Spatiotemporally-controlled droplet merging reveals 2D diffusion dynamics of multi-component surfactants

Investigating the two-dimensional (2D) diffusion dynamics of multi-component surfactant systems is challenging due to the difficulty in establishing well-defined initial boundaries both temporally and spatially. To overcome this challenge, we employed two key strategies. First, we utilized a merging droplet technique which provides controlled spatiotemporal initial conditions, enabling in-situ measurement of multi-component surfactant diffusion. Second, we designed a model multi-component surfactant system comprising oxidized dioleoylphosphatidylcholine (oxDOPC) and cholesterol (Chol) through controlled UV-induced oxidation of DOPC. By using ¹H nuclear magnetic resonance (NMR) spectroscopy and numerical simulations of diffusion with a moving boundary, we quantified the molecular profiles and diffusivities of miscible and immiscible species in oxDOPC-Chol system. Furthermore, we determined the line tension of oxDOPC domain from the elastic relaxation of its deformed shape. This work provides significant methodological advancements for studying molecular mixing in multi-component interfacial materials, including systems involving membrane proteins and lipid rafts.