Materials Science

A Measured Flow

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Science  26 Oct 2007:
Vol. 318, Issue 5850, pp. 535
DOI: 10.1126/science.318.5850.535b

Measurement of the flow of thin films is complicated by the coupling of frictional forces and the driving force (pressure) at the molecular level. To this end, Xu et al. designed a set of brush like polymers, with a flexible backbone connected to a dense shell of rigid side chains. When the polymers are driven across a substrate, the reduced interaction of side groups with the substrate causes the backbone to coil, and the extent of this compacting can be used to gauge local variations in film pressure. Conformational changes can also reveal the friction coefficient at the substrate. The authors tracked polymer spreading on mica or graphite using atomic force microscopy (AFM) to probe the local chain conformations. The friction coefficient showed strong humidity dependence on the hydrophilic mica substrate, though not on hydrophobic graphite. The limitations of AFM detection notwithstanding, the authors envision a range of situations where these polymer probes could be useful pressure sensors. — MSL

Adv. Mater. 19, 2930 (2007).

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