Real-Time Visualization of Solutal Marangoni Convection in Sub-100 nm Precursor Films Using Laser-Generated Droplet Probes.

We present a tracer-free method for the real-time visualization of flow in nanometer-thick liquid films─an essential scale for controlling wetting and interfacial transport. We demonstrate that focusing a 532 nm continuous-wave laser (≈1 mW) on a bubble region within a 5 μm gap, filled with a binary mixture of liquids with different volatilities and surface tensions (ethanol/PEG-200 mixture), spontaneously generates PEG-rich microdroplets on demand. Here, we show that immediately after the laser is switched off, the droplet depins and travels rectilinearly away from the reservoir at ∼30 μm/s. Coupled laminar-flow/species-transport simulations reveal that the ∼50 nm-thick precursor film sustains a surface flow driven by solutal Marangoni convection arising from spontaneous ethanol evaporation; the droplet is passively conveyed by this flow. This laser-droplet platform thus provides the first tracer-free technique to observe interfacial flow in precursor films thinner than 100 nm─including steady-thickness states─using a standard optical microscope. Furthermore, when an artificial, high-concentration PEG region is created by laser manipulation, a subsequently generated droplet is attracted toward it, confirming that this method can also visualize local concentration gradients on the substrate. The method exceeds the thickness limits of conventional micro-PIV (Particle Image Velocimetry) and offers a new real-time probe for ultrathin-film transport phenomena. These findings enhance our understanding of concentration-gradient-driven transport at the nanoscale and inform multiscale designs for heat- and mass-transfer devices.