Rheo-SANS at the NIST Center for Neutron ResearchB Greenwald1,2 , L Porcar1,2, C Glinka2, N Wagner3, R Egres3, A Nandi3, SM Choi4, JC Shulz5

1University of Maryland; College Park, MD 20705

2NIST Center for Neutron Research; Gaithersburg, MD 20899

3University of Deleware; Newark, DE 19716

4Korean Advanced Institute of Science and Technology; Gusong-dong, Yusong-gu Daejon, Korea 305-701

5Australian Nuclear Science and Technology Organization; Menai NSW, Australia 2234

Shear flow presents a great technological interest because shear-induced complex fluid phases have profound effects on industrial material processing. In order to correlate the unique flow properties exhibited by these complex fluids with their structural deformation, we have developed a device to perform simultaneous measurement of rheology and structure by Small Angle Neutron Scattering (SANS).

Quartz and titanium Couette type flow cells have been designed to fit into a commercial rheometer (Paar Physica USD200*). The setup allows scattering measurements from a range of configurations using both the so-called radial configuration, where the incident neutron beam is parallel to the velocity gradient, to the tangential, where the incident neutron beam is parallel to the flow direction.

Couette cells with gaps of 0.5 or 1mm are available and can operate at temperatures ranging from –20°C to 150°C with a solvent trap preventing sample evaporation. The apparatus is installed on the NG7 SANS spectrometer and allows us to perform many different rheological tests as e.g. simple shear flow either at constant shear stress or constant shear rate, creep and creep recovery as well as amplitude oscillatory shear deformation. All while simultaneously recording scattering data.

To illustrate the performance of our equipment, we give a brief report below of recent experiments carried out on PCC/PEG suspensions and wormlike micelles solution.

* The mention of commercial equipment does not imply endorsement by the National Institute of Standards and Technology

NeutronsNeutrons

Abstract

For Alignment of the Translation

For Alignment of the Tilt

Cup alignment is an iterative two-step process performed with a dial indicator mounted on the rheometer shaft. First, the bottom of the cup is aligned relative to the shaft by rotating the dial indicator and minimizing deviations until it is centered in the plane of the photograph shown on the left. This is done using the four brass screws which translate the cup. Second, the top of the cup is aligned by tightening or loosening three screws which compress an o-ring, changing the tilt angle of the cup. Changing the tilt will alter the planar alignment, therefore, the two processes are repeated until the cup is aligned to a concentricity of 0.0005”. 

Cup Alignment

Small angle neutron scattering was used to investigate microstructure changes in PCC(7:1)/PEG dispersions when subjected to flow. From on-axis (radial) and off-axis scattering patterns we observe that long axis alignment of the particles is maintained during shear thickening

Couette shear cell

NIST, NG-7, stress control rheometer

radial

tangential

Rheo-SANS Investigation of PCC(7:1)/PEG Suspensions

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),(

)2(),()(

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Discontinuous shear thickening PCC(7:1)/PEG suspension

PCC(7:1)/PEG, = 0.35Radial directionAnnular average, q=.01 Å-1

Shear thickening in concentrated colloidal dispersions occurs when the hydrodynamic lubrication forces due to shear flow overcome interparticle stabilization forces, resulting in the formation of transient aggregates or “hydroclusters”. Discontinuous shear thickening behavior in anisotropic particle dispersions is observed to occur at significantly lower particle loadings than reported for spherical particle dispersions. One hypothesis for this observed phenomenon is that at the shear thickening transition, anisotropic particles rotate out of flow-field alignment, resulting in the formation of randomly packed aggregate structures.

Rheo-SANS experiments at NIST have been conducted in order to observe the microstructural changes that occur in polyethylene glycol based dispersions of acicular (rod-like) precipitated calcium carbonate particles during shear thickening. Through neutron scattering experiments concurrent with real time, stress controlled rheological measurement, we observe that the high degree of long-axis particle alignment that occurs during shear thinning is maintained at higher shear stresses associated with shear thickening behavior. We conclude that shear thickening in these anisotropic particle dispersions is not the result of particle misalignment. Rather, the observed shear thickening results from the formation of transient hydroclusters of flow-oriented particles.

Neutrons enter the rheometer from the smaller front opening. This opening is wide enough to allow neutrons to pass through the cup in both the radial (perpendicular to the flow direction) and tangential (parallel to the flow direction) configuration. The rear, larger, opening allows ample angle for scattered neutrons from either configuration to reach the detector.

Temperature is controlled by heating liquid Nitrogen vapor to a set temperature and blowing the gas between the cup and a quartz shield used to insulate the gas from the environment. The temperature can be varied between –20°C and 90°C with an accuracy of ±0.1°C. For low temperature there is an inlet which allows nitrogen gas to be blown onto the quartz shield to avoid condensation from forming on the face of the shield.

Liquid Nitrogen Inlet

Over Pressure Valve

Nitrogen Gas Inlet

Front Opening

Rear Opening

Base Insert

The bob is made of quartz and measures 70 mm in height 48 mm in diameter. The cups are made from three different materials: quartz, aluminum and titanium. The cups measure 94 mm in height and either 50 mm or 49 mm inner diameter, giving gaps of 1 mm and 0.5 mm respectively.

The cup and bob can be used in either a Mooney-Ewart or simple cylinder geometry. In the Mooney-Ewart geometry the 1 mm or 0.5 mm gaps require, respectively, 12 mL or 8 mL of sample.

The Bob and Cups

The rheometer is equipped with a radial aperture and tangential slit package cut into boron aluminum backed with cadmium. The aperture is a 1 cm diameter circle with available slits for the tangential position measuring 0.3, 0.5, 0.6 and 0.8 mm in width. The slit/aperture package is mounted to two micrometer screws placed in the X and Y planes to ease alignment of the slit.

The tangential slit is aligned by comparing transmission runs across the gap. As the beam traverses from the outer edge of the cup through the gap into the bob, the transmission changes. By looking at the transmission vs. position (see right) we can determine the proper alignment of the tangential slit.

The exact distance between the center of the tangential slit and the center of the radial aperture is fixed. Therefore, proper alignment of the slit also gives proper alignment of the aperture.

*Alignment of slit for measurement position.

Slit Packages and Alignment

0.1

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Tra

nsm

issi

on (

%)

Position (mm)

Aluminum Cup Quartz BobGap

NG7 SANS Beam LineAccuracy 1% of maximum value

Minimum Torque 150 mNm

Torque Resolution 0.01 Nm

Speed Range 10-4 to 1000 min-1

Shear Rate Range 1.3*10-4 to 4.8*103 s-1

Shear Stress Range 0.67 to 3.5*104 Pa

Viscosity Range 1.7*10-3 to 2.7*108 Pas

Temperature Range -20 to 150 C

Volume 8, 12 mL

Gap Size 0.5, 1 mm

Technical Specifications

The Rheometer

Rheometer installed on Huber table at the NCNR’s NG7 SANS beam line.

1E-4 1E-3 0.01 0.1 1 10 100 1000

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In the case of wormlike micelles, history effects are critical to an understanding of the flow behavior. In the present case, the system exhibits a pronounced shear thinning regime over a wide range of stress. Shear thinning is directly related to the breakage of the micellar network and subsequent orientation of wormlike micelles in the flow direction. The dependence of the rheological properties on the shear history of the sample has been investigated using simultaneous SANS and rheology. We learnt that the hysteresis was the result of the persistence of micellar alignment. Once aligned, the micelles remain in that aligned state even at low values of stress. This phenomenon is still under investigation.

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Wormlike Micelles Solution

*

Quarrtz Bob Aluminum cup Quartz Cup Titanium Cup