3D-printed biomimetic surface structures with abnormal friction properties

Our daily experiences suggest that friction would increase when a sliding interface is pressed harder. At the nanoscale, however, recent experiments showed that friction on chemically modified graphite can decrease with increasing normal load due to atomic interlayer delamination. In this paper, we report that, through rational design of surface components, the friction of a macroscopic surface can also counterintuitively decrease with increasing normal load, resulting in a so-called macroscopic negative friction coefficient. This unusual feature is enabled by the coupling of contact pressure and deformation of the microstructured surface, which is achieved via two distinct microscale architectures, referred to as ‘lollipop’ and inverted ‘Y’ structures respectively. This work offers a novel strategy for designing meta-surfaces that possess unusual tribological properties, which may eventually lead to revolutionary engineering applications.