A research team at TU Darmstadt has gained new insights into the evaporation behavior of binary liquid droplets. The study, published in PNAS, provides the first marker-free, high-resolution data on the internal dynamics of glycerine-water droplets – a liquid mixture that is also used in inkjet and 3D printing processes. The aim was to better understand transport processes in evaporating droplets in order to enable their targeted use or avoidance in technical applications.
The study focuses on the so-called “coffee ring effect”, in which particles migrate to the edge of a droplet during evaporation and are deposited there. This behavior is undesirable in many technical processes as it leads to uneven material distribution – a well-known problem in coatings, inks or functional printing processes, for example. Homogeneous drying profiles are crucial for additive manufacturing techniques where droplet deposition needs to be controlled in a targeted manner.
The researchers analyzed the droplets using two complementary spectroscopic methods: Magnetic Resonance Imaging (MRI) and Raman microscopy. Both methods allow a detailed concentration analysis without the use of marker molecules that could influence the physico-chemical behavior of the droplet. MRI provided a two-dimensional resolution of the concentration distribution in the droplet cross-section, while Raman spectroscopy provided high-resolution information on local gradients. Despite the methodological differences, the results were consistent and in agreement with external numerical models.
The findings are not only relevant for 3D printing, but also for the further development of droplet-based processes in microfluidics, cooling technology and functional surface coating. The interdisciplinary collaboration between the departments of materials science and chemistry enabled a holistic view of droplet behavior – from molecular transport to optical simulation.
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