The overall goal of this procedure is to use small angle neutron scattering sands with a sheer cell sample environment to study the microstructure of complex fluids in the velocity velocity gradient plane of shear. This is accomplished by first assembling a well sealed shear cell sample environment. The second step is to attach the shear cell to the cell mounting bracket located on the red board in the sample environment stage of the neutron beam line.
Next, the sample is carefully loaded into the shear cell so as to avoid air bubbles being introduced into the experimental volume, the final step is to run the experiment first by defining the shear rate at which the sample is sheared using motor control software. And second, to set up the desired sands experiments according to the standardized sands procedures. Ultimately, the sands shear cell sample environment is used to measure the spatiotemporal microstructure of a shearing complex fluid.
In this example, we investigate the microstructure of a surfactant solution with sheer banding flow instabilities in the velocity gradient direction of sheer. I'm Paul Butler, team leader of the Macromolecular and Micro Structural Sciences team here at the NIST Center for Neutron Research demonstrating the experiment. Today will be Kate Kernan, a graduate student in Norm Wagner's Group at the University of Delaware.
Visual demonstration of this experiment is critical because there are many steps and techniques necessary in order to assemble the shear cell and load the sample. Once the shear cell is placed onto the beamline complex fluids can be interrogated under shear flow using small neutron scattering. The first step after fabricating the parts is to assemble the sheer cell.
Begin by cleaning the middle plate, including sample loading and set screw pathways. Identify the top of the plate indicated by a score mark. Wrap a set screw in thread tape and use an Allen wrench to screw it into a hole at the bottom.
Wrap and insert the two remaining set screws in the other bottom hole and the hole on the side. Next place the round white O-rings in the grooves on both sides of the plate. Now begin work on the front plate.
Insert the ceiling spring loaded bushing into the plate so that the spring side will open toward the sample. Place the small and large square Double seal O-rings into the grooves of the plate. Complete the work on the front plate by placing the quartz window on top of the square O-rings.
Prepare the back plate in the same manner as the front plate. At this point, start assembling the front and middle plates by placing the front plate on a flat surface. With the bushing spring facing up, align the score on the top of the middle and front plates and place the middle plate onto the front plate.
Now work with the back plate. Take the mandrel shaft and use evenly applied force to insert it into the back plate. The mandrel should click into position and hold the quartz window and square O-rings in place.
Set the back plate aside. The next step is to raise the front and middle plate assembly onto a platform with enough clearance below the assembly. For the mandrel, align the score on the top of the front plate assembly with the score on the back plate assembly and insert the long part of the mandrel shaft into the front plate assembly.
The cell will slide together and click when properly assembled. Now screw the assembly together in a cross pattern using the four socket head cap screws for each of the access ports. Wrap thread seal tape around the threads and screw it into the top of the middle plate.
Tighten with a wrench. Place the cadmium mask into the receiving slot machined at the front of the front plate. Finally, use the quick connectors to cross connect the coolant hose between the top ports on the front and back plates.
Continue preparing for the experiment by transporting the cell to the beam line to place the cell in the beam line. First, cover the sands detector window with the safety shield, with the sample environment stage prepared and properly aligned. Identify the cell mounting bracket and shaft coupler attached to the baseline.
Make sure the set screws for the shaft coupler are loosened. Align the shaft coupler and the mandrel shaft so that the set screws on the coupler will screw into the flat part of the mandrel shaft. Carefully slide the shear cell horizontally into the cell mounting bracket.
Use two socket head cap screws to attach the shear cell assembly to the cell. Mounting bracket tightened securely. Always making sure the shear cell is flush against the cell mounting bracket.
Connect the mandrel shaft to the drive assembly by tightening the two set screws on the shaft connector. After the cell has been mounted, aligned and calibrated, the next step is to load the sample. Make sure the stop cocks are in the closed position.
Preload the sample into a 10 milliliter threaded syringe. Make sure the sample is free of bubbles. Place an empty syringe without a plunger on the connector in the middle of the cell to collect overflow.
When everything is ready, open both stopcocks slowly inject the sample until it begins to enter the empty syringe. Once this is done, turn the motor control off to allow the belt to be manually moved. Shear the sample by hand to help move bubbles to the top of the shear cell.
Inject additional sample as needed to push the bubbles out of the shear cell gap. With the air bubbles removed, close the stopcock to lock the sample in the cell to run Simple, steady sheer experiments. Set up the desired small angle neutron scattering experiments.
Set the sheer rate of interest in the control file associated with the motor control software. Select the sheer direction of the sample During the experiment, start the shear cell motor and the neutron scattering experiment. Check the detector counts and observe the small angle neutron scattering two dimensional pattern to be.
Certain results are being properly recorded during shearing. Shown here is a scattering pattern obtained under sheer flow using the sheer cell. The sample studied is a viscoelastic worm-like my cell solution of subtle trimethyl ammonium bromide.
The solution contains long entangled thread like self-assembled amphiphilic molecules when sheared the sample exhibits sheer thinning. These solutions also show the onset of sheer banding when the flow field segregates into two or more bands, each with a characteristic sheer rate in the COE geometry at sufficiently high shear rates. This sample exhibits two bands, one with a higher than expected shear rate and one with a lower than expected shear rate.
The new shear cell instrument can be used to study the micro structural state of the surfactant when shear banding is observed. Systematic measurements in the one millimeter coquet gap are performed using a 0.1 millimeter slit aperture at different sheer rates. The intensity rings are correlation peaks due to segment segment interactions, and isotropy in a ring indicates segmental flow alignment with high alignment typical for a pneumatic phase.
A significant difference in scattering anti isotropy is observed between positions in the low shear and high shear bands. This technique paves the way for researchers in radiology soft materials and non-equilibrium thermodynamics in order to explore smart materials and the structure property relationships of complex fluids.
