Shear of confined perfluorinated molecules: Effect of side branching
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- Shear of confined perfluorinated molecules: Effect of side branching
- Cho, Yoon-Kyoung; Granick, Steve
- Issue Date
- ELSEVIER SCIENCE SA
- WEAR, v.200, no.1-2, pp.346 - 352
- To understand the dependence of nanorheology on molecular architecture for perfluorinated molecules, studies were performed using two lubricants commonly used in the magnetic storage industry. Experiments mimicked a single asperity by confining liquids to molecular spacings between atomically smooth mica and applying oscillatory shear forces. The force-distance profiles were compared. Large differences were obtained for the linear shear response. The elastic and dissipative shear moduli of a branched perfluoropolymer (Krytox-AZ) were comparable even at high loads, signifying that the fluid remained fluid-like over experimental frequencies. However, linear perfluoropolymer (Demmim S-20) was easily solidified. The nonlinear responses were also surprising. Previously, we (G. Reiter et al., Science 263 (1994) 1741) reported that the shear-induced transition from an elastic state (at rest) to a dissipative state (during sliding) was discontinuous and hysteric for various hydrocarbon fluids but not for the perfluorinated chain, perfluoroheptaglyme. However, the stick-slip transition was discontinuous, and even more hysteretic for the linear perfluoropolymer (Demnum S-20).
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