Spreading of droplets of complex fluids

The behaviour of Newtonian fluids during spreading onto smooth solid substrates is well known. It results from the competition between viscous dissipation and capillary driving forces yielding Tanner's law. We study how a shear-rate dependence of the viscosity and surfactant addition (acting on the capillary forces) affect the spreading kinetics. In the case of shear-thinning fluids, the spreading behaviour does not deviate strongly from Tanner's law. This is quite surprising considering that the viscosity decreases because of the shear undergone during spreading. Viscous dissipation getting smaller, the spreading rate should be enhanced, which it is not observed experimentally. The robustness of Tanner's law is furthermore surprising if one notes that the contact line singularity disappears due to the shear-dependent viscosity. On the other hand, we observe extremely fast spreading in the case of Trisiloxane surfactants: the radius of the drop grows linearly in time. This suggests that Marangoni effects are important for this "superspreading".