The fluidity of lipid membranes is an essential property for their biological function. Here we use a dynamic surface forces apparatus to probe flow at the nanoscale on supported phospholipid bilayers and we present direct mechanical measurements of their fluidity. We show that gel-phase DPPC bilayers deposited on glass behave as a rigid surface for the flow, whereas fluid DOPC bilayers are mobile and slide with a very low friction coefficient. These findings provide new perspectives for the investigation of the lubricant properties of phospholipid bilayers and for the design of biomaterial and biomedical devices of low frictional properties.