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

Nanorhéomètre pour l’étude des liquides confiné

International audienceNanorhéomètre pour la mesure des propriétés mécaniques sans contac

Real-time single analog output for quadrature phase interferometry

International audienceWe present a dynamic displacement sensor based on a quadrature phase interferometer, providing a real-time analog output of the differential displacement between two mobile surfaces. The sensor offers a sub-picometer resolution with a sensitivity essentially uniform over a distance range extending to several micrometers, and can be used to measure the amplitude and phase of very small oscillations in nano-mechanical testing. We demonstrate its use in nano-rheology, by studying the flow in nanometric liquid films at very small strain rates

Wall slip of complex fluids: Interfacial friction versus slip length

International audienc

A micro-nano-rheometer for the mechanics of soft matter at interfaces

International audienceWe present a nano-rheometer based on the dynamic drainage flow between a sphere and a plane from bulk regime to highly confined regime. The instrument gives absolute measurements of the viscosity of simple liquids in both regime. For complex fluids, the measurements involves the viscosity and the elastic modulus. The device operates on distances ranging over four orders of magnitude from 1 nanometer to 10 micrometers, bridging rheological properties from the macroscopic to the molecular scale. This allows to measure an hydrodynamic or visco-elastic boundary condition and to explore the causes of the boundary condition at the microscopic level

Wall slip of complex fluids: Interfacial friction versus slip length

International audienc

Large slippage and depletion layer at the polyelectrolyte/solid interface

The slippage of polymer solutions on solid surfaces is often attributed to a depletion layer whose origin, thickness, and interaction with the flow are poorly understood. Using a Dynamic Surface Force Apparatus we report a structural and nanorheological study of the interface between hydrolyzed polyacrylamide solutions and platinum surfaces. Polyelectrolyte chains adsorb on the surfaces in a thin charged layer, acting as a nonattractive wall for the bulk solution. We investigate the flow of the viscoelastic solution on the adsorbed layer from the nanometer to 10 micrometers, bridging microscopic to macroscopic properties. At distances larger than 200 nanometers, the flow is well described by an apparent slip boundary condition. At smaller distance the apparent slip is found to decrease with the gap. In contrast to the apparent slip model, we show that a 2-fluids model taking into account the finite thickness of depletion layers at the non-attractive wall describes accurately the dynamic forces over 4 spatial decades of confinement. Depletion layers are found to be an equilibrium property of the interface, independent on the flow and on the confinement. Their thickness is phenomenologically described by x + 2l D with x the correlation length of the semi-dilute solutions and l D the Debye length. We interpret this result in terms of screened repulsion between the charged adsorbed layer and the bulk polyions

Nanorhéomètre pour l’étude de fluides confinés des échelles macroscopiques vers les tailles moléculaires

International audienceWe present a unique rheometer allowing the study of fluid systems from the macrosco-pic scale to the microscopic scale. This rheometer developed from a surface forces apparatus geometry allows the equilibrium measurement of the mechanical response of the studied system. From this state of equilibrium, we impose a small harmonic perturbation of the gap between surfaces to measure the rheological response in linear regime. It is possible for continuous confinement from 10 micrometers to nanometer in a single experiment, the study of the transition from macrocopic to the microscopic. MOTS-CLÉS : rhéomètre, confinement, nanofluidique, SFANous présentons ici un rhéomètre original permettant l'étude de systèmes fluides de l'échelle macroscopique à l'échelle microscopique. Ce rhéomètre développé à partir d'une géo-métrie de machine à forces de surface permet la mesure de la réponse mécanique à l'équilibre du système étudié en fonction du confinement. À partir de cet état d'équilibre, nous imposons une petite oscillation de la distance entre les surfaces afin de mesurer la réponse rhéologique proche de l'équilibre. Cette mesure est possible pour des confinements variant de 10 micro-mètres au nanomètre permettant en continu et dans une même expérience, l'étude du passage du macrocopique au microscopique