Anomalous friction at a 'Janus interface': the possible role of blisters

When a surface (glass, mica...) is silanized, the resulting hydrophobic sheet may be poorly stuck to the solid and 'blisters' can occur in water (without any gas phase). If a second, hydrophilic, surface is lying (in water) at a small distance D above, the blister may stick to the upper side, giving (a) an attraction between the walls (b) a transient mechanical bridge opposing shear. Assuming a Poisson distribution for the blister area, we predict a mechanical relaxation spectrum with a temperature dependent exponent beta. If beta similar to 1/2, this could account for some of recent results by Granick and coworkers on 'Janus' systems. It cannot, however, account for similar observations by the same group using thiol monolayers on silver, where the thiol/silver binding is usually considered as strong. (C) 2002 Elsevier Science B.V. All rights reserved.