Newtonian fluids were placed between molecularly smooth surfaces whose spacing was vibrated at spacings where the fluid responded as a continuum. Hydrodynamic forces agreed with predictions from the no-slip boundary condition only provided that flow rate (peak velocity normalized by spacing) was low, but implied partial slip when it exceeded a critical level, different in different systems, correlated with contact angle (surface wettability). With increasing flow rate and partially wetted surfaces, hydrodynamic forces became up to 2–4 orders of magnitude less than expected by assuming the no-slip boundary condition that is commonly stated in textbooks.

• Received 6 February 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.096105