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Microscale transport phenomena in complex fluids
Microscale transport phenomena in complex fluids
Surfactant molecules are amphiphilic in nature, and when these molecules are dissolved in water above a critical concentration, they spontaneously self-assemble into large and flexible aggregates which are known as the wormlike micelles. These wormlike micellar (WLM) solutions are widely used in many pragmatic applications, for instance, in environmental remediation, in cosmetic industries as gelation and shear-thinning fluids, as drag reducing and cooling agents, etc. In particular, the rheological properties of these wormlike micellar solutions were found to be interesting and intricate as compared to those of a polymer solution due to their continuous breaking and reformation dynamics under an imposed flow field. Nowadays, these wormlike micellar solutions are also widely used as fracturing fluids in enhanced oil recovery. In this particular application, a wormlike micellar solution is forced to flow through a complex micropore network and/or porous medium in order to displace the residual heavy oils for their enhanced recovery. Therefore, a thorough understanding of the flow of a WLM solution through these complex micropore networks is extremely vital for their better use.
The VCM (Vasquez-Cook-McKinley) constitutive model has been used for the WLM solution for predicting its rheological behavior and the governing equations, namely, mass and momentum equations along with the VCM constitutive model equations have been solved using the finite-volume method based open source code OpenFOAM.
Single polymer dynamics study
Single polymer dynamics study
In order to design a desired product in many industries, especially in cosmetic, pharmaceutical and plastic industries, a thorough knowledge is necessary at the molecular scale, for instance, the topology and conformation of molecules, their packing at the molecule scale and how they interact with other surrounding molecules under the influence of various intra and intermolecular forces. For accruing these informations, DNA molecule is very often used as model system, in particular, for polymer solutions. This is mainly because of the fact that these DNA molecules can be easily prepared with similar molecular weights, and can be easily stained with a suitable fluorescent dye, and thereby offering a way for its visual observation under a microscope. Using DNA as a model polymer, a voluminous body of studies available in the literature but mostly related to either dilute or concentrated polymer solutions. However, recently, this single molecule study has been extended to the semidilute regime which lies in between the dilute and concentrated regimes. This particular regime of polymer solutions has many industrial applications, for instance, in ink-jet printing and spinning of nanofibers.
A typical microfluidic set up for carrying out the single molecule study of DNA molecules in an extensional flow field is shown in Figure 1. In a flow field, the fluid molecules exert a drag force on the DNA molecules, and therefore the DNA molecules tend to unravel and stretch out. However, all the DNA molecules will not behave identically as they have different initial configurations and they are also subjected to constant thermal fluctuations due to the bombardment by the solvent molecules. This results a rich varieties of molecular configurations as schematically shown in Figure 2 in case of a semidilute polymer solution which are different to that seen in case of either dilute or concentrated polymer solution. In addition to this, it has been found that the molecules in the semidilute regime stretch less than that in the dilute regime. All these experimental findings are further corroborated by performing highly advanced mesoscopic simulations based on the Brownian dynamics method. For more details, please see the references below.
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