Concentration gradients in evaporating binary droplets probed by spatially resolved Raman and NMR spectroscopy

Abstract

Significance Imagine you spill your drink and miss some spots when cleaning up. The next morning you notice that the stains look quite different on different surfaces. What has happened? In droplets of liquid mixtures, the components evaporate at different rates, which leads to gradients in concentration and surface tension. These gradients can cause, for example, so-called Marangoni flows, which in turn affect the evaporation process. To better understand evaporation-induced liquid flows, the concentration gradients have to be measured without disturbing the liquid. Marker molecules might be surface-active or even may affect the evaporation process. We report here on marker-free and contactless measurements of concentrations by spatially resolved Raman and NMR spectroscopy in evaporating binary droplets.