Dynamic wetting of dilute polymer solutions: the case of impacting droplets.

The moving contact line of a dilute polymer solution that advances over, or recedes from a solid substrate, is a fundamental problem of fluid dynamics with important practical applications. In particular, the case of droplets impacting on hydrophobic surfaces received much attention in the recent past. Experiments show that while the advancing motion proceeds as with Newtonian liquids, recession is severely inhibited. This phenomenon was initially understood as an effect of elongational viscosity, which was believed to cause large energy dissipation in the fluid. Later on, a hydrodynamic mechanism was proposed to suggest that the slowing down of the contact line is due to non-Newtonian normal stresses generated near the moving droplet edge. Recent experiments however ruled out the role of elongational viscosity, showing that the fluid velocity measured inside the droplet during retraction is the same in water drops and polymer solution drops. Direct visualization of fluorescently stained λ-DNA molecules showed that polymer molecules are stretched perpendicularly to the contact line as the drop edge sweeps the substrate, which suggests an effective friction arises locally at the drop edge, causing the contact line to slow down.

Dilatancy in the flow and fracture of stretched colloidal suspensions.

Concentrated particulate suspensions, commonplace in the pharmaceutical, cosmetic and food industries, display intriguing rheology. In particular, the dramatic increase in viscosity with strain rate (shear thickening and jamming), which is often observed at high-volume fractions, is of practical and fundamental importance. Yet, manufacture of these products and their subsequent dispensing often involves flow geometries substantially different from that of simple shear flow experiments. In this study, we show that the elongation and breakage of a filament of a colloidal fluid under tensile loading is closely related to the jamming transition seen in its shear rheology. However, the modified flow geometry reveals important additional effects. Using a model system with nearly hard-core interactions, we provide evidence of surprisingly strong viscoelasticity in such a colloidal fluid under tension. With high-speed photography, we also directly observe dilatancy and granulation effects, which lead to fracture above a critical elongation rate.

Effect of polymer additives on the wetting of impacting droplets.

When a droplet of water impacts a hydrophobic surface, the drop is often observed to bounce. However, for about 10 years it has been known that the addition of very small quantities (approximately 100 ppm) of a flexible polymer such as poly-(ethylene oxide) can completely prevent rebound. This effect has for some time been explained in terms of the stretching of polymer chains by a velocity gradient in the fluid, resulting in a transient increase in the so-called "extensional viscosity." Here we show, by measuring the fluid velocity inside the impacting drop, that the extensional viscosity plays no role in the antirebound phenomenon. Using fluorescently labeled lambda DNA we demonstrate that the observed effect is due to the stretching of polymer molecules as the droplet edge sweeps the substrate, retarding the movement of the receding contact line.

Wicking with a yield stress fluid.

This work presents an experimental investigation of the flow of a model yield stress fluid (yield stresses between 5 and 21 Pa) driven by capillarity in horizontal glass tubes with diameters ranging from 0.46 to 1.5 mm. It is shown that the liquid penetration stops after typically a few centimeters. The results disagree with a simple model based on the balance between capillary and frictional forces, suggesting that the yield stress fluid constitutive equation may not be valid in the immediate vicinity of the wall. Scaling is proposed with respect to a dimensionless number comparing the yield stress with the capillary pressure.

Impact of shear-thinning and yield-stress drops on solid substrates.

The behaviour of shear-thinning and viscoplastic fluid drops impacting on solid substrates as compared with that of Newtonian drops is studied experimentally by means of high-speed imaging. In particular, the investigation focuses on the morphological aspects of drops after inertial spreading. While the impact morphology of drops of shear-thinning fluids turns out to be qualitatively similar to that of Newtonian fluids, viscoplastic drops can exhibit central drop peaks at the end of inertial spreading. The influence of yield-stress magnitude on drop impact behaviour is qualitatively established by measuring the size of these central drop peaks. The peaks indicate that drop deformation during impact is localized: within a threshold radius, shear-stress effects will not be large enough in magnitude to overcome yield-stress effects, and therefore viscoplastic fluids within this region will not deform from the drop shape prior to impact.

A note on the effects of liquid viscoelasticity and wall slip on foam drainage.

A foam drainage model is modified to attempt the description of foams made of viscoelastic liquids (such as polymer solutions). In particular, the standard approach to foam drainage dominated by viscous dissipation in Plateau borders is modified to take into account the elastic forces acting on the fluid within Plateau borders, and slipping of the polymer solution at the walls of Plateau borders. It is shown that, in the case of forced drainage, the resulting differential equations reduce to the same one obtained in the case of Newtonian liquids, which is satisfied by the well-known solitary wave solution. According to these results, the fluid elasticity has no effect on the drainage velocity, while the wall slip assumption is compatible with recent observations showing a faster drainage velocity in the forced drainage experiment.