EPL, 81 (2008) 28002
DOI: 10.1209/0295-5075/81/28002

Cyclic motion and inversion of surface flow direction in a dense polymer brush under shear

M. Müller¹ and C. Pastorino<sup>2

1</sup>  Institut für Theoretische Physik, Georg-August Universität - Friedrich Hund-Platz 1, 37077 Göttingen, Germany
²  Departamento de Física, Centro Atómico Constituyentes, CNEA-CONICET - Av. Gral. Paz 1499, 1650 Pcia. de Buenos Aires, Argentina

mmueller@theorie.physik.uni-goettingen.de

received 31 August 2007; accepted in final form 8 November 2007; published January 2008
published online 4 December 2007

Abstract
Using molecular simulations, we study the properties of a polymer brush in contact with an explicit solvent under Couette and Poiseuille flow. The solvent is comprised of chemically identical chains. We present evidence that individual, unentangled chains in the dense brush exhibit cyclic, tumbling motion and non-Gaussian fluctuations of the molecular orientations similar to the behaviour of isolated tethered chains in shear flow. The collective molecular motion gives rise to an inversion of the hydrodynamic flow direction in the vicinity of the brush-coated surface. Utilising Couette and Poiseuille flow, we investigate to what extent the effect of a brush-coated surface can be described by a Navier slip condition.

PACS
83.50.Lh - Slip boundary effects (interfacial and free surface flows).
61.25.Hq - Macromolecular and polymer solutions; polymer melts; swelling.
82.20.Wt - Computational modeling; simulation.

© EPLA 2008