Research

Synopsis:

My scientific interest involves extending the understanding of physics of complex fluids such as colloidal suspensions, emulsions, polymer solutions and polymer nanocomposites to design new processes and multifunctional materials that are more energy-efficient, environmentally friendly, clinically safe and cost-effective. These studies are becoming particularly challenging when inherent material properties (e.g. opaqueness, thermal stability, photobleaching) limit the utilization of conventional characterization techniques. Thus the nature of my research is based on simultaneous utilization of numerous characterization techniques, e.g. light scattering, optical microscopy, rheology, and microfluidics in conjunction with theoretical models. I attempt to answer fundamental questions such as how do materials’ pristine properties, the processing conditions and molecular/mesoscopic interactions affect the bulk properties? What are the mechanisms of structure formation under the influence of various parameters? What is the time scale of structural evolution under different processing conditions? How to control material bulk properties by manipulating the properties at smaller scales? To what extent do different forces contribute to the transport of soft materials (such as vesicles, double emulsions, colloidal suspensions, etc.)?

 

Current Research Projects:

1) Design, fabrication, and optimization of milli-channels using fast prototyping techniques

2) Investigating steady and transient flow behavior of Newtonian and non-Newtonian fluids in bi-furicating and impinging jet flows

3) Computational modeling and design of experiments to study extensional flow properties of dilute polymer solutions

 

Past Research Projects:

1) Analyzing the formation of inertio-elastic vortices of shear-thinning fluids in planar microfluidic abrupt contractions (University of California, Berkeley)

Flow instabilities arise in high-speed industrial processes such as electrospraying, inkjet printing, roll-coating, and, fiber spinning. The instabilities can compromise the quality of the final product in various forms: formation of bead structures in fiber spun fibers, the emergence of wrinkles in roll coating and, etc. The origin of flow instabilities stems from the extreme extension rates that a fluid (such as a dilute polymer solution, a polymer melt or an ink suspension) experiences during the high-speed flow in a nozzle throat (i.e. a small length scale) over a very short period of time. In the first project, we studied the formation of flow instabilities in microfluidic sudden contractions and investigated the effect of flow rate, channel geometry and fluid elasticity on the flow instability.

Relevant publications:

• Khalkhal, Fatemeh, Kendrick Chaney and Susan Muller (2016), Optimization and application of dry film photoresist for rapid fabrication of high-aspect-ratio microfluidic devices, Microfluidics and Nanofluidics, 20 (11), 153.

• Khalkhal, Fatemeh and Susan Muller, (in preparation), Formation of inertio-elastic vortices of a shear-thinning fluid in planar microfluidic abrupt contractions.

 

2) Microfluidic studies of emulsions and suspensions in wall-bounded shear flow (University of California, Berkeley)

Monodisperse double emulsion droplets (~100 um in diameter) were generated in flow-focusing microfluidic devices. Due to their monodispersity (polydispersity index of 1.0005), they are excellent candidates for drug delivery agents. They can also be used as models for cells for in vitro studies, in the absence of actual cells. We studied the inertial migration behavior of double emulsion droplets in straight channels (at Re ~ 17-18) in comparison with rigid spheres. The elastic droplets were more focused on the center of the channels near the outlet while the rigid particles traveled very close to the channel walls. This is a work in progress and can help us to better understand the mechanisms of blood cell migration at different flow rates.

Relevant publications:

• Hidema, Ruri, Fatemeh Khalkhal, and Susan Muller, Optimizing a microfluidic device to produce double emulsions, International Congress on Rheology (ICR), Kyoto, Japan, August 2016.

Khalkhal, Fatemeh, and Susan Muller, Dynamics of Double Emulsion Droplets in a Wall-Bounded Shear Flow, American Institute of Chemical Engineers Annual Meeting, Salt Lake City, UT, November 2015.

Khalkhal, Fatemeh, and Susan Muller, Microfluidic Studies of Emulsions and Suspensions in Wall-Bounded Shear Flow, American Institute of Chemical Engineers Annual Meeting, Salt Lake City, UT, November 2015.

 

3) A fundamental study to address carbon (soot) buildup in car engines by analyzing the efficacy of newly designed surfactants (Yale University)

Carbon (soot) which is a byproduct of incomplete fuel combustion, needs to be collected by engine oil before it can accumulate and damage the engine. Once it is introduced into the oil, carbon can increase the viscosity of oil which makes it more difficult to flow. Several surfactants could be designed and added to facilitate the dispersion of carbon in oil while it circulates and cleans the engine. In this project, the efficacy of several surfactants in dispersing soot in engine oil was evaluated at room and elevated temperatures by designing a set of bench experiments to simulate engine oil tests with high fidelity. 

Relevant publications:

Khalkhal, Fatemeh, Ajay Singh Negi, James Harrison, Casey D. Stokes, David L. Morgan and Chinedum Osuji (2017), Evaluating dispersant stabilization of colloidal suspensions from the scaling behavior of gel rheology and adsorption measurements, Langmuir, 34 (3), 1092-1099.

Khalkhal, Fatemeh, and Chinedum Osuji, Scaling behavior of colloidal gel elasticity in the context of dispersant surface activity, Society of Engineering Science (SES) 50th Annual Technical Meeting, Brown University, Providence, RI, July 2013.

 

4) Characterization of flow-induced structures in carbon nanotube suspensions (Ecole Polytechnique de Montreal)

Developed structure-property relationships in model carbon nanotube-epoxy suspensions in collaboration with a multidisciplinary team of industrial partners, government agencies and Concordia University as a part of a large scale project to develop novel nano-composites with high electrical conductivity and electromagnetic shielding effectiveness for more fuel-efficient and cost-effective aircraft.
The macroscopic properties of suspensions are inherited from their structure which can be easily influenced by various parameters such as processing condition, particle-fluid, and particle-particle interactions and, particle flexibility. Since carbon nanotube suspensions are very opaque, the application of light scattering techniques in structure characterization is very challenging. A set of resolute (mostly rheological) experiments were proposed to characterize the flow-induced structures quantitatively.

Relevant publications:

Khalkhal, Fatemeh and Pierre J. Carreau (2012) Critical shear rates and structure build-up at rest in MWCNT suspensions, J. Non-Newtonian Fluid Mechanics, 171-172, pages 56-66.

Khalkhal, Fatemeh and Pierre J. Carreau (2011) Scaling behavior of the elastic properties of non-dilute MWCNT-epoxy suspensions, Rheologica Acta, 50(9), 717-728.

Khalkhal, Fatemeh, Pierre J. Carreau and Gilles Ausias (2011) Effect of flow history on linear viscoelastic properties and the evolution of the structure of MWCNT suspensions in an epoxy, J. Rheology, 55(1),153-175.

Khalkhal, Fatemeh, Characterization of flow-induced structures in carbon nanotube suspensions, Ph.D. thesis, Ecole Polytechnique de Montreal, Montreal, 2011.