# Complex Wave Packet Dynamics Induced by Marangoni Stresses

## TL;DR

This paper studies how surfactant-laden drops spread on thick films, revealing new wave dynamics caused by Marangoni stresses.

## Contribution

The study identifies previously unobserved wave merging and splitting phenomena during surfactant-driven Marangoni spreading.

## Key findings

- Wave merging near the deposition point occurs early during surfactant drop spreading.
- Splitting of the innermost wave happens later due to speed differences between surfactant front and wave.
- These dynamics are specific to low viscosity thick subphases and not observed with pure fluid drops.

## Abstract

New features emerge during Marangoni spreading of a surfactant-laden
drop on a thick film, where inertia plays a significant role in the
hydrodynamics relative to the more commonly studied low inertia spreading
phenomenon. We uncover these features using high-speed imaging and
understand their dynamics using numerical simulations on subphases
of varying viscosity and depths. Deposition of a drop of surfactant
solution drives the formation of a packet of waves moving across the
surface. Waves closest to the deposition point are partially covered
with the surfactant layer and are directly affected by Marangoni stresses;
waves at larger distances are not. A previously unobserved event,
merging of peaks close to the deposition point, is detected at early
times in experiments and replicated in simulation. Such an event does
not occur if a pure fluid drop of the same composition as the subphase
is deposited on the surface. At later times, the difference in speed
between the surfactant front and the innermost wave causes splitting
of the innermost wave, creating a wave covered with surfactant and
another wave not covered with surfactant. These features are detectable
for thicker subphases of low viscosity liquids, commonly found in
laboratory studies and technological settings where Marangoni spreading
is present.