Spreading of complex fluids with a soft blade.

The spreading of complex fluids is a part of our everyday life: we spread jam, cosmetics, paint, etc. It is also central in industry, mostly to make paper or protective coatings. Here, we consider the spreading of polymer solutions with an elastic blade that is soft enough to deform during the spreading (similarly to a brush, a finger or rubber squeegees). We compare fluids with identical shear-thinning rheology, but which generates – or not – normal stresses when flowing. This allows us to disentangle the effects of viscosity, shear-thinning and normal stresses on the spreading process. We thus reveal two counter-intuitive results: first, shear-thinning does not significantly modify the film thickness compared with an equivalent Newtonian fluid, but increases the force that has to be applied to spread the liquid. In addition, the geometry of the experiment can make normal stresses negligible compared with the viscous forces even if they are dominant over the shear stress. We combine experiments, a numerical model and a scaling analysis to evidence and explain these observations.