Clemens-Schöpf-Institut für Organische Chemie und Biochemie, Technische Universität Darmstadt, Alarich-Weiss-Str. 16, D-64287 Darmstadt, Germany.
Institut für Technische Thermodynamik, Technische Universität Darmstadt, Alarich-Weiss-Str. 10, D-64287 Darmstadt, Germany.
J Magn Reson. 2023 Mar;348:107389. doi: 10.1016/j.jmr.2023.107389. Epub 2023 Jan 31.
Controlling and improving processes like for example the production of organic semiconductors via printing depends on understanding the interplay of wetting and evaporation of complex fluids. Therefore, examination of the time dependent composition of complex fluid droplets during wetting or evaporation is of interest. The evaporation rate of sessile droplets containing largely water depends on the vapor pressures of the individual components and on the humidity (or partial pressure) of the surrounding gas phase. Hence, for a complete picture of an evaporation process and the comparability of the results of different measurements, it is essential to measure and control the humidity and temperature in the measurement compartment. Accordingly, climate chambers are available in different scales to fit a variety of techniques like contact angle goniometry to obtain results in a controlled atmosphere. We recently reported the application of MRI (Magnetic Resonance Imaging) and spatially resolved NMR (Nuclear Magnetic Resonance) spectroscopy for the examination of the evaporation of sessile droplets on surfaces in 10 mm NMR tubes. These are considered to be closed compartments. Here, we present an apparatus to a) measure and b) control the relative humidity within the sample compartment of the NMR setup by introducing preconditioned gas into the NMR tube. We monitored the evaporation of water droplets using RARE images and compared the volume decay with a) a simple diffusive evaporation model and b) with detailed FEM (finite element numerical model) simulations using COMSOL for validation. We find three evaporation regimes depending on the flow rate as well as on the distance of the gas outlet and the evaporating droplet. In one of the sample configurations tested the evaporation takes place in such a way that it can be described with the help of the simple diffusive model without convection. Thus, the presented approach opens comparative measurements with other methods as well as the observation of droplet evaporation in very dry or very humid environments with and without the influence of convection. Finally, using PRESS spectra, it is shown that the evaporation rate of water from a water/DMSO droplet can be controlled. This shows how the setup presented here can be used to study the evaporation of droplets of more complex mixtures.
控制和改进工艺，例如通过印刷生产有机半导体，取决于对复杂流体的润湿性和蒸发相互作用的理解。因此，在润湿或蒸发过程中检查复杂液滴随时间的组成是很有意义的。含有大量水的不流动液滴的蒸发速率取决于各个组分的蒸气压以及周围气相的湿度（或分压）。因此，为了全面了解蒸发过程以及不同测量结果的可比性，必须测量和控制测量室中的湿度和温度。因此，不同规模的气候室可用于各种技术，例如接触角测角法，以在受控气氛中获得结果。我们最近报告了 MRI（磁共振成像）和空间分辨 NMR（核磁共振）光谱在 10mm NMR 管表面上不流动液滴蒸发的应用。这些被认为是封闭的隔室。在这里，我们提出了一种装置，通过向 NMR 管中引入预处理气体，a）测量和 b）控制 NMR 装置样品室中的相对湿度。我们使用 RARE 图像监测液滴的蒸发，并将体积衰减与 a）简单的扩散蒸发模型和 b）使用 COMSOL 进行的详细有限元数值模型（FEM）模拟进行了比较，以进行验证。我们发现，根据流速以及气体出口和蒸发液滴的距离，存在三种蒸发状态。在所测试的样品配置之一中，蒸发以这样的方式发生，即可以借助于没有对流的简单扩散模型来描述。因此，所提出的方法不仅可以进行与其他方法的比较测量，而且可以在有或没有对流影响的情况下观察非常干燥或非常潮湿环境中的液滴蒸发。最后，使用 PRESS 光谱表明，可以控制水/DMSO 液滴中水的蒸发速率。这表明了这里介绍的装置如何用于研究更复杂混合物液滴的蒸发。
