Measuring complex fluids under extreme flow conditions

R. P. Murphy, P. F. Salipante, S. D. Hudson, T. B. Martin, P. A. Beacauge, E. G. Kelley, K. M. Weigandt

Utilizing the unique focusing optics, flexible sample space, and SAXS capabilities at the FMB-beamline, a group of researchers from the National Institute of Standards and Technology measured the rheology and structure of complex fluids subjected to extreme flow velocities while confined within micrometer-sized capillaries.

abstract image of x-ray scattering

What did the scientists do?

A capillary rheometer capable of producing high shear rates at the wall, previously developed for neutron scattering, was modified to expand the accessible shear rates up to 107 s-1 when using a high-flux x-ray source with small spot sizes, such as the FMB-beamline at CHESS. Using the new setup optimized for x-ray scattering, the structure and rheology of worm-like micelle solutions were measured at high shear rates to better understand the microstructural alignment, breakdown, and shear thinning rheology of these widely utilized surfactants.

Why is this important?

Worm-like micelle surfactant systems have numerous applications ranging from pharmaceutical formulations to enhanced oil recovery. The simultaneous rheology and x-ray scattering measurements will help link the changes in macroscopic rheological properties to the changes in nanoscale fluid structure such as micelle orientation and length distribution. These measurements are also important to improve rheological models, which currently fail to accurately predict the viscosity of complex fluids at high shear rates.

What are the broader impacts of this work?

Industrial processing methods such as injection, spraying, coating, and jetting require high flow rates in confined geometries. These two flow requirements produce high shear rates and shear stresses within the fluid near the stationary wall, which can lead to reversible and irreversible changes to the complex fluid microstructure.

High-shear flows are also encountered in everyday life such as pharmaceuticals or paints that are sprayed out of a nozzle, vaccines that are injected through a small needle, blood flowing through micrometer capillaries, and polymers fuel additives lubricating and squeezing between the piston walls inside a combustion engine.

The experimental methods and x-ray scattering analysis from this work will provide practical tools to better measure and understand the extreme flow conditions that can alter the nanostructure of a wide range of complex fluids. Expanding the range of extreme flow measurements will be helpful to design and engineer new pharmaceuticals, coatings, lubricants, and fuel additives with improved flow-stability and rheological properties.

image of experimental setup, description in caption
The high-shear capillary rheometer cell positioned at the FMB-ID3B beamline at CHESS (a). Sample viscosity is determined by controlling the flow rate and measuring the pressure drop across a small, fused silica capillary with an inner diameter of 75 µm. Water flowing at a maximum fluid velocity of (b) 68 m s^-1 and (c) 136 m s^-1 through a 25 µm and 50 µm capillary, respectively, corresponds to a wall shear rate of approximately 107 s^-1. Conditions of (b) laminar flow or (c) turbulent flow can be achieved depending on the sample response to flow. An x-ray beam is focused on the small inner diameter center to probe the center line region of the velocity field (d). SAXS measurements at the FMB-beamline showed distinct changes in the worm-like micelle structure at rest (e) and under flow (f).

Why did this research need MSN-C & CHESS?

Since smaller diameter capillaries are required to achieve higher shear rates, the ability of the FMB-beamline to create small incident x-ray spots at the sample was key to obtain sufficient scattering signal from less than 1 nanoliter of sample volume. The extremely small x-ray spots were also utilized to scan different positions of the axis-symmetric velocity field, which provided different projections of the sample structure under flow within the same capillary cell.

How was the work funded?

This work was supported by the National Institute of Standards and Technology, NIST Center for Neutron Research, and the NIST nSoft consortium. RPM and EGK acknowledge support from the Center for High Resolution Neutron Scattering (CHRNS) and the National Science Foundation.

References:

R. P. Murphy, et al. (2020) ‘Capillary RheoSANS: measuring the rheology and nanostructure of complex fluids at high shear rates.’ Soft Matter, 16 (27), 6285-6293. http://dx.doi.org/10.1039/D0SM00941E