Erb Alexander, Kind Jonas, Zankel Timon L, Stark Robert W, Thiele Christina M
Physics of Surfaces, Institute of Materials Science, Technical University of Darmstadt, Darmstadt 64287, Germany.
Clemens-Schöpf-Institute for Organic Chemistry and Biochemistry, Technical University of Darmstadt, Darmstadt 64287, Germany.
Proc Natl Acad Sci U S A. 2025 May 20;122(20):e2423660122. doi: 10.1073/pnas.2423660122. Epub 2025 May 14.
Evaporation of sessile multicomponent droplets is ubiquitous in many common processes, such as printing, cooling, and coating. The evaporation rate is not evenly distributed across the surface of the droplet. As a result, there are regions with high evaporation rates in which the more volatile part evaporates preferentially; this can induce local surface tension gradients, which, in combination with density differences, can lead to flows within the droplet originating from local concentration gradients. Current experimental methods are based on the addition of markers that can alter liquid properties. Thus, marker-free experimental evidence for the concentration fields proposed is needed. In this work, we use Raman imaging and MRI to quantify concentration gradients in 4.2 μL droplets of 90 mol% water and 10 mol% glycerol. MRI concentration maps with 33 μm resolution enable the investigation of local concentrations as close as 100 μm to the 3-phase contact line. Raman imaging allows even higher resolution and longer observation times. The results of both methods are in excellent agreement. In accordance with the simulations, an increasing glycerol content close to the 3-phase contact line was found. Close to the droplet apex, the glycerol content decreased. The horizontal and vertical concentration gradients were on the order of 1 × 10 mol%μm. These findings can be used for the development and optimization of inks, medical diagnostic devices, food processing procedures, 3D bioprinting, and many more; they might provide the experimental concentration fields sought for the optimization of simulations.
固着多组分液滴的蒸发在许多常见过程中普遍存在，如印刷、冷却和涂层。蒸发速率在液滴表面分布不均。因此，存在蒸发速率高的区域，其中挥发性更强的部分优先蒸发；这会引发局部表面张力梯度，该梯度与密度差异相结合，会导致液滴内源于局部浓度梯度的流动。当前的实验方法基于添加会改变液体性质的标记物。因此，需要针对所提出的浓度场的无标记实验证据。在这项工作中，我们使用拉曼成像和磁共振成像来量化由90摩尔%的水和10摩尔%的甘油组成的4.2微升液滴中的浓度梯度。分辨率为33微米的磁共振成像浓度图能够研究距离三相接触线近至100微米处的局部浓度。拉曼成像具有更高的分辨率和更长的观测时间。两种方法的结果高度一致。与模拟结果相符，发现靠近三相接触线处甘油含量增加。靠近液滴顶点处，甘油含量降低。水平和垂直浓度梯度约为1×10摩尔%/微米。这些发现可用于油墨、医学诊断设备、食品加工工艺、3D生物打印等的开发和优化；它们可能会提供模拟优化所需的实验浓度场。
