Dynamics of Stratification in Multicomponent Colloidal Films During Evaporative Drying

纳米颗粒薄膜具有广泛的应用，包括抗菌涂料、耐腐蚀涂料和电池隔膜。通常，某些元件在胶片中的位置必须仔细控制。传统上，这是通过昂贵的多步骤制造工艺实现的。最近纳米颗粒薄膜形成的数学模型已经确定了一种方法，可以在一个步骤中创建复杂的垂直结构涂层，这个过程被称为分层。该工艺有可能减少多功能纳米颗粒膜的生产时间和成本。然而，对分层过程的实验研究很少。在这个项目中，一种新的实验技术将在分层过程中使用窄聚焦x射线束扫描薄膜。这些实验将在整个薄膜形成过程中跟踪颗粒的运动，并最终提供关于分层如何取决于纳米颗粒浓度、纳米颗粒大小、蒸发速率和其他处理参数的信息。这项研究将有助于改进多组分薄膜的制造工艺，这些多组分薄膜将用于造福人类福祉和健康的应用，例如用于下一代电池的无机聚合物薄膜，用于医疗植入物的载药涂层，以及用于潮湿环境的防霉涂料。与STEM劳动力发展相关的影响包括对研究生和本科生进行胶体材料新实验技术的培训。研究者还将致力于从代表性不足的人群中招募学生进行研究。最后，研究的概念将被纳入高中流体力学的新课程材料和视频讲座中。最近的研究已经确定了一种在分层过程中一步创建垂直结构涂层的方法。二元胶体混合物的理论描述表明，随着大小颗粒的体积分数和Peclet数的变化，二元胶体混合物的分层情况也不同。然而，由于难以(i)量化膜中颗粒的垂直浓度分布，以及（ii）跟踪膜形成过程中颗粒的运动，尚未实现模型预测与实验的全面比较。该项目将使用窄聚焦光束的小角度x射线散射（SAXS），在薄膜的不同垂直位置进行，以充分探索最近模型预测的复杂分层行为。本工作的目的是：(1)建立一个分层行为与颗粒体积分数和Peclet数关系的实验过程图；(2)通过时间分辨微束SAXS对成膜过程进行首次分层测量；(3)进行x射线光子相关光谱（XPCS）研究，了解粒子在成膜过程中的扩散率和动力学变化；(4)将研究整合到面向高中受众的流体力学新教材和视频讲座中。这项工作将提供在多组分系统中薄膜形成过程中颗粒浓度的第一次现场测量，以及在最终干燥薄膜和薄膜形成过程中对最近分层模型的第一次直接实验测试。该项目还将有助于建立新的先进的散射技术，以表征胶体材料。最后，获得的数据和见解将有助于推动分层理论描述的改进。该研究将广泛影响单步工艺的发展，以创造具有复杂浓度分布的多组分薄膜，减少多功能薄膜的生产时间和成本。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Nanoparticle films have wide applications, including antibacterial paints, corrosion-resistant coatings, and battery separator membranes. Often, the locations of certain components in the film must be carefully controlled. This has traditionally been achieved with expensive multi-step fabrication processes. Recent mathematical models of nanoparticle film formation have identified a means to create complex, vertically-structured coatings in a single step, a process referred to as stratification. This process has the potential to decrease production time and costs of multifunctional nanoparticle films. However, there are few experimental studies of the stratification process. In this project, a new experimental technique that involves scanning through the film with a narrowly-focused X-ray beam will be performed during the stratification process. These experiments will follow motion of particles throughout the film formation process, and will ultimately provide information on how stratification depends on the nanoparticle concentration, nanoparticle size, evaporation rate, and other processing parameters. The research will help improve manufacturing processes for multicomponent films used in applications that will benefit human welfare and health, such as inorganic-polymer films for next-generation batteries, drug-loaded coatings for medical implants, and mold-resistant paints for humid environments. Impacts related to STEM workforce development include training of a graduate student and undergraduate students in novel experimental techniques for colloidal materials. The investigator will also aim to recruit students from underrepresented populations for the research. Finally, concepts from the research will be incorporated into new curriculum materials and video lectures on fluid mechanics for high school audiences. Recent research has identified a means to create vertically-structured coatings in a single step during stratification. Theoretical descriptions of binary colloidal mixtures show a variety of stratification regimes as the volume fraction and Peclet number of small and large particles are varied. However, comprehensive comparison of model predictions with experiments has not yet been achieved because of difficulties in (i) quantifying the vertical concentration profile of particles in the film, and (ii) following motion of particles during the film formation process. This project will use small-angle X-ray scattering (SAXS) with a narrowly-focused beam, performed at varying vertical positions in the film, to fully explore the complex stratification behavior predicted by recent models. The objectives of the work are to: (1) create an experimental process diagram of stratification behavior as a function of particle volume fraction and Peclet number; (2) conduct the first measurements of stratification during film formation by performing time-resolved microbeam SAXS; (3) conduct X-ray photon correlation spectroscopy (XPCS) studies to yield knowledge of how particle diffusivity and dynamics vary during film formation; and (4) integrate research into new curricular materials and video lectures on fluid mechanics for high school audiences. The work will provide the first in situ measurements of particle concentration during film formation in multicomponent systems and the first direct experimental test of recent stratification models in both final dried films and during the film formation process. The project will also help establish new advanced scattering techniques for characterization of colloidal materials. Finally, the data and insight gained will aid in driving improvements to theoretical descriptions of stratification. The research will broadly impact development of single-step processes to create multicomponent films with complex concentration profiles, decreasing production time and costs of multifunctional films.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.