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Although many theories exist to understand the transport of surfactants to and along interfaces under flow, few techniques are available to directly monitor surfactant dynamics at interfaces. Due to the broad use of surfactants, from oil spill remediation to healthcare and household products, a better means of assessing surfactant dynamics would be widely applicable. Here, we describe and demonstrate the use of fluorescent surfactant-dyes as a means of dynamic visualization of surfactant transport and have synthesized Nile Red/polyethylene glycol (NR-PEG) derivatives for this purpose. These NR-PEG surfactant-dyes fluoresce differently at oil-water interfaces than in bulk aqueous solution, allowing for the selective monitoring of bound and free surfactant using confocal microscopy. We have visualized the transport of surfactants to and along interfaces of droplets in a pipe flow and at the interfaces of liquid-infused surfaces undergoing shear-driven drainage. Imaging surfactant adsorption to interfaces at these time scales will allow for the development of improved dispersant systems for applications such as oil spill remediation and can also be used in understanding other fundamental transport phenomena involving dispersants.

Targeted nanoparticles often require conjugating targeting ligands to polyethylene glycol (PEG) chains of a nanoparticle's dense protecting corona. "Click" chemistries are commonly employed for their bioorthogonality, with strain-promoted azide-alkyne cycloadditions (SPAAC) increasingly chosen to avoid cytotoxic copper catalysts. However, conjugation becomes compromised if reactive PEG chain ends cannot encounter their reaction counterparts. We use fluorescence to probe the location of Nile Red, methylpyrene, and butylpyrene, dyes with comparable hydrophobicities to SPAAC alkynes (logP = 3.2-5.7), tethered to PEG chains on 100 nm NPs. Using fluorescence peak shifts, we find that Nile Red resides 43% of the time in the 5k PEG corona and 57% at the more hydrophobic nanoparticle core. Increasing the PEG MW to 67k doubles the corona dye fraction to 86% (14% core). More hydrophobic methylpyrene and butylpyrene, monitored with I1/I3 ratios, reside 1% in the corona (99% core). These results explain difficulties with using SPAAC reactions for conjugating large ligands to nanoparticles with PEG coronae.