(195o) Confinement and Wettability Effects on the Observed Rheology in Microcapillary Flow

Properly characterizing the rheology of non-Newtonian fluids is a key step when designing any process where the fluid will be flowing. Numerous well-designed rheometers and viscometers can perfectly characterize the bulk rheology of even the most complex non-Newtonian fluids. However, in various applications, complex fluids flow in small capillaries (30-200 µm in diameter), where deviations from bulk rheology due to surface effects and confinement become much more important to understanding the behavior and path of the fluid flow. ‘Confinement’ occurs when confining dimensions become comparable to the length scale of cooperative motion in the fluid. (Voronov, Papavasiliou and Lee 2008) Confinement manifests itself as deviations from expected steady-state behavior and, often the development of unusual dynamic flow instabilities. (Cohen, Mason and Weitz 2004) (Lin, Cheng and Cohen 2014) (Bakli and Chakraborty 2015) (Brennan, et al. 2015) (Ramaswamy, et al. 2017) For most Newtonian fluids, confinement does not set in until the confining dimension approaches the molecular length scale of Å or nm. However, many complex, non-Newtonian fluids contain much larger molecules, particles, and self-assembled structures with sizes ranging from 10’s of nanometers to 100’s of microns.

This work uses a microcapillary rheometer device to characterize the steady state and dynamic flow behavior of model complex fluids (wormlike micelle solutions and semi-dilute polymer solutions) under confining conditions and compares them to bulk rheological measurements made using a conventional Couette rheometer. This research also uses chemical surface modification to hydrophobize the surface of the fused silica microcapillaries in order to mimic both ‘water-wet’ and ‘oil-wet’ surface conditions, in order to further understand how these surface properties affect the observed rheology of complex fluids in real systems such as oil reservoirs or other porous media. The experimental results are compared to theoretical predictions and mathematical models that have been developed to describe complex fluid flow through small capillaries or pores.

References:

Bakli, C., and S. Chakraborty. 2015. "Slippery to Sticky Transition of Hydrophobic Nanochannels." Nano Lett. 15: 7497-7502. doi:10.1021/acs.nanolett.5b03082.

Brennan, J.C., N.R. Geraldi, R.H. Morris, D.J. Fairhurst, G. McHale, and M.I. Newton. 2015. "Flexible conformable hydrophobized surfaces for turbulent flow drag reduction." Scientific Reports 5: 10267. doi:10.1038/srep10267.

Cohen, I., T.G. Mason, and D.A. Weitz. 2004. "Shear-Induced Configurations of Confined Colloidal Suspensions." Phys. Rev. Lett. 93: 046001. doi:10.1103/PhysRevLett.93.046001.

Lin, N.Y.C., X. Cheng, and I. Cohen. 2014. "Biaxial shear confined colloidal hard spheres: the structure and rheology of the vorticity-aligned string phase." Soft Matter 10: 1969-1976. doi:10.1039/C3SM52880D.

Ramaswamy, M., N.Y.C. Lin, B.D. Leahy, C. Ness, A.M. Fiore, J.W. Swan, and I. Cohen. 2017. "How Confinement-Induced Structures Alter the Contribution of Hydrodynamic and Short-Ranged Repulsion Forces to the Viscosity of Colloidal Suspensions." 7: 041005. doi:10.1103/PhysRevX.7.041005.

Voronov, R.S., D.V. Papavasiliou, and L.L. Lee. 2008. "Review of Fluid Slip over Superhydrophobic Surfaces and Its Dependence on the Contact Angle." Ind. Eng. Chem. Res. 47: 2455-2477. doi:10.1021/ie0712941.