Drops spreading and retracting and forming a rim

Surfactant laden droplets spreading on a solid may spontaneously retract due to interfacial energy gradients at the solid surface. We explain this retraction by considering fluctuations near the triple line, along with an instability due to these fluctuations and the surface gradients. We provide a theoretical description of this phenomenon along with experimental evidence.

The spreading of the drop is modelled by lubrication theory followed by scaling analysis. At low surfactant concentrations, spreading conforms to Tanner’s law whereby the base radius scales as r_B~t^1/10. Surfactant adsorption is modelled by Langmuir kinetics and the dynamic solid-vapor surface energy, γ_SV, and solid-liquid interfacial energy, γ_SL, are both modelled in accordance with the dynamic adsorption coupled with spreading of the drop.

The retraction process is then considered qualitatively, where fluctuations near the triple line are propagated by hydrodynamic instability. At low surfactant concentrations, this instability is also responsible for a noticeable rim which is left behind after retraction. We also show that the width of the rim increases with decreasing surfactant concentration.

We provide experimental support to the theoretical explanation with octadecylamine-laden tetradecane drops on mica. The drops are filmed with a high-speed camera and then analyzed via frame-by-frame software. For all concentrations tested, we find good agreement with both the analytical and qualitative portions of the theory.