(14be) Flow and Jamming of Particulate Materials

The majority of industrial processes involve particulate forms of material. The grain sizes range from nano and mico-scale (e.g. colloids) to macro-scale (e.g. granular materials, emulsions, and foams). Even with a comprehensive knowledge of the particle-particle interactions, the prediction of mesoscale and large scale behavior of a collection of these particles is not trivial. A significant effort has been dedicated to understanding the phenomenology, and more importantly, unifying the laws which govern the collective dynamics of particulate systems. These studies cross between several fields such as Physics, Chemical and Mechanical Engineering, and material science. I am in particular interested in the flow of particulate systems, and I would like to pursue this field with experimental studies on the rheology of granular materials and colloids.

Postdoctoral Research:

(i) Reversibility of a Colloidal System: I am currently studying a colloidal system adsorbed at an oil-water interface, which is subject to cyclic shear. The colloidal particles are sub-micron size, such that Brownian motion has significant contribution to the single particle dynamics. Through this study, we found that thermal noise significantly affects the reversibility of material in a nontrivial manner.

[1] S. Farhadi and P. Arratia, â??Shear-induced Reversibility for Thermal Colloidsâ?, to be submitted (2016).

(ii) Dynamics of Active Systems: In active particulate systems, in contrast to passive systems, single particle dynamics is independently present even without external driving. We have designed two species of active granular systems: spinners, and translators. The intra-structure of particles (which are 3D printed) is designed such that they either spin or translate in the presence of an upward air flow. In this ongoing study, we aim at describing the collective dynamics, and in particular, we search for an effective temperature which governs the dynamics. Subsequently, we will be able to extract an equation of state for these systems.

PhD Research:

My PhD research was an experimental study on the jamming and rheology of granular materials, with a specific focus on elliptical-shaped particles. I performed continuous Couette shear (rheology) and cyclic compression (jamming-unjamming) on both systems of circular and elliptical shaped particles. The identical experiment, on both circles and ellipses, enabled us to understand the role of particle anisotropy on jamming and the bulk rheology.

[2] S. Farhadi, A. Zhu, RP. Behringer, â??Stress and Structure Instability in Cyclic Compression of Ellipses and Disksâ?, Physical Review Letters, (2015).

[3] S. Farhadi, RP. Behringer, â??Dynamics of sheared ellipses and circular disks: effects of particle shapeâ?, Physical Review Letters 112 , 148301 (2014).

[4] S. Farhadi, RP. Behringer, Alex Zhu, â??Slow dynamics for elliptical particles under continuous shear and cyclic compressionâ?, AIP Conf. Proc., 1542, 879-882 (2013).

Future Directions:

I would like to pursue my future research in two directions:

1- I am interested in developing techniques to directly measure force distribution for a disordered matter, in its grain scale. There have been few successful experimental techniques such as photoelasticity and contact area size to measure grain scale stress. However, the filed of colloids lacks experimental tools for this purpose so far. Knowledge of the grain scale forces is useful in connecting the structural properties to the dynamics, and eventually obtaining insight on problems such as self assembly, where both structure and grain scale forces play role.

2- An emerging filed in physical sciences is 'machine learning with big data'. The idea is to extract information by obtaining a large collection of data from a specific system, and create learning models to predict the dynamical behavior. I would like to apply these techniques to understand the underlying mechanisms of flow and transport in particulate systems.

Teaching Interests:

As a graduate student of Physics, I have been involved with teaching several introductory courses, which included lab instruction. I also have experience of teaching nonlinear dynamics, which is a higher level undergraduate course. I should emphasize that I have an undergraduate major degree in Chemical Engineering, which helps me with teaching courses such as thermodynamics, fluid dynamics, and heat and mass transfer. Also, as an expert in granular materials, I can develop new courses such as granular matter, transfer in porous materials, and more mathematical based courses such as nonlinear dynamics, and chaos.