criteria for the macroscopic failure of the wet pile to the cohesion between grains, which in turn was attributed to the formation of liquid menisci with radii of curvature that were determined by the surface roughness characteristics of individual grains, has provided a satisfying quantitative explanation of the increase of θ c with the liquid volume fraction and air-liquid interfacial tension [5,4].…”

Section: Introductionmentioning

confidence: 99%

Stability of monomer-dimer piles

et al. 2002

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We measure how strong, localized contact adhesion between grains affects the maximum static critical angle, θc, of a dry sand pile. By mixing dimer grains, each consisting of two spheres that have been rigidly bonded together, with simple spherical monomer grains, we create sandpiles that contain strong localized adhesion between a given particle and at most one of its neighbors. We find that tan θc increases from 0.45 to 1.1 and the grain packing fraction, Φ, decreases from 0.58 to 0.52 as we increase the relative number fraction of dimer particles in the pile, ν d , from 0 to 1. We attribute the increase in tan θc(ν d ) to the enhanced stability of dimers on the surface, which reduces the density of monomers that need to be accomodated in the most stable surface traps. A full characterization and geometrical stability analysis of surface traps provides a good quantitative agreement between experiment and theory over a wide range of ν d , without any fitting parameters. 45.70.Cc,61.43.Gt

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“…Studies of avalanches [6,7,8] and surface flows [9] in granular media have largely focused on dry grains. By wetting such media, however, one introduces controllable adhesive forces between the grains [10, 11] which lead to qualitatively new behavior [12,13,14,15,16,17,18]. In our previous work [13] we identified three fundamental regimes for the repose angle of wet granular materials as a function of the liquid content.…”

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confidence: 99%

Avalanche Dynamics in Wet Granular Materials

2002

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We have studied the dynamics of avalanching wet granular media in a rotating drum apparatus. Quantitative measurements of the flow velocity and the granular flux during avalanches allow us to characterize novel avalanche types unique to wet media. We also explore the details of viscoplastic flow (observed at the highest liquid contents) in which there are lasting contacts during flow, leading to coherence across the entire sample. This coherence leads to a velocity independent flow depth at high rotation rates and novel robust pattern formation in the granular surface.PACS numbers: 45.70.Ht, 45.70.Mg Avalanches and landslides [1,2,3] are among the most dramatic of natural catastrophes, and they also provide an evocative metaphor for a wide range of propagating breakdown phenomena from impact ionization in semiconductors [4] to magnetic vortex motion in superconductors [5]. On the other hand, the existence of avalanches, i.e. the sudden collapse of the system previously frozen into a high energy state, is a fundamental manifestation of the metastable nature of granular materials. Studies of avalanches [6,7,8] and surface flows [9] in granular media have largely focused on dry grains. By wetting such media, however, one introduces controllable adhesive forces between the grains [10, 11] which lead to qualitatively new behavior [12,13,14,15,16,17,18]. In our previous work [13] we identified three fundamental regimes for the repose angle of wet granular materials as a function of the liquid content. The granular regime at very low liquid contents is dominated by the motion of individual grains; in the correlated regime corresponding to intermediate liquid contents, a rough surface is formed by the flow of separated clumps; and the repose angle of very wet samples results from cohesive flow with viscoplastic properties.Here we report investigations of the avalanche dynamics and flow properties of wet granular materials, employing a rotating drum apparatus (a cylindrical chamber partly filled with a granular medium and rotated around a horizontal axis) [6,7]. At low rotation rates, the medium remains at rest relative to the drum while its surface angle is slowly increased by rotation, up to a critical angle (θ max ) where an avalanche occurs, thus decreasing the surface angle to the repose angle (θ r ). The flow becomes continuous at high rotation rates, but the transition between avalanching and continuous flow is hysteretic in rotation rate in dry media [6,19].Previous studies of cohesive granular media in a rotating drum [15,16,17] have focused on the surface angles of the medium before and after avalanches. For example, Quintanilla et al. recently performed a statistical analysis of avalanche size based on these angles, and showed that the average clump size increased with cohesion [20]. In our measurements, we focus instead on characterizing the dynamics of cohesive flow. We quantitatively investigate the flow dynamics during avalanches at different liquid contents by analyzing the time evolution of the avera...

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What keeps sandcastles standing?

Cited by 294 publications

References 4 publications

Capillary adhesion at the nanometer scale

Capillary adhesion at the nanometer scale

Stability of monomer-dimer piles

Avalanche Dynamics in Wet Granular Materials

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