Engineering
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Dielectric RheoSANS — Simultaneous Interrogation of Impedance, Rheology and Small Angle Neutron Scattering of Complex Fluids
Chapters
Summary April 10th, 2017
Here, we present a procedure for the measurement of simultaneous impedance, rheology and neutron scattering from soft matter materials under shear flow.
Transcript
The overall goal of this procedure is to measure the simultaneous impedance, rheology and neutron scattering from a conductive carbon black suspension as it undergoes deformation in response to an applied shear field. This method can help answer key questions in the soft matter and applied materials fields about the intrinsic link between material microstructure and macroscopic properties such as conductivity and rheology. The main advantage of this technique is that the measurements are performed simultaneously, allowing the entire time evolution of mechanical, structural and electrical response to be reconstructed and compared.
The implications of this technique extend to the development of advanced conductive materials for electrochemical and optoelectronic applications. Visual demonstration of this method is critical owing to the complex nature of the experimental protocol. NIST professionals will assist in some key aspects of the experiment.
Before beginning the procedure, ensure that the neutron beam and the rheometer are both off, the transducer is locked and the motor bearing lock is installed. Clean the dielectric cup and bob assemblies with a detergent solution. Thoroughly rinse the assemblies with deionized water and allow the assembles to air dry.
Unlock the transducer and remove the motor bearing lock. Then, turn on the rheometer and start the control software. Mount the dielectric geometry and bob assembly on the lower and upper tool mounts of the rheometer respectively.
Use a three milometer allen key to loosen the dielectric geometry set screws. Mount the cup assembly on the dielectric geometry. Then, in the rheometer control software, zero the gap and apply 10 newtons of normal force.
Under this axial compression, tighten the set screws to secure the cup assembly to the geometry. Then, set the gap to the measurement gap width. Close the oven door and verify that the oven fully encloses the dielectric cell assembly.
With sufficient vertical clearance above and below the geometry to allow a full revolution without contacting the oven walls. Open the oven and remove the dielectric assemblies. Thread the geometry shaft through the slip ring and join the dielectric cup and slip ring connectors.
While holding the slip ring, concentric with the shaft above the flange, secure the slip ring adapters on top of the flange. Gently slide the slip ring down to fit over the adapters. Next, mount the rheometer alignment tool on the lower tool head.
Install the truncated snout and set the sample aperture to one millimeter by eight millimeters. In the rheometer control software, set the geometry displacement angle to 0.49 radians. Then in the SANS control software, verify that all neutron guides have been removed.
Open the oven door to view the beam alignment laser. Use the SANS software to adjust the height and angle of the sample stage until the laser beam is directed along the oven center line and though the sample aperture without intersecting the aperture walls. After rheometer alignment, remove the alignment tool and perform a series of SANS measurements to calibrate the instrument.
Then, use the rheometer LCD screen, to set the gap to 100 millimeters. Mount the dielectric cell with the bob assembly on the upper tool head and the cup geometry and slip ring assembly on the lower tool head. Use the instrument software to zero the gap.
Next, mount the carbon brush assembly on the brush adapter. Line up the carbon brushes with the grooves on the slip ring and then secure the adapter to the rheometer with screws. Connect the carbon brush assembly to the top bus bar and the dielectric bob assembly to the bottom bus bar.
Check the connections of the shielded BNC cables to the bus bars and the LCR meter. Connect the SANS and Trigger BNC cables to the DAQ card. Connect the rheometer to the DAQ card.
Verify that the LCR meter and the rheometer are successfully communicating with the computer. Before loading the sample, verify that the gap is set to 100 millimeters. Then, load four millimeters of a carbon black dispersion in propylene carbonate into the dielectric cup assembly, minimizing the amount of sample left on the cup wall.
Lower the geometry to 40 millimeter. Then in the rheometer control software, set the motor velocity to one radian per second. Lower the dielectric bob assembly to achieve a gap width of 0.1 millimeters and then set the motor velocity to zero radians per second.
Check that the sample levels has reached the Couette wall but is not over filled. Fill the inner dielectric bob assembly with the chosen solvent for the experiment. Place the solvent trap on the rim of the dielectric cup assembly.
Next, configure the virtual instrument file for the desired experimental conditions. Set the appropriate run times and procedures for the SANS instrument and the rheometer. Click parameters set to execute the experiment.
A conductive carbon black suspension was evaluated by RheoSANS. Rheology, dielectric data and neutron scattering were measured continuously as the shear rate decreased logarithmically in steps, holding at each step for a specific time interval. The steady state, rheological dielectric and SANS data were processed and analyzed at each shear rate step.
A period of decreasing conductivity and effective volume fraction with increasing shear rate was attributed to yielding of the microscopic gel. A subsequent period of increasing conductivity and decreasing effective volume fraction with increasing shear rate was attributed to shear thickening caused by hydrodynamic forces drawing aggregated particles together. Micro structural transitions at higher shear rates were represented with two dimensional plots.
Dielectric RheoSANS provides a flexible platform capable of the interrogation of nonequilibrium behavior of soft matter systems. Using a wide variety of potential experimental protocols, this instrument is available for broad use by the scientific community at the NCNR.
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