Correction to: Nature Materials https://doi.org/10.1038/s41563-019-0425-1, published online 22 July 2019

In the version of this article initially published, equation (1) in the Methods section, now reading $$F = \frac{4}{3}\frac{E}{{\left( {1 - v^2} \right)}}\sqrt {R\delta ^3}$$, was missing a delta symbol following “R”. The symbol has now been restored in the HTML and PDF versions of the article.