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.

Molecular Rotors for In Situ Viscosity Mapping during Evaporation of Confined Fluid Mixtures

“…Various other techniques, including infrared spectroscopy [47], Laser Induced Fluorescence and Raman scattering [48], acoustic reflectometry and spectroscopy [49,50], Injected Gas Chromatography [51], refractometry [52] and tensiometry [53] allowed to determine the evolution of concentration in time, yet without any spatial resolution. Finally, very few studies managed to measure concentration fields with both spatial and temporal resolution: they for instance used spatially resolved NMR [54,55] possibly coupled with Raman spectroscopy [56], interferometry to detect changes of refractive index (mostly in the vapor surrounding the droplet) [57,58] or fluorophores sensitive to local solvent environment [59]. While these techniques offer interesting opportunities to characterize the evaporation of liquid film and droplets, they all suffer some restrictions either on the geometry of the system or on the studied compound and alternative measurement technologies are still of high interest.…”