Marangoni instabilities in spreading of polymer solutions on a suspended liquid film*

Spreading of multi-component droplets is a rich physics phenomenon that has attracted much attention in the past years. Particularly because of its prevalent presence in many technological applications like those involving coating, surface cleaning, surface patterning, material deposition, and many others. Here, we report a novel instability that occurs during the spreading of a polymer-surfactant solution on a soap film. We previously have shown that after deposition of a droplet on a soap film, the droplet forms a new suspended layer in the center of a soap film. When the droplet's surface tension is lower than the soap film, a rapid spreading takes place in the order of milliseconds. Such geometry enables us to study the spreading behavior of liquids in the absence of high shear viscous forces induced by solid boundaries. When the deposited droplet is a polymer solution, the spreading dynamics can show instabilities. We show that at low polymer concentration, the spreading front remains stable and perfectly circular. However, above the entanglement concentration of the polymers, it destabilizes into a daisy shape pattern. We discovered that the enhanced elastic response of the solution due to the formation of polymer networks triggers the instabilities. We introduce a critical length scale based on balancing elastic and capillary forces which its growth rate perfectly predicts the onset of the instability.