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Deutsche Physikalische Gessellschaft IOP Institute of Physics

On the role of capillary instabilities in the sandcastle effect

Binyang Du1,2, Alexander Martin König1 and Diethelm Johannsmann1,3

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Using acoustic resonators, we have investigated the mechanical stiffness of contacts between rough surfaces. In the first part, the underlying acoustic model is validated with experiments showing a transition from elastic to inertial loading. The second part is concerned with the increase in contact stiffness induced by transient exposure to a humid environment. A novel mode of surface deformation is proposed, which builds on a capillary instability. Under certain conditions, a slight decrease in the mean distance between the two surfaces may induce a rather strong increase in capillary attraction, while leaving the elastic forces of repulsion almost constant. The thus-created negative differential spring constant induces a collapse of the gap in-between neighboring load-bearing asperities. The initial decrease in distance may either be induced by local asperity creep or by distortions of the contacting surfaces on a larger scale, which improve the interlock of the asperities at a small cost of strain energy.


PACS

68.35.Gy Mechanical properties; surface strains

68.35.B- Structure of clean surfaces (and surface reconstruction)

62.20.D- Elasticity

81.40.Jj Elasticity and anelasticity, stress-strain relations

81.40.Lm Deformation, plasticity, and creep

62.20.F- Deformation and plasticity

Subjects

Surfaces, interfaces and thin films

Condensed matter: structural, mechanical & thermal

Dates

Issue 5 (May 2008)

Received 12 March 2008

Published 13 May 2008



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