Measuring Material Microstructure Under Flow Using 1-2 Plane Flow-Small Angle Neutron Scattering

Summary

A shear cell is developed for small-angle neutron scattering measurements in the velocity-velocity gradient plane of shear and is used to characterize complex fluids. Spatially resolved measurements in the velocity gradient direction are possible for studying shear-banding materials. Applications include investigations of colloidal dispersions, polymer solutions, and self-assembled structures.

Abstract

A new small-angle neutron scattering (SANS) sample environment optimized for studying the microstructure of complex fluids under simple shear flow is presented. The SANS shear cell consists of a concentric cylinder Couette geometry that is sealed and rotating about a horizontal axis so that the vorticity direction of the flow field is aligned with the neutron beam enabling scattering from the 1-2 plane of shear (velocity-velocity gradient, respectively). This approach is an advance over previous shear cell sample environments as there is a strong coupling between the bulk rheology and microstructural features in the 1-2 plane of shear. Flow-instabilities, such as shear banding, can also be studied by spatially resolved measurements. This is accomplished in this sample environment by using a narrow aperture for the neutron beam and scanning along the velocity gradient direction. Time resolved experiments, such as flow start-ups and large amplitude oscillatory shear flow are also possible by synchronization of the shear motion and time-resolved detection of scattered neutrons. Representative results using the methods outlined here demonstrate the useful nature of spatial resolution for measuring the microstructure of a wormlike micelle solution that exhibits shear banding, a phenomenon that can only be investigated by resolving the structure along the velocity gradient direction. Finally, potential improvements to the current design are discussed along with suggestions for supplementary experiments as motivation for future experiments on a broad range of complex fluids in a variety of shear motions.

Introduction

Developing a scientific understanding of a natural phenomenon requires accurate and precise measurements. Metrology is also the basis of successful engineering and design of new processes and materials. Rheology is the science of the deformation and flow of matter. Rheology is central in our ability to process a wide variety of materials and is also used by product formulators to target specific material properties. Typical examples of the former include molding polymers or forming composites, whereas the latter includes the development of everyday consumer products such as paints, shampoos, and foods. Whether the viscosity of a molten polymer is controlled s....

Protocol

Figure 2 shows an assembled shear cell attached to the baseplate, which is mounted to the breadboard on the sample environment stage and aligned in the neutron beam for a SANS experiment. The stepper motor, gear box and belt drive, slit motor stage, shear cell and direction of the neutron beam are labeled in Figure 2. The present protocol provides directions for assembling the shear cell (section 1), mounting the shear cell onto the sample environment stage (section 2), calibrating the geometry for a SANS experiment (section 3), loading a sample (section 4), running an experiment and data collection (section 5) and ending an experimen....

Results

Representative results of a successful flow-SANS experiment are given in Figures 9, 10, and 11. These examples are from investigations made on a wormlike micelle solution (WLM) (Table 1) known to exhibit shear banding during certain conditions of shear. A complete discussion of the scientific findings can be found in references^15-17.

Figure 10 represents results of a scattering pattern obtained under shear flow usin.......

Discussion

A new instrument capable of measuring the microstructure of shearing complex fluids in the velocity-velocity gradient plane of shear via small angle neutron scattering is developed and validated. The shear cell design complements other instruments using radiation sources, such as X-ray and light scattering, as well as rheo-SANS instruments capable of characterizing the microstructure in the two other planes of shear (velocity-vorticity and velocity gradient-vorticity)^8,10. This instrument.......

Materials

List of materials used in this article

Name	Company	Catalog Number	Comments
Deuterated Water (99.9%)	Cambridge Isotopes	7789-20-0	83.3 wt % in formulation D2O
CTAB- Cetyltrimethylammonium Bromide	Sigma-Aldrich	57-09-0	16.7 wt % in formulation CH3(CH2)15N(Br)(CH3)3
1/16 in Allen wrench
3/16 in Allen wrench
3/8 in Open end wrench
Tape
Thread seal tape
Syringes (2)