EAGER: Microscale Fingering Instabilities in Drying Colloid and Polymer Films

当像石油这样的粘性流体被困在像土壤或沙子这样的多孔材料中时，很难将其清除。试图通过强迫空气进入材料来取代被困的油是失败的，因为低粘度的空气会穿透高粘度的油。空气会在油中形成长长的手指，留下大部分的油。这个视觉上引人注目的过程，被称为Saffman-Taylor不稳定性，已经在宏观系统中得到了很好的记录。它强烈影响能源生产和环境修复的采油过程。在一次偶然的发现中，人们在纳米颗粒和聚合物的干燥薄膜中发现了手指，这与建筑、汽车和许多其他消费品的油漆中发现的成分相似。令人惊讶的是，这些手指形成的尺度要小得多，而且它们不遵循萨弗曼-泰勒不稳定性所确定的规则。这个项目将研究这些微观手指形成的物理原理与宏观现象的不同之处。这种指法不稳定性将在控制良好的实验和使用专门显微镜获得的图像的计算分析中定量表征。所获得的知识将用于控制或抑制手指的形成，以改变这些薄膜的性质。在干燥黏性液体中的颗粒薄膜时，这些手指可能无处不在，并且可能对这些薄膜对其基底的附着力产生强烈的影响。本研究将研究使用高速共聚焦激光扫描显微镜发现的干燥过程中胶体-聚合物薄膜中视觉上引人注目的流体流动不稳定性。这是首次观察到聚合物溶液在微米或亚微米长度尺度上的指指不稳定性，如胶体晶体等多孔材料。指指不稳定性，如Saffman-Taylor不稳定性，对多孔介质中二次采油和含水层运移产生了广泛的影响。大多数关于Saffman-Taylor不稳定性的研究集中在Hele-Shaw细胞或多孔材料中，一种高粘度牛顿流体与一种不混溶的低粘度牛顿流体的置换。在这项研究中，手指在常规光学显微镜下是不可见的，因为它们的波长在微米尺度上，违背了宏观系统中发现的典型的Saffman-Taylor尺度。该项目将研究这种不稳定性在不同聚合物浓度、聚合物分子量和粒径下的模式，其中聚合物的旋转半径范围从非常小到与胶体晶体的孔隙尺度相当。目的是确定这种指控不稳定性是否与之前的研究不同，这是由于纳米尺度上流体的非牛顿行为或界面特性所致。由于聚合物被留下，它在薄膜-衬底界面形成了明确的固体结构。这些结构将被研究并用于制造纳米级或微尺度的流体通道网络。此外，指入不稳定性可能通过减少接触面积和/或产生微纤化结构而对薄膜粘附在基材上产生显著影响。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 a viscous fluid such as oil is trapped within a porous material like soil or sand, it is very difficult to remove. Trying to displace the trapped oil by forcing air into the material fails, because the lower viscosity air will penetrate through the higher viscosity oil. The air will form long fingers into the oil, leaving most of the oil behind. This visually striking process, known as the Saffman-Taylor instability, has been well documented in macroscopic systems. It strongly affects oil recovery processes for energy production and environmental remediation. In a serendipitous discovery, fingers were found in drying films of nanoparticles and polymer, similar to that found in paints used on buildings, cars, and many other consumer goods. What is surprising is that these fingers form at much smaller scales, and they do not follow the rules determined for the Saffman-Taylor instability. This project will investigate how the physics surrounding the formation of these microscopic fingers differs from macroscale phenomena. This fingering instability will be characterized quantitatively in well-controlled experiments and computational analysis of images obtained using specialized microscopy. The knowledge gained will be used to control or suppress the formation of fingers in order to alter the properties of these films. These fingers may be ubiquitous in drying thin films of particles in viscous liquids and may have a strong impact of the adhesion of these films to their substrates.This research will investigate a visually striking fluid flow instability in colloid-polymer thin films during drying that was found using high-speed confocal laser scanning microscopy. This is the first observation of a fingering instability of polymer solutions at micron or sub-micron length scales in porous materials such as colloidal crystals. Fingering instabilities, such as the Saffman-Taylor instability, have had broad impacts for transport in porous media for secondary oil recovery and aquifer transport. Most studies of the Saffman-Taylor instability focus on the displacement of one higher viscosity Newtonian fluid with an immiscible lower viscosity Newtonian fluid in either a Hele-Shaw cell or a porous material. Fingers in this study are not visible using regular light microscopy because they have wavelengths on the micron scale, defying the typical Saffman-Taylor scaling found in macroscopic systems. This project will investigate the modes of this instability across various polymer concentrations, polymer molecular weights, and particle sizes where the polymer radius of gyration ranges from very small to that which is comparable to the pore scale of colloidal crystals. The goal is to determine whether this fingering instability differs from previous studies due to the non-Newtonian behavior or the interfacial properties of the fluid at the nanoscale. Because the polymer is left behind, it forms well-defined solid structures at the film-substrate interface. These structures will be investigated and utilized in the fabrication of nanoscale or microscale networks of fluid channels for devices. Moreover, the fingering instability may have a significant effect on film adhesion to the substrate through reduction of contact area and/or creating microfibrillated structures.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.