Self-organized spreading of droplets to fluid toroids.

HYPOTHESIS: Mixing of a chemical trigger of lower surface tension into a microdroplet with relatively higher surface tension can cause a rapid spreading of the droplet on a liquid-sublayer to form a host of metastable liquid morphologies such as sheets, toroids, threads, or droplets. Subsequently, such metastable fluidic objects break into a collection of droplets to form microemulsions. EXPERIMENTS: Introduction of surfactant loaded water or long-chain alcohols into an oleic acid microdroplet stimulate a rapid spreading of the same on a water sublayer, which helps in the formation of a metastable liquid sheet connected to a liquid toroid. Much like slipping films, the liquid sheet dewets the water underlayer through the formation of holes before they grow and coalesce to form liquid ribbons. While such liquid structures eventually break into an array of microdroplets, the liquid toroid expands before undergoing a Plateau-Rayleigh instability to form microdroplets. FINDINGS: A single step self-organization process in which a chemical trigger can convert a microdroplet into a liquid-toroid on a water surface, in absence of any rotational influence. A symmetric to asymmetric transition in toroid morphology is observed due to the changeover of laminar to turbulent flow regimes with the reduction in viscosity of fluid-sublayer or variation in chemical triggers. The toroid cross-section and droplet spacing after the toroid breakup follow a length scale evaluated from a linear stability analysis.