Spreading of non-Newtonian fluids and surfactant solutions on solid surfaces

The spreading of Newtonian fluids onto smooth solid substrates is well known; the speed of the contact line is given by a competition between capillary driving forces and viscous dissipation, yielding Tanner's law R∝t1/10R∝t1/10. Here, we study the spreading of droplets of complex fluids having either surfactants or polymers dissolved in the fluid. It turns out that both surfactants and polymers slow down the spreading. A special type of surfactants (trisiloxanes), however, leads to a “superspreading”, in which the droplet spreads out orders of magnitude quicker than with usual surfactants. The slowing down of the spreading can be explained quantitatively; however, the mechanism of the speeding up remains a puzzle.