Deformation of solids with nanoscale pores by the action of capillary forces

We analyze the various strain measures by which nanoporous materials react to changes in their surface stress and surface stretch at the solid-pore interface. Results of continuum mechanics are given for model geometries, arrays of spheres and fibres as well as solids with spherical and cylindrical voids. The findings are supported by atomistic simulation. The results for the mean stress agree with predictions of the generalized capillary equation of Weissmueller and Cahn, and they are incompatible with the notion of Laplace pressure. By contrast, no general relation between the mean stress and the macroscopic dimensional change could be identified. Even though certain microstructures with quite different geometry exhibit strikingly similar behaviour, others differ significantly. The macroscopic dimensional change can be largely enhanced compared with the local strain of the nanoscale building blocks. This fact can be exploited to amplify the stroke of nanoporous metal actuators.