EAGER: Microscale Fingering Instabilities in Drying Colloid and Polymer Films

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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.