In-situ imaging to investigate the concentration profile and morphology formation of single droplets during convective drying

喷雾干燥以其快速、温和的干燥特性在食品和医药生产中得到广泛应用。近年来，研究的目的是使用喷雾干燥作为工具来生产具有限定形态的颗粒。最终的颗粒形态取决于待干燥溶液的组成、粒度和干燥条件。颗粒形态影响喷雾干燥过程和最终粉末的性能。液滴到颗粒的转变通常在定义的设置中用单液滴干燥方法进行研究。颗粒形态形成的决定性参数是局部结壳形成和取决于局部浓度的结壳的机械性质。因此，在干燥过程中结壳形成的知识是理解颗粒形态的关键，因为局部浓度、分层现象和局部不稳定性影响颗粒形态。因此，在液滴干燥过程中测量局部浓度是必要的。到目前为止，还没有技术可以以足够的分辨率测量干燥过程中3D液滴的浓度分布。在该项目中，计划使用2D设置测量浓度分布和结壳的形成，其中液滴在两个双折射玻璃载玻片之间的受限几何形状中干燥。通过光学显微镜测量液滴的尺寸，而用荧光显微镜测量浓度分布。除了二元体系外，还将研究三元体系。首先，2D方法将通过在20至120 ° C的温度范围内的水干燥进行验证，并与理论值进行比较。在第二个工作包中，该方法将应用于含有分子和胶体溶解组分的二元和三元溶液的干燥。将在较宽的温度和浓度范围内研究结壳的形成。在第三个工作包中，将开发图像分析算法，以量化干燥动力学的液滴面积以及局部浓度的荧光强度。最后一个工作包致力于将结果从2D设置转移到3D。在这里，CSLM和时间分辨微计算机断层扫描将用于在单滴干燥期间原位测量结壳形成。这些知识是预测形态形成的关键。

Spray drying is a widespread drying technology in food and pharmaceutical production due to its fast and gentle drying characteristics. In recent years, research aims at using spray drying as tool to produce particles with defined morphology. The final particle morphology depends both on the composition of the solution to be dried, the particle size and the drying conditions. Particle morphology influence the spray drying process and the final powder properties. Droplet-to-particle transition is usually investigated with single droplet drying methods in defined setups. The decisive parameter for the formation of the particle morphology is the local crust formation and the mechanical properties of the crust that depend on the local concentration. Therefore, the knowledge of crust formation during drying is key for the understanding of particle morphology, as local concentration, demixing phenomena and local instabilities influence particle morphology. Therefore, it is essential to measure the local concentration during droplet drying. So far, no technologies exist to measure the concentration profile in a 3D droplet during drying with a sufficient resolution. In this project it is planned to measure the concentration profile and the formation of the crust with a 2D setup, where the droplet is dried in a confined geometry between two hydrophopic glass slides. The size of the droplet is measured by light microscopy whereas the concentration profile is measured with fluorescence microscopy. Next to binary systems, also ternary systems will be studied. At first, the 2D method will be validated by means of water drying in a temperature range between 20 and 120 °C and compared to theoretical values. In the second work package, the method will be applied to the drying of binary and ternary solutions with molecular and colloidal dissolved components. The crust formation will be studied in a broad temperature and concentration range. In the third work package, an image analysis algorithm will be developed to both quantify the droplet area for drying kinetics as well as the fluorescence intensity for local concentration. The last work package is dedicaded to the transfer of the results from the 2D setup to 3D. Here, CSLM and time resolved µ-Computertomography will be employed to in situ measure the crust formation during single droplet drying. This knowledge is key for the prediction of morphology formation.