(2gc) Elasticity Induced Dynamics of Complex Fluids and Filaments

After completing my Ph.D. in 2022 under the guidance of Prof. Arezoo M. Ardekani (Mechanical Engineering, Purdue University), I joined Prof. Michael D. Graham's lab as a postdoctoral research associate (Chemical and Biological Engineering, University of Wisconsin-Madison). My research interests are complex fluids and microorganisms. Biological and industrial fluids are often complex due to the presence of long-chain macromolecules - for example, mucus and polymer solutions - and they exhibit mechanical responses between elastic solids and viscous fluids and hence known as viscoelastic fluids (Fig. 1a). The stretching of molecular chains in the flow of such fluids leads to anomalous flow dynamics, which regulate a wide range of industrial, environmental, and biological processes (Fig. 1b). Fluid-structure interactions between background fluids and elastic filaments are important for flagellated micro-organisms locomotion, mucociliary transport, and polymer processing of composites. My Ph.D. research focused on viscoelastic flows through porous media (Fig. 1c) and motile flagellar cells in complex flows (Fig. 1d). My current research as a postdoctoral researcher is focused on viscoelastic flow through channels (Fig. 1e) and the development of a quantitatively representative model of complex fluids using data-driven methods. My research has been published in leading peer-reviewed journals such as Proceedings of the National Academy of Sciences, Soft Matter, and Physical Review Fluids, and some of them also appeared on the cover page of the journals.

Research Background

Complex flows through porous media are relevant in a host of industrial applications such as groundwater remediation, microbial mining, enhanced oil recovery (EOR), and biological processes such as targeted drug delivery, infectious biofilm transport in the body, particles and cells transport during respiration and fertilization. Large elastic stresses induced due to confinement in the porous media lead to elastic instabilities in the viscoelastic flows. In the significant part of my Ph.D., I studied pore-scale viscoelastic instabilities and their effects on the sample scale transport of fluids and particles in porous media (Fig. 1c). The accumulation of polymeric stresses as polymeric chains advect through closely located pores creates streaks characterized by high polymeric stress. These streaks act as a barrier for the flow crossing the regions and lead to flow separation, which induces distinct pore-scale flow states in porous media. At the sample scale, viscoelastic flow enhances transverse transport in ordered porous media through the lateral fluctuations of these streaks and longitudinal transport in disordered porous media through the formation of conduits. Thus, the topology (structure) of the polymeric stress field controls the flow patterns and ultimately material transport in complex flows. However, direct measurements of the stress field are challenging, often inaccurate, and limited to simple geometries and steady flows. Using the Lagrangian description of flow, we developed a framework, which provides direct access to the topology of the polymeric stress field from readily measured flow fields in arbitrary geometries and even in unsteady flows. My presentation based on these findings won the best poster award at the 92nd Annual Meeting of The Society of Rheology (2021) and was selected for Purdue Engineering Graduate Showcase-2021. The paper published in PNAS reporting the connection between the polymeric stress field and the Lagrangian stretching field was also featured in Purdue ME news and other news outlets such as Phys.org.

Many micro-swimmers use long, thin elastic structures generally called flagella or cilia to propel themselves through background liquids. Their locomotion plays a critical role in fertilization, waste treatment, and environmental remediation. Particularly, the sperm are single flagellated cells, and they travel a distance of more than a thousand times their body length to reach the eggs through highly chaotic flows and complicated pathways for successful fertilization. During my Ph.D., I also studied sperm motility and flagellar elastohydrodynamic interactions in different external flows (Fig. 1d). In Poiseuille flow present in the reproductive tracts of internal fertilizing mammalians, the sperm swim downstream in strong flow and upstream in weak flow. The sperm also exhibit a net migration toward the centerline of the Poiseuille flow. Strong flows mechanically inhibit flagellar motility through elastohydrodynamic interactions and lead to rich buckling dynamics of the flagellum beyond a critical flow strength. These results offer insights into applications like sperm sorting and in-vitro-fertilization (IVF).

Adding a tiny amount of polymers (a few parts per million) dramatically reduces turbulent drag. Therefore, polymeric additives are commonly used in pipeline transport of liquids such as crude oil, water heating and cooling systems, and airplane tank filling. Polymeric additives have been also used in sewer systems for improving drainage capacity and are envisioned to be used in irrigation canals and flood control. My current research as a postdoctoral researcher is focused on viscoelastic flow in open and closed channels (Fig. 1e). Polymeric injection in an open channel leads to a decline in water level just downstream of the injection point due to the decreased friction at a fixed volumetric flow rate. However, unexpectedly far downstream of the channel, the water height increases which is detrimental to practical applications. We have investigated the mechanism of water rise and also suggested a technique to mitigate it. The polymeric addition can lead to elasticity-induced a new type of turbulence known as elastoinertial turbulence (EIT), which is suspected to limit the polymeric drag reduction. My current research is focused to investigate the dynamics of EIT.

Future Research Plan

My scientific vision is to establish a theoretical and computational lab to study viscoelastic flows relevant to energy, biomedical, and environmental applications. The flow of viscoelastic fluids such as mucus, biofilm, and interstitial fluid through tissues (poroelastic material) controls biological processes such as bacterial infection and targeted drug delivery. I plan to investigate instability and transport in viscoelastic flow through poroelastic materials. Often viscoelastic fluids interact with flexible filaments in biological systems. Several epithelial surfaces inside the human body such as the lungs’ airways and reproductive tracts are covered with arrays of elastic cilia through which viscoelastic mucus flows. Viscoelastic fluid-structure interaction is also important for flagellated microorganisms’ locomotion in their natural environment. I plan to investigate the effect of viscoelastic flow instabilities on the dynamics of flexible filaments and microorganisms’ motility. The investigation of pore-scale viscoelastic instabilities even in rigid geometries is mainly focused on single-phase flow through symmetric 1-D porous geometries. However, the natural porous geometries contain 3D asymmetric pores, and viscoelastic flows through them in realistic applications (i.e., EOR, Groundwater remediation) are often multi-phase flows. I plan to investigate viscoelastic instabilities in 3D asymmetric geometries considering polymeric degradation and multi-phase flow. Viscoelastic flows are important for electrochemical applications such as electrodialysis, desalination, electrodeposition, and electrical energy storage batteries. Liquid electrolytes used in these applications are complex fluids, where the fluid’s elasticity plays an important role-for example viscoelastic electrolytes in batteries can mitigate dendrites growth. My lab will investigate viscoelastic flows relevant to electrochemical applications.

Teaching Interests

Teaching and mentoring are an integral part of the academic career. I am keen to offer undergraduate and graduate-level courses in Fluid Mechanics, Transport Phenomena, and Numerical Methods. I have served as a teaching assistant for the Fluid Mechanics course during my Ph.D. at Purdue University. I have also delivered guest lectures in graduate-level courses in complex fluid mechanics at Purdue University. I also have experience mentoring a master's student and two Ph.D. students.