Session U6: Physics of Slip Phenomena at Liquid/Solid Interfaces

Slip Behavior in Liquid Nanoscale Films: Influence of Molecular Ordering, Wall Roughness and Patterned Surface Energy

The development of micro- and nanofluidic devices for actuation of liquid films, drops and bubbles requires detailed knowledge of the interfacial forces affecting transport. The small dimension size guarantees that all tranport properties are strongly dominated by boundary effects. The large surface to volume ratios, however, also cause significant frictional losses which can be reduced by generating slippage at the liquid-solid interface. Slippage can be enhanced by surface chemical treatments, textured substrates and nucleation of nanobubbles. High molecular weight polymers also generate large slip lengths, defined as the distance within the solid phase where the extrapolated flow velocity vanishes. While hydrodynamic analyses are useful in providing a continuum description of fluidic response at the microscale, molecular dynamics (MD) simulations offer detailed resolution of the molecular behavior near chemically or topologically modified surfaces, a necessity in constructing nanofluidic devices. In this talk we show how the slip length in nanoscale liquid films is affected by the amplitude and wavelength of surface roughness. We also consider periodic variations in the liquid-solid interaction potential mimicking regions of no-shear and no-slip, as with surfaces covered by nanobubbles. A detailed comparison between hydrodynamic predictions and MD simulations elucidates what geometric and molecular parameters govern the slip length at different length scales. Excellent agreement is obtained when the system size is about an order of magnitude larger than the molecular size. We end this talk with discussion of a simplified model for predicting the dynamic exponent observed in the MD simulations for the power law increase in slip length with shear rate. These studies clearly pinpoint the molecular origin of the dynamic exponent and help explain the different slip laws expected for liquid versus gas flow.   

Low Friction Flow of Liquid at Smooth and Nanopatterned Interfaces

With the recent important development of microfluidics systems, miniaturization of flow devices has become a real challenge. Microchannels, however, are characterized by a large surface to volume ratio, so that surface properties strongly affect flow resistance in submicrometric devices. Although the no-slip boundary condition used for describing simple liquids flows at a macroscopic scale is very robust, it is now admitted that simple liquids may undergo substantial slip on solid surfaces, which cannot be neglected at the scale of tenth of micrometers. However, experimental results on this topic are still controversial~: slip effects reported vary quantitatively (over order of magnitudes) as well as qualitatively (regarding their linear or non-linear variation with the shear rate), without clearcut relation with expected relevant parameters for interfacial hydrodynamics, i.e. liquid-surface interactions and surface roughness. We first report an accurate determination of what we expect to be an \textit{intrinsic} slip length of water and organic solvants on smooth hydrophilic and hydrophobic surfaces. This boundary slip is well defined, does not depend on the scale of investigation (from 1 to several hundreds of nanometers) neither on shear rate (up to 5.10$^{3}$ s$^{-1})$. On smooth highly hydrophobic surfaces, the magnitude of slip is 20 nm, in good agreement with theory and numerical simulations. We then present results showing that the concerted effect of wetting properties and surface roughness may considerably reduce friction of the fluid past the boundaries. The slippage of the fluid is shown to be drastically reduced by using surfaces that are patterned at the nanometer scale. This effect occurs in the regime where the surface pattern is partially dewetted, in the spirit of ``~superhydrophobic' effect that has been discovered at the macroscopic scale. Our results show that in contrast to the common belief, surface friction may be reduced by surface roughness. They also open the possibility of a controlled realization of the ``~nanobubbles~'' that have long been suspected to play a role in interfacial slippage.   

Polymer/Nonpolymer Interactions and Apparent Wall Slip During Flow at High Stresses: An Historical Perspective

The no-slip boundary condition is a valuable empiricism derived from 19$^{th}$ Century experiments on low molar-mass liquids. Data that suggest deviations from the no-slip condition have long been available, but convincing evidence came only through experiments with entangled molten polymers, where the molecular scale over which slip might occur is large enough to result in macroscopic effects. The mechanisms for apparent slip in entangled polymers remain unclear; there is evidence to support both adhesive failure at the melt/metal interface and cohesive failure within the entangled melt. This talk will provide an historical overview and address critical experiments.   

Liquid Slippage over a Hydrophobic Surface: The Effect of Nanobubbles and Nanoroughness.

Micro and nanofluidic devices for manipulating fluids are widespread and are finding uses in many scientific and industrial contexts. Their design and small length scale introduce new research questions and themes to consider, first of all the impact of surface phenomena in controlling the flow. The present talk focuses on the combined effect of roughness and (partial) wettability on the flow, the circumstances that lead to interesting modification of old hydrodynamic problems and new flow responses. New high-precision quantitative methods based on confocal and atomic force microscopy - double focus confocal fluorescence cross correlation and thin film drainage measurements - are detailed. Also covered is the design of the model surfaces with the controlled nanoroughness and wetting properties. A discussion of the theoretical models suggested for a description of experimental configurations is given. Special emphasis is given to the analysis of experimental data. This covers the formation and role of nanobubbles, a contribution from surface forces into accelerating the flow, an interplay between nanoroughness and slippage, and more.   

Slippage, Cavitation and Sharkskin in Polymer Melts

Slippage in polymeric materials has been a subject of intense interest for three primary reasons. First, it is strongly interconnected with extrusion instabilities that commonly occur in polymer manufacturing (sharkskin, gross melt fracture, stick-slip). Second, the effect can be quite strong, the magnitude of slippage can become an appreciable fraction of the largest velocity of the flow. Third, molecular scale theoretical models have been developed that relate the slippage to shear stress, polymer molecular weight and polymer-surface interactions, all of which the experimentalist can control. We have utilized near-field velocimetry to demonstrate slippage within the first 100 nm from a solid wall and found a stress dependent transition from weak to strong slippage as well as a dependence on interfacial interactions. We have also shown that slippage can occur at a polymer-polymer interface when the interaction between them is weak. Allowing slippage at a polymer-polymer interface dramatically reduces the undesirable flow instability known as sharkskin. In the case of mixed flow boundary conditions, we have observed that the polymer can cavitate at the wall.