Shear-induced dilation of confined liquid films

We demonstrate that sheared molecularly-thin fluid films dilate at the poin t of stick-to-slip (which is the transition from static to kinetic friction ), indicating that density decreases when sliding occurs. This contrasts wi th incompressibility characteristic of bulk fluids when they are deformed. The magnitude of dilation was less than the size of the molecule and was la rger in a polymer system (large molecules) than for small-molecule fluids. The experiments employed a surface forces apparatus modified to measure, us ing piezoelectric methods, sub-angstrom variations of him thickness during dynamic shear excitations that were performed at rates too rapid to allow f luid to enter and exit the zone of shear contact during the period of shear excitation. To demonstrate generality of the dilation effect, the specific systems studied included nonpolar fluids whose complexity was varied (a gl obular-shaped molecule, OMCTS; a branched alkane, squalane; a tethered dibl ock copolymer, polyvinylpyridine-polybutadiene) and also an aqueous electro lyte, MgCl2 dissolved in water. Extensive analysis is also presented of the piezoelectric methods that were employed to detect volume changes too smal l to observe by the methods of multiple beam interferometry that are tradit ional for thickness measurement in a surface forces apparatus.