CAREER: Decoding the dynamics of complex fluids near surfaces and interfaces

当液滴接触固体表面时，它们可以湿润表面并扩散到表面上，这在各种工业和技术过程中都很重要，例如冷却工业反应器，向植物叶片施用杀虫剂，从地下水库采油以及喷墨印刷。了解生物液体在固体表面的润湿和蒸发是解决与人类健康有关的问题的关键一步，例如防止病毒飞沫传播疾病。液滴的润湿、扩散和蒸发会受到液体中表面活性剂（如表面活性剂或纳米颗粒）以及固体表面性质的强烈影响。在这种情况下，预测和控制液体扩散具有挑战性，配方通常基于反复试验。这个CAREER项目将研究液体的组成，包括简单液体、含有表面活性剂和/或纳米颗粒的液体，以及生物液体，如何影响液滴润湿、扩散和蒸发。将对不同组成的液滴进行一系列实验，以揭示液体中的不同成分如何单独和协同地对一系列固体表面上的润湿和扩散动力学的物理特性做出贡献。结果将确定湿润、扩散和蒸发的关键参数，并揭示影响这些过程的界面复杂流动的细节。这项研究将用于培训不同类型的本科生和研究生。它还将允许创建教育视频和演示，让初高中学生参与科学活动，吸引女性退伍军人参与科学并激励她们在向平民生活过渡时接受高等教育，并提高公众的科学素养。关于简单流体的润湿、扩散和蒸发，已有大量文献；然而，为简单流体建立的概念不能直接应用于携带表面活性剂和纳米颗粒的复杂流体的液滴。对于颗粒/表面活性剂混合体系中扩散和蒸发的关键因素以及颗粒/表面活性剂协同作用在这些情况下的影响的理解是有限的。本CAREER项目的科学目标是通过阐明胶体颗粒界面吸附和固体表面能在动力学中的作用，促进对多组分流体系统润湿和扩散的基本理解。颗粒的表面性质将被修改，以改变纳米颗粒吸附到流体/流体界面的能量屏障。固体表面能的极性和色散分量的大小将被系统地调整，以检查复杂流体和衬底之间的相互作用力在润湿行为中的作用。这些研究将确定纳米颗粒界面吸附的影响，以及潜在的颗粒层的界面流变学，对无底液滴的扩散动力学和蒸发动力学的影响。将研究混合颗粒/表面活性剂分散体，以说明纳米颗粒的界面约束与马兰戈尼效应是否会影响润湿行为，以及界面流变性是否会影响多组分体系的干燥。人工唾液模型将作为基础流体来研究黏液在流体-流体界面上对各种表面扩散和蒸发的重要性。本研究解决了对表面和界面附近复杂流体行为的经验主义理解的需要，以及表面特征在改变此类系统润湿性中的作用。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。

When liquid drops contact a solid surface, they can wet the surface and spread over it, which is important in a variety of industrial and technological processes such as cooling industrial reactors, applying pesticides to the leaves of plants, oil recovery from underground reservoirs, and inkjet printing. Understanding wetting and evaporation of biological fluids on solid surfaces is a critical step toward solving problems related to human health, such as preventing disease transmission by virus-loaded droplets. Wetting, spreading and evaporation of liquid drops can be strongly affected by the presence of surface-active agents in the liquid, such as surfactants or nanoparticles, as well as the properties of the solid surface. Predicting and controlling spreading of liquids is challenging in this case and formulations often are based on trial-and-error experiments. This CAREER project will investigate how the composition of liquids, including simple liquids, liquids containing surfactants and/or nanoparticles, and biological fluids, affect droplet wetting, spreading, and evaporation. A series of experiments will be carried out on droplets of various composition to reveal how various components in the liquid contribute individually and synergistically to the physics of wetting and spreading dynamics on a series of solid surfaces. The results will identify key parameters in wetting, spreading and evaporation, and also uncover details of complex flows at interfaces that affect these processes. The research will be used in training a diverse group of undergraduate and graduate students. It will also enable the creation of educational videos and demonstrations to engage middle school and high school students in scientific activities, attract female veterans to science and inspire them to pursue higher education as they transition to civilian life, and enhance the science literacy of the public.A large body of literature is available on the wetting, spreading, and evaporation of simple fluids; however, concepts established for simple fluids cannot be directly applied to droplets of complex fluids carrying surfactants and nanoparticles. The understanding of key factors in spreading and evaporation in a mixed particle/surfactant system and the impact of particle/surfactant synergism in these cases is limited. The scientific goal of this CAREER project is to advance fundamental understanding of the wetting and spreading in case of multicomponent fluidic systems by elucidating the roles that colloidal particle interfacial adsorption and solid surface energies play in the dynamics. Surface properties of the particles will be modified to alter the energy barrier for nanoparticle adsorption to the fluid/fluid interface. Magnitudes of the polar and dispersive components of the solid surface energy will be systematically tuned to examine the role of interaction forces between the complex fluid and the substrate in the wetting behavior. These studies will identify the influence of nanoparticle interfacial adsorption, and potentially the interfacial rheology of the particle layer, on spreading dynamics and evaporation kinetics of a sessile drop. Mixed particle/surfactant dispersions will be examined to illustrate whether the interfacial confinement of nanoparticles combined with Marangoni effects influences wetting behavior and whether interfacial rheology affects drying in multicomponent systems. Artificial saliva models will be used as a base fluid to investigate the importance of mucin at the fluid-fluid interface on spreading and evaporation on various surfaces. This research addresses the need to develop an empiricism-free understanding of the behavior of complex fluids near surfaces and interfaces, and the function of surface characteristics in altering wettability in such systems.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.