Direct observation of Marangoni effect in picoliter droplets colliding in mid-air

Picoliter droplets of ethanol and distilled water, each with a diameter of approximately 50 µm, were generated in air using on-demand inkjet technology. The behavior of the liquid following droplet collision was directly observed using a stroboscopic technique with a time resolution better than 100 ns. By visualizing the flow of dye-colored ethanol in a unified droplet, it was confirmed that the ethanol surface completely covered the distilled water droplet within approximately 30 µs. The timescale of this coverage, driven by the difference in surface tension, was much shorter than that of molecular diffusion. As a result, we successfully observed the elementary process of Marangoni flow, which occurs on the microsecond timescale.