Frédéric Restagno (Laboratoire de Physique des Solides, Orsay). Slip of polymeric liquids on a solid surface.

A viscous fluid flowing near a wall can dissipates energy through its viscosity or through slip at the wall due to the friction of the last layer of fluid on the solid surface. For simple fluids, the adhesion of the molecules at the wall is so large than the slip effect is completely negligible. This is the classical non slip boundary condition. De Gennes showed [1] that, assuming that the friction of a linear polymer melt on a surface is not different from the one of a simple fluid, an entangled melt would present a large slip at the surface.

Using a velocimetry technique based on fluorescent photobleaching [3], we systematically measured the slip length of different PDMS polymeric liquids on a non-adsorbing surface made of non-entangled short chains of PDMS grafted to the surface. We observed a constant slip length, independent of the shear rate. Moreover, we observed a linear variation of the slip length with the fluid viscosity, as predicted by de Gennes allowing a precise determination of the solid-liquid coefficient. We showed that the friction coefficient is equal to the one we previously measured by studying the solid friction of an elastomer of PDMS on the same surface [2]. This allowed to directly connect for the first time solid-solid friction to solid-fluid friction in a direct measurement [4] and to get a better understanding of the molecular mechanisms of friction of liquids. Finally, we will present some new results on the friction of polymer solutions [5] extending our understanfing of slip in the case of viscoelastic liquids.

[1]   de Gennes, CR Acad Sc Paris 288B, 219 (1979).

[2]   Cohen, Céline, et al. "Incidence of the molecular organization on friction at soft polymer interfaces." Soft Matter 7.18 (2011): 8535-8541.
[3]   Hénot, Marceau, et al. "Comparison of the slip of a PDMS melt on weakly adsorbing surfaces measured by a new photobleaching-based technique." Macromolecules 50.14 (2017): 5592-5598.

[4]   Hénot, Marceau, et al. "Friction of polymers: from PDMS melts to PDMS elastomers." ACS Macro Letters 7.1 (2018): 112-115.

[5]   Cross, Benjamin, et al. "Wall slip of complex fluids: Interfacial friction versus slip length." Physical Review Fluids 3.6 (2018): 062001.