Concentration Gradients in Evaporating Binary Droplets Probed by Spatially Resolved Raman and NMR Spectroscopy

TL;DR

This study demonstrates the use of confocal Raman microscopy and spatially resolved NMR spectroscopy to measure and analyze vertical concentration gradients in evaporating binary droplets, providing insights into evaporation dynamics.

Contribution

It introduces combined spectroscopic methods as non-intrusive tools to quantify concentration gradients in evaporating binary droplets.

Findings

Vertical concentration gradients are observed during evaporation.

Raman and NMR methods provide complementary local composition data.

Deuteration enables selective analysis of mixture components.

Abstract

Understanding the evaporation process of binary sessile droplets is essential for optimizing various technical processes, such as inkjet printing or heat transfer. Liquid mixtures whose evaporation and wetting properties may differ significantly from those of pure liquids are particularly interesting. Concentration gradients may occur in these binary droplets. The challenge is to measure concentration gradients without affecting the evaporation process. Here, spectroscopic methods with spatial resolution can discriminate between the components of a liquid mixture. We show that confocal Raman microscopy and spatially resolved nuclear magnetic resonance (NMR) spectroscopy can be used as complementary methods to measure concentration gradients in evaporating 1-butanol/1-hexanol droplets on a hydrophobic surface. Deuterating one of the liquids allows analysis of the local composition through the comparison of the intensities of the CH and CD stretching bands in Raman spectra. Spatially resolved NMR spectroscopy is used to measure the composition at different positions of the droplet. Confocal Raman and spatially resolved NMR experiments show the presence of a vertical concentration gradient as the 1-butanol/1-hexanol droplet evaporates.