Nanoscale water capillary bridges under deeply negative pressure

Negative Laplace pressure inside water capillary bridges between two bodies contributes to the pull-off adhesion force that can be measured with the atomic force microscope (AFM). We measure the pull-off force between a nanoscale AFM tip and a silicon wafer in air and in ultrahigh vacuum (UHV). The difference gives us the capillary force contribution, which yields the pressure drop when divided by the foundation area of the bridge. We show that the pressure can be deeply negative, down to −160 MPa.