(127c) Dynamics of Complex Fluids and Soft Materials

Dense suspensions, i.e., solid particles suspended in a liquid are ubiquitous systems encountered in many forms, where particle size can range from nm of proteins and lipids in cells all the way to mm in case of mud slurries [1, 2]. At the largest length scale, geological hazards like landslides involve flow of mud slurries whereas cement and concrete flows that arise in construction and landscaping are also contributors to climate and energy problems [3]. Confined macromolecules like proteins and lipids found in cells, tissue engineering, flow of blood cells, and jamming of tumorous cells are biologically relevant examples of such suspensions [4]. Understanding the mobility of colloids or nanoparticles is crucial to various physical, chemical, biological as well as industrial systems [5,6]. Examples of these suspensions range from protein suspension, transport through membranes, to flow of fine particles dispersed in a fluid. Such highly concentrated suspensions under confinement can shed light into dynamics of crowded macromolecules, as such proteins inside biological cells. These confined systems often display anomalous diffusion. The understanding of diffusion of biomolecules is critical since it underpins a variety of intracellular metabolic, translational and locomotion processes [2,4]. On the other hand, mud slurries, cement and concrete flows, ceramics associated with industrial processing that are energy inefficient and major contributors to CO₂ emission. The flow behavior of such suspensions often displays non-Newtonian behaviors such yield stress, normal stress differences, shear tinning, shear thickening or even shear jamming.

To tackle these challenging issues numerically, I have developed two simulation schemes. The first numerical scheme (COPSS) is a prototype that considers hydrodynamic interactions (both near-and-far field) and couple it with Brownian dynamics under confinement. Precise modeling of the hydrodynamic interactions under confinement is a challenging issue [5] and to solve this we relied on an Immersed Boundary General geometry Ewald-like method to capture lubrication and long-range hydrodynamics and include the appropriate non-slip conditions at the confining walls [6,7]. Using this scheme, we made the first step of qualitatively reproducing the experimental observed anomalous diffusion as observed in cells and is crucial for the understanding of diffusion of biomolecules that underpins a variety of intracellular metabolic, translational and locomotion processes [2,4]. In the cellular environment, the lipids and proteins are often squishy and often attractive interactions are also found. The first step would be to model the soft, squishy particles with full hydrodynamics under confinement. This implementation can further be used to study the cell jamming observed, where the role of hydrodynamic interactions has not yet been explored. Further in the project, we would also consider the attractive or adhesive interactions between particles and particles and fluid.

To tackle the other challenging issue of the macroscopic flow behavior of dense suspensions as observed in natural settings such as landslides or geological faults and industrial settings such as flow of slurries in pipelines, we will rely on the latter approach lf_dem [8]. We have so far only considered the ideal, monodisperse spherical particles [7-14]. However, in the industrial or natural settings, the particles are often non-spherical, highly rough and polydisperse [3]. The aim of this part of the project is to develop a faster algorithm that can simulate large number of particles in a reasonable time frame to simulate industrial systems. The speed up will also help us to simulate highly polydisperse (size ratio>10) systems in a reasonable time frame.

The above-mentioned challenges, the non-equilibrium, driven problems such as flowing sand, mixing cement and concrete, mudflow, slurries; and equilibrium problems like diffusion of proteins, and drug delivery are all studied individually. Current theories are patchworks of phenomenological models, but we lack a comprehensive picture to predict the flow behavior of such systems. Developing a unified picture for complex fluids holds the key to potentially revolutionizing not only engineering practices but also our fundamental understanding of multiphase fluid flow. In terms of overarching goal, we aim to bridge the gap between the disparate disciplines such as biology, fluid dynamics, contact mechanics and network theory. These numerical and theoretical efforts will highlight the intricate role played by hydrodynamics and colloidal-scale motion that appear in common across cell functions, types, and conditions. The proposed efforts on one hand explore not only the diffusion of a mixture of shape and size polydisperse macromolecules inside the cells but also the jamming of cells. On the other hand this work will deal with larger (granular or non-Brownian) dispersions and highlight the important role of contact mechanics at the particle level [15] and the network theory [16]. Further, establishing the relationship between network structure to the noted microscopic contact interactions is challenging in terms of the nonequilibrium statistical mechanics and will be explored.

References

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