MRI and localised NMR spectroscopy of sessile droplets on hydrophilic, hydrophobic and superhydrophobic surfaces - Examination of the chemical composition during evaporation.

Evaporation of droplets is a process important in many different areas of science, technology and also everyday life. The understanding of droplet evaporation of homogeneous and heterogeneous substance mixtures is important, for example, to explain the formation of coffee stains or to optimize the results in offset printing. For a detailed understanding of the evaporation of complex mixtures from structured surfaces, such as inks used in offset printing, a time-resolved analysis of the droplet composition is essential. Measurement of (local) concentrations may deepen the understanding of wetting phenomena and their connection with transport phenomena. Therefore, we demonstrate in this paper that magnetic resonance methods can be used to (a) image sessile droplets on structured surfaces and (b) investigate their composition in a time-resolved manner. First it is shown that water droplets on superhydrophobic, hydrophobic and hydrophilic surfaces, despite the large liquid/gas interface, can be imaged well and without interfering artefacts using RARE. Further, the signals are examined in localised PRESS NMR spectra with respect to line shape and quantifiability. Finally, it is demonstrated that non-localised NMR spectra can be used to track the droplet composition during evaporation.