Research areas in Complex Fluids Lab (CFL):
1) Development of structure-property relationship in complex fluids
One of the research goals of the Complex Fluids Lab is focused on uncovering the mechanisms involved in the formation of different flow structures in complex fluids, particularly in milli- and micro-channels. Complex fluids are mixtures of a fluid (liquid or gas) and another phase (liquid, gas, or solid) that exhibit unusual mechanical responses to applied stress or deformation due to the geometrical constraints (e.g., the shape of the microstructure) that the phase coexistence imposes. Examples of the applications of these materials are to be found in printing inks, food products, personal care products, lubricating oils, liquefied natural gas, and medicine, to name a few.
The development of new multi-functional complex fluids highly relies on our ability to accurately measure, characterize, and control the microstructure often by studying the mechanical and physical behavior of complex fluids in bulk. Given the non-trivial nature of the relationship between the microstructure and mechanical properties of complex fluids, we pursue to establish this relationship by exploring the correlations between comprehensive experimental and theoretical results. By combining computational fluid dynamics (CFD) with experimental techniques such as rheometry, optical microscopy, and flow visualization, we 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.)?
- 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, 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.
2) Fabrication of low-cost microfluidic devices using fast prototyping techniques
Microfluidic devices (or chips) are miniaturized devices containing micron size chambers with dimensions as small as the thickness of a human hair (~ 75 microns) or smaller, through which fluids flow or are confined. Due to their small size, they can operate at minimal quantities of samples, which is critical in many biomedical applications, including lab-on-a-chip (e.g., blood glucose test strips), diagnostics (e.g., cancer, HPV), organ-on-a-chip (e.g., an artificial heart) and drug delivery (e.g., insulin patches). Such devices can annually save us millions of dollars due to much shorter time of experiments (by parallelization of experiments on a single device), and by saving the lab space through performing the experiments on a small chip. Microfluidic devices can facilitate early cancer diagnostics and reduce the risk of human error as a result of low concentration of cancer cells in the blood at the very early stages of the disease.
Microfluidic devices are traditionally fabricated using soft photolithography techniques. For specific applications, these devices need to have a relatively high channel depth or high aspect ratio [aspect ratio = channel depth/channel width]. Fabrication of high-aspect-ratio microfluidic devices with conventional soft photolithography such as SU-8 based techniques is very challenging. On the other hand, molds with a wide range of aspect ratios can be prepared by laminating a single or multiple layers of a thin, dry film photoresist onto metal wafers; we can make devices as deep as 500 um and with aspect ratios as high as 10.
Similarly, milli-channels can be designed and fabricated using additive manufacturing techniques for desired applications such as analyzing the bifurcating flow of complex fluids. The fast prototyping lab in the school of engineering is currently equipped with four Ultimaker 2+ and two Creality 3D printers and 16 workstations complete with 3D modeling software including SolidWorks/Simulation, AutoCAD, Creo, and Fusion360.
- 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.
- Hidema, Ruri, Fatemeh Khalkhal, and Susan Muller, Optimizing a microfluidic device to produce double emulsion droplets, International Congress on Rheology, Kyoto, Japan, August 2016.
3) Generation and migration behavior of monodisperse double emulsion droplets
Double emulsion droplets are used in many applications, including drug delivery and models for cells for in vitro studies, in the absence of actual cells. In drug delivery applications, it is crucial to generate droplets that have the same size (i.e., are monodisperse) to ensure uniform distribution of drugs to different parts of an organ and different organs. The uniform size can be measured using a polydispersity index (PDI); larger values of PDI correspond to a broader size distribution. PDI can be defined as the standard deviation (σ) of the particle diameter distribution divided by the mean particle diameter. We use flow-focusing microfluidic devices to generate mono-disperse (uniform size, ~100 um in diameter) droplets of water in oil in water (W/O/W) with PDI of 1.0005.
Monodisperse double emulsion droplets can also be used as models for cells for in vitro studies, in the absence of actual cells; an example includes using these droplets to mimic the migration behavior of red blood cells in microfluidic channels. In a preliminary study, we examined the inertial migration behavior of double emulsion droplets in straight channels (at Re ~ 17-18) compared to 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 work in progress can help us to better understand the mechanisms of blood cell migration at different flow rates and uncover the underlying mechanisms in blood clotting.
- 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 (oral presentation).