Wall slip of complex fluids: interfacial friction or slip length?

Using a dynamic Surface Force Apparatus, we demonstrate that the notion of slip length used to describe the boundary flow of simple liquids, is not appropriate for viscoelastic liquids. Rather, the appropriate description lies in the original Navier's partial slip boundary condition, formulated in terms of an interfacial friction coefficient. We establish an exact analytical expression to extract the interfacial friction coefficient from oscillatory drainage forces between a sphere and a plane, suitable for dynamic SFA or Atomic Force Microscopy non-contact measurements. We use this model to investigate the boundary friction of viscoelastic polymer solutions over 5 decades of film thicknesses and one decade in frequency. The proper use of the original Navier's condition describes accurately the complex hydrodynamic force up to scales of tens of micrometers, with a simple "Newtonian-like" friction coefficient, not frequency dependent, and reflecting closely the dynamics of an interfacial depletion layer at the solution/solid interface.Comment: 7 pages, 5 figure

Friction mechanisms at polymer–solid interfaces

International audienceWe present series of experiments conducted on model systems consisting of smooth solid surfaces covered with end grafted polymer chains at controlled surface density, and put into contact with either it polymer melt or it crosslinked elastomer of the same chemical species, and devoted to the characterization of friction as a function of the molecular parameters of the interface. Two techniques were used to do so: Near-Field Laser Velocimetry. which allows us to directly measure the local velocity at the interface when the melt is sheared against the grafted surface, and friction force measurements in it JKR like apparatus in the case of elastomers. Results will be discussed in the framework of molecular models of chain interdigitation and chain pull-out mechanisms

Slip and friction mechanisms at polymer semi-dilute solutions / solid interfaces

The role of the polymer volume fraction, $\phi$, on steady state slippage and interfacial friction is investigated for a semi-dilute polystyrene solutions in diethyl phthalate in contact with two solid surfaces. Signicant slippage is evidenced for all samples, with slip lengths b obeying a power law dependence. The Navier's interfacial friction coecient, k, is deduced from the slip length measurements and from independent measurements of the solutions viscosity $\eta$. The observed scaling of k versus $\phi$ clearly excludes a molecular mechanism of friction based on the existence of a depletion layer. Instead, we show that the data of $\eta$($\phi$) and k($\phi$) are understood when taking into account the dependence of the solvent friction on $\phi$. Two models, based on the friction of blobs or of monomers on the solid surface, well describe our data. Both points out that the Navier's interfacial friction is a semi-local phenomenon

Effect of Surface Elasticity on the Rheology of Nanometric Liquids

International audienceThe rheological properties of liquids con ned to nanometer scales are important in many physical situations. In this paper we demonstrate that the long range elastic deformation of the con ning surfaces must be taken into account when considering the rheology of nanometric liquids. In the case of a squeeze- ow geometry, we show that below a critical distance Dc the liquid is clamped by its viscosity and its intrinsic properties cannot be disentangled from the global system response. Using nanorheology experiments, we demonstrate that picometer elastic de ections of the rigid con ning surfaces dominate the overall mechanical response of nanometric liquids con ned between solid walls

Chemical modification of PDMS surface without impacting the viscoelasticity: Model systems for a better understanding of elastomer/elastomer adhesion and friction

The influence of both viscoelastic and interfacial parameters on the surface properties of elastomers is difficult to study. Here, we describe a simple route to achieve surface modification of PDMS without impacting the viscoelastic properties of the bulk. PEG modified PDMS surfaces were synthesized by two step surface modification based on hydrosilylation. The covalent grafting of PEG on the surface has been evidenced by AFM and ATR-FTIR, and its effect on the hydrophilicity characterized by static and dynamic contact angle. The static water contact angle of the PEG-modified PDMS decreases from 110° (for unmodified PDMS) to 65°. Dynamic contact angles also show a significant decrease in both advancing and receding contact angles, along with a significant increase in the contact angle hysteresis, which can be related to an increase in the surface energy as estimated by JKR measurements. The viscoelastic properties of modified PDMS are found to be quantitatively comparable to those of the unmodified PDMS. This simple method is an efficient way to prepare model materials which can be used to get a better understanding of the exact contribution of the surface chemistry on surface properties of elastomers

Nanorheology with a Conventional Rheometer: Probing the Interfacial Properties in Compatibilized Multinanolayer Polymer Films

Measuring the viscoelastic behavior of polymers in the vicinity of a surface or under confinement is an experimental challenge. Simple rheological tests of nanolayered films of polyethylene/polyamide 6 compatibilized in situ during the coextrusion process enabled the probing of these interfacial properties. Taking advantage of the different melting points and of the multiplication of the number of interfaces, a drastic increase of dynamic moduli was reported when increasing the interphase volume fraction in the films. A solid-like behavior for the interphase was identified. The complex viscosity of nanolayered films as a function of angular frequency was quantitatively captured for all samples using a weighted mixing law of bulk and interphase viscosities, without additional adjusting parameters, highlighting the interfacial synergy developed in nanolayered polymer films