(6kf) Soft and Biological Materials: Simulation and Theory (SoftBiM)

Collective motion in dense packings of cells occurs in wound healing, embryonic development, and cancerous tumor growth. Most current computational models of dense cell packings either treat the system as collections of spherical particles or assume that the system is confluent, with no extracellular space. We have developed a new model for dense cell packings in two spatial dimensions, where the cells are modeled as deformable particles that have a preferred area and perimeter. We measure the packing fraction, φj at jamming onset as a function of the asphericity, α, which is the ratio of the perimeter square to the area of the particle. We find that the jammed packing fraction increases monotonically with α and that the system becomes confluent with φ_j = 1 for α > 1.16. Using surface Voronoi analysis, we show that this value for α corresponds to the case when the cells completely fill their Voronoi-tesselated regions. We also demonstrate that the free area per cell obeys a k-gamma distribution, which has been found for jammed packings of non-deformable particles. Finally, we will describe results from our model concerning the mobility of deformable particles subjected to applied forces, as well as diffusion of deformable particles subjected to active forces to discuss the effect of geometry in active jamming of deformable particles.

Keywords: Soft materials, Granular materials, Complex fluids, Jamming of Cell monolayers.

Research Interests:

Soft Materials such as complex fluids, granular materials, and soft solids are ubiquitous in industrial applications and frequently used in the food, consumer, oil, and biotechnology industries. Their processability and stability are mainly affected by their flow behavior and rheological properties. To explain their complex rheological behaviors one needs to understand their microstructure and its evolution under applied deformation. In addition, there is a spectrum of length and time scales that must be resolved to explain their macroscopic flow behavior such as shear-thinning in polymer solutions and biological fluids like blood, shear-thickening in colloidal suspensions, and jamming and yielding in epithelial cell monolayers. Particulate systems composed of soft particles, such as emulsions, foams, bubbles, hydrogels, grafted core-shell particles, dendrimers, and star polymers can be used as the building blocks novel multifunctional materials, with optimized energy absorption, self-healing behavior, high mechanical strength, and other desirable properties. In addition, many biological systems such as biofilms, cell colonies, and tissues can also be considered as collections of soft and deformable particles. In my future research, I will develop a theoretical framework that will enable the design of new multiscale soft materials with optimized and multifunctional properties.

Postdoctoral Project: “Jamming of soft deformable particles: From bubbles to tissues”. This research was conducted under the mentorship of Prof. Corey S. O’Hern in the Department of Mechanical Engineering & Material Science, Yale University, New Haven, CT.

Ph.D. Dissertation: “Computational studies on multi-phasic multi-component complex fluids”. This research was conducted under the mentorship of Prof. Joao M. Maia from the Department of Macromolecular Science and Engineering, Case Western Reserve University, Cleveland, OH.

Research Experience:

My academic training in mechanical engineering, chemical engineering, rheology, and macromolecular science and engineering enabled me to study a wide range of materials using experimental, computational, and theoretical techniques. During my postgraduate studies in Europe (KU Leuven, Belgium, University of Ljubljana, Slovenia, and University of Minho, Portugal), I focused on the rheological properties of polymeric materials in both solid and liquid states with an emphasis on designing and fabricating novel polymeric nanofiltration membranes with antifouling properties. During my PhD training, I developed simulation techniques to investigate the rheological properties of a variety of soft materials such as colloidal suspensions, colloidal gels, emulsions, and polymeric blends. I have expertise in using micro- and mesoscale simulation techniques, such as molecular dynamics, dissipative particle dynamics, smooth particle hydrodynamics, and Lattice Boltzmann methods and macro-scale techniques such as finite element and discrete element method simulations. I have also developed multi-threaded, parallel, and GPU algorithms for high performance computing clusters.

Teaching Interests:

While at Yale, I served as a research mentor for several high school and undergraduate students during the summer through the Science, Technology, and Research Scholars (STARS) program and Research & Engineering Apprenticeship program (REAP), which is supported by the Army Educational Outreach Program (AEOP). These programs seek to encourage and prepare disadvantaged students and students from underrepresented groups to pursue careers in science, engineering, technology and mathematics (STEM) fields. I have also served as a teaching assistant for undergraduate and graduate courses in the rheology of complex fluids, theory of viscoelasticity, and computational methods in chemical and mechanical engineering. As a faculty member, I am interested in teaching courses at both the undergraduate and graduate levels in fluid mechanics, transport phenomena, microhydrodynamics, and mechanics of materials.

