Interfacial instabilities in Marangoni-driven spreading of polymer solutions on soap films

Tuning and controlling the flow behavior of multi-component liquids has been a long-lasting struggle in various technological applications. Here, we studied Marangoni spreading of a polymer-surfactant ternary solution when deposited on a soap film with higher surface tension. The spreading front becomes unstable into a fingering pattern above the entanglement concentration of the polymer solution, indicating that the interplay between the elastic and interfacial properties drives the instability. Balancing these terms results in a critical length scale for the onset of the instability.

To investigate the connection between the rheological characteristics of the samples and the origins of the instabilities, various rheological tests were performed. Elastic and loss modulus of the samples were measured within the linear viscoelastic regime. The spreading behavior of the solutions was studied using high-speed imaging techniques.

At low concentrations of polymers, spreading dynamics are governed by surface tension gradient and viscous dissipation leading to a stable front growing linearly in time. However, above the entanglement concentration of polymers spreading front destabilizes into a daisy shape pattern suggesting the elastic forces dominating the spreading dynamics. We introduced a length scale that precisely predicts the onset of the instability.