Evidence of contact line waves driving sawtooth patterning from evaporating thin films

Study of non-linear dynamics driving thin film evolution is complicated by the small lengthscales and rapid timescales involved. In prior work, we demonstrated that sparingly soluble salts in water can leave behind a crystalline “fossil record” of contact line instabilities arising during evaporation of thin films to enable retrospective study of their dynamics. In those experiments, triangular patterns composed of locally uniform thin and thicker domains emerged with remarkable consistency. Here, we examine the physics driving formation of such sawtooth patterns. Linear stability analysis of the thin-film equation suggests that a uniformly thin film can become unstable to azimuthal perturbations when evaporation-driven stress exceeds a critical threshold, nucleating counter-propagating shock-hole pairs that generate triangle-shaped thicker domains. We observe a constant propagation velocity across experimental conditions, locally uniform domain heights, and straight trench-like domain boundaries, which are all consistent with these dynamics. We further demonstrate that surfactant addition can alter pattern morphology, with anionic surfactants decreasing azimuthal propagation speed while cationic surfactants disrupt formation of triangles altogether. These findings are consistent with surfactant-specific effects on wave stability, providing a potential explaination for the emergence of triangular patterns from thin-film evaporation.