Future Directions in SoftBiM:

The main direction of my future research will be the development of highly efficient hierarchal computational methods to understand the mechanical and rheological properties of soft materials. My computational studies will be complemented by theoretical calculations to investigate the flow behavior of soft particulate systems with tunable interactions relevant to a wide range of industrial applications such as flow assurance, mixing, and separations. These studies will accelerate the development of novel multifunctional materials by tuning particle interactions, geometries, and tribological properties such as surface friction.

Fluid mechanics and rheology of complex fluids: As a faculty member, I will use mecoscopic simulation methods to study flows of multiphasic systems such as suspensions and emulsions in bulk and under confinement. These systems are of immediate importance in a wide range of industries such as oil, food, consumer products, and biotechnology. Flows in micro-channels are used to model biological phenomena, such as blood flow and cell migration where the suspended particles are soft and deformable. For example, in patients with sickle cell anemia, some red blood cells (RBC) stiffen into irregular shapes as a result of oxygen depletion (through mutant hemoglobin (HbS) nucleation and polymerization), which blocks small blood vessels. The blockages further deplete oxygen, which promotes vasoocclusion. To model sickle cell anemia, a multiscale approach is required since the problem involves a wide range of length scales (from nanoscale nucleation and growth to microscale shape changes and macroscopic clogging of cells). I will investigate the role of particle geometry, surface properties, interactions, and mechanical properties in determining the flow mechanics of a wide range of multiphasic systems ranging from colloidal suspensions and gels to emulsions and bubble flows to cell monolayers and tissues.

Granular materials: From sand and staples to animal swarms: While a pile of dry sand can flow freely like a fluid upon removing the container walls, the same is not true for highly nonspherical particles. In fact, piles of long rods, flexible fibers, and staples can behave like cohesive solids and form a free-standing stable column with finite tensile and compressive strengths. The cohesive behavior of the pile is purely geometrical and is not currently understood. Geometric cohesion is used by swarms of insects as a survival mechanism to build stable structures such as rafts to survive floods or adaptable clusters of insects to protect themselves during their reproductive cycle against environmental, mechanical (gravity, wind), and thermal perturbations. My research group will investigate the physical mechanisms that give rise to geometrical cohesion and how it can be used to create and develop new materials that utilize geometric cohesion.

Selected Publications:

1. A. Boromand, A. Signoriello, F. Ye, C.S. O’Hern, M.D. Shattuck, “Jamming of Deformable Polygons”, Phys. Rev. Lett. Under review, (2018).
2. A. Boromand, S. Jamali, B. Grove, J. Maia, “A generalized frictional and hydrodynamic model of the dynamics and structure of dense colloidal suspensions”, J. Rheol. Cover Image 62, 905 (2018).
3. A. Boromand, S. Jamali, J. Maia, “Structural fingerprints of yielding in short-range attractive colloidal gels”, Soft Matter. 13, 458 (2017).
4. A. Boromand, S. Jamali, J. Maia, “Gaussian-inspired auxiliary non-equilibrium thermostat (GIANT) for Dissipative Particle Dynamics simulations”, Comput. Phys. Commun. 197, 27 (2015).
5. A. Boromand, S. Jamali, J. Maia, “Viscosity measurement techniques in Dissipative Particle Dynamics”, Comput. Phys. Commun. 196, 149 (2015).
6. S. Jamali, A. Boromand, N. Wagner, J. Maia, “Microstructure and rheology of soft to rigid shear-thickening colloidal suspensions”, J. Rheol. 59, 1377 (2015).
7. 7. S. Jamali, A. Boromand, S. Khani, J. Wagner, M. Yamanoi, J. Maia, “Generalized mapping of multi-body dissipative particle dynamics onto fluid compressibility and the Flory-Huggins theory”, J. Chem. Phys. 142, 164902 (2015).