On the impact of capillarity for strength at the nanoscale

TL;DR

This paper investigates how capillary forces at the nanoscale influence the strength of nanomaterials, emphasizing the dominant role of surface tension over surface stress through theoretical analysis and experimental validation.

Contribution

It clarifies the relative impact of surface tension and surface stress on nanoscale strength, demonstrating surface energy's significant contribution and challenging previous assumptions.

Findings

Surface tension significantly affects nanoscale strength.

Surface stress has a negligible impact according to the study.

Experimental data supports the theoretical prediction.

Abstract

The interior of nanoscale crystals experiences stress that compensates the capillary forces and that can be large, in the order of 1 GPa. Various studies have speculated on whether and how this surface-induced stress affects the stability and plasticity of small crystals. Yet, experiments have so far failed to discriminate between the surface contribution and other, bulk-related size effects. In order to clarify the issue, we study the variation of the flow stress of a nanomaterial while distinctly different variations of the two capillary parameters surface tension and surface stress are imposed under control of an applied electric potential. Our theory qualifies the suggested impact of $\textit{surface stress}$ as not forceful and instead predicts a significant contribution of the surface energy, as measured by the $\textit{surface tension}$. The predictions for the combined potential- and size dependence of the flow stress are quantitatively supported by the experiment. Previous suggestions, favoring the surface stress as the relevant capillary parameter, are not consistent with the experiment.