Particle-Induced Fingering and Pattern Formation

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Non-technical Description:The dynamics of interfaces between two fluids, such as air and water, exhibit diverse physical phenomena. Known as interfacial instabilities, fluid-fluid interfaces can deform and generate intricate and regularized patterns, such as tears of wine, fingers of paint as it coats a wall, and droplets forming from a liquid jet. In particular, when one of the fluid phases comprises particles, such as mud (a water and sand mixture), the dynamics of fluid-fluid interfaces can be even more complex and difficult to predict. In addition to their ubiquity, controlling interfacial instabilities is of critical importance in many engineering applications, ranging from droplet microfluidics, nanoparticle coating and printing, to manufacturing new functional materials. While many of these diverse applications involve a liquid phase comprising particles (also known as suspensions), the effects that suspended particles have on the interfacial instabilities are yet unknown. Hence, understanding the coupled dynamics between fluid-immersed particles and fluid-fluid interfaces can impact a wide range of applications and even lead to innovations that harness the instabilities for new applications. In addition, interfacial instabilities constitute a class of visually stunning phenomena that can be easily demonstrated in tabletop experiments or as part of an art project. Hence, the visual aspects of the research are incorporated into an educational module and outreach activities that connect art to core fluid mechanics concepts for students of diverse backgrounds and ages. These educational activities are designed to engage artistic students in science, to increase the diversity and retention of student in STEM, and to enhance awareness and appreciation for fundamental science and fluid mechanics in general public. Technical Description:The overall objective of this proposal is to understand the physical mechanisms and conditions for instabilities on the interface between the particle-liquid mixture and gas. Despite the ubiquity of processes that involve suspensions and fluid-fluid interfaces, the effects of fluid-immersed particles on the interfacial instabilities remain poorly understood, with opportunities for making fundamental advances. In this proposal, the PI experimentally considers two complementary flow configurations that interface gas with the mixture of non-colloidal particles and viscous liquid: 1) the suspension displacing air, and 2) air displacing the suspension inside a vertical Hele-Shaw cell. Distinct from the current state of the art, the two flow configurations are inherently stable without particles, so that they can isolate the effects of suspended particles on the fluid-fluid interface. The PI’s preliminary experiments have demonstrated the emergence of interfacial deformations and new particle-laden structures, which are induced by the coupling of particles with the fluid-fluid interface. Experimental measurements of the novel particle-induced instabilities are combined with reduced mathematical modeling and additional parametric studies, with the aim of discovering the basic mechanisms required to exploit the instability and lead to new pattern generation. The results of this fundamental study have the potential to transform the current understanding of the interfacial dynamics of suspensions and enable the use of suspended particles to control interfacial instabilities.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.