Hypothesis: Self-driven actions, like motion, are fundamental characteristics of life.
Today, intense research focuses on the kinetics of droplet motion. Quantifying macroscopic
motion and exploring the underlying mechanisms are crucial in self-structuring and
self-healing materials, advancements in soft robotics, innovations in self-cleaning
environmental processes, and progress within the pharmaceutical industry. Usually,
the driving forces inducing macroscopic motion act at the molecular scale, making
their real-time and high-resolution investigation challenging. Label-free surface
sensitive measurements with high lateral resolution could in situ measure both molecular-scale
interactions and microscopic motion. Experiments: We employ surface-sensitive label-free
sensors to investigate the kinetic changes in a self-assembled monolayer of the trimethyl(octadecyl)azanium
chloride surfactant on a substrate surface during the self-propelled motion of nitrobenzene
droplets. The adsorption–desorption of the surfactant at various concentrations, its
removal due to the moving organic droplet, and rebuilding mechanisms at droplet-visited
areas are all investigated with excellent time, spatial, and surface mass density
resolution. Findings: We discovered concentration dependent velocity fluctuations,
estimated the adsorbed amount of surfactant molecules, and revealed multilayer coverage
at high concentrations. The desorption rate of surfactant (18.4 s−1) during the microscopic
motion of oil droplets was determined by in situ differentiating between droplet visited
and non-visited areas.
