Since 1980's, tiny fingers were observed at the Marangoni spreading front on a water thin film of several micrometers. Today, we found evidence for a new interfacial instability on a thin film of viscoelastic fluid. We can see finger-like patterns at the spreading front and rib-like filaments at the drop-film interface.
Marangoni spreading on thin films is widely observed in nature and applied in industry. It seriously affects airway drug delivery, especially in surfactant displacement therapy. This video illustrates a surfactant-laden droplet spreading on films of viscoelastic fluids. The experiments used particle seeding, and particle tracking velocimetry (PTV) to determine the deformation of the film–droplet interface and to measure velocity fields. Radially aligned patterns were observed on viscoelastic films in combination with rapid azimuthal film thickness variations. The pattern formation on viscoelastic films is correlated with instability at the film–droplet–air contact line when the liquid is expelled radially by the spreading droplet. PTV revealed azimuthal variations of the velocity field near the contact line. The observed contact line instability differs from Newtonian thin films' fingering instabilities.
Detailed information in Ma et al. (2023). J. Fluid Mech., 958, A33.