Evaporation of Sessile Binary Mixture Droplets: Time Dependence of Droplet Shape and Concentration Profile from One-Dimensional Magnetic Resonance Microscopy

Many technological applications like inkjet printing, coating, or cooling processes rely on the evaporation of sessile droplets. Regarding liquid mixtures, the understanding of the underlying physics is still incomplete and process optimization requires trial and error. Our main goal is to establish a novel method in this field, one-dimensional magnetic resonance microscopy, to investigate the evaporation of sessile binary mixture droplets in the microliter range. It allows us not only to determine the droplet volume and shape, including contact angle, but also to measure concentration profiles with a spatial resolution of a few micrometers. These capabilities are demonstrated for a mixture of 1-octanol (OCT) and pentadecafluoro-1-octanol (F-OCT) by combining spatially resolved 1H and 19F nuclear magnetic resonance measurements. We clearly observe three evaporation regimes for the OCT/F-OCT mixture. The first and second regimes are characterized by the predominant evaporation of F-OCT and are separated by a depinning event. The third regime starts when no F-OCT is left and, thus, features the evaporation of a pure OCT droplet. During all stages, concentration gradients perpendicular to the substrate are weak in the studied binary droplet.