SOFT MATTER IN A TIGHT SPOT - NANORHEOLOGY OF CONFINED LIQUIDS AND BLOCK-COPOLYMERS

The linear frequency-dependent shear theology and force-distance profi les of molecularly-thin fluids of very different structure were contra sted: a globular molecule octamethylcyclotetrasiloxane (OMCTS), branch ed alkanes (3-methylundecane and squalane), and a polymer brush in nea r-theta solution (polystyrene-polyvinylpyridine). In each case the dat a suggest a prolongation of the longest relaxation time (tau(1)) with increasing compression. At frequencies omega > 1/tau(1) the shear resp onse was ''solid-like'', but at omega < 1/tau(1) it was ''liquid-like' '. OMCTS under mild compression exhibited seeming power-law viscoelast ic behavior with G'(omega) approximate to G ''(omega) over a wide freq uency range. Of the branched-molecule fluids, 3-methylundecane exhibit ed oscillatory force-distance profiles; this confirms prior computer s imulations. But squalane (6 pendant methyl groups in an alkane chain 2 4 carbons long) showed one sole broad attractive minimum. Polymer brus hes in a near-theta solvent exhibited changes qualitatively similar to those OMCTS, in particular, a smooth progression of longest relaxatio n time, generating a transition from ''liquid-like'' to ''solid-like'' shear theology with decreasing film thickness. The common trend of sh ear response in these systems, in spite of important differences in mo lecular structure and force-distance profiles, is emphasized.