Shear History Extensional Rheology Experiment - II (SHERE-II) - 12.03.13
Science Objectives for Everyone
The Shear History Extensional Rheology Experiment - II (SHERE-II) investigation involves a non-Newtonian fluid that will undergo preshearing (rotation) for a specified period of time, followed by stretching. This combination of shearing and extensional deformations is common in many earth-based polymer processing and manufacturing operations such as extrusion, blow-molding and fiber spinning. However, in order to accurately predict the flow behaviour of polymeric fluids under such deformation histories, an accurate knowledge of the extensional viscosity of a polymer system and its variation with strain rate is critical and will be measured during this experiment. The fundamental understanding and measurement of these complex fluids is important for containerless processing, a key operation for fabrication of parts, such as adhesives or fillers, using elastomeric materials on future exploration missions.
Science Results for Everyone
ZIN Technologies Incorporated, Cleveland, OH, United States
National Aeronautics and Space Administration (NASA)Sponsoring Organization
Human Exploration and Operations Mission Directorate (HEOMD)Research Benefits
Information PendingISS Expedition Duration
March 2011 - September 2011Expeditions Assigned
27/28Previous ISS Missions
SHERE, the precursor to SHERE-II was performed on ISS Expeditions 17 and 18.
- The Shear History Extensional Rheology Experiment - II (SHERE-II) is designed to investigate the effects of a pre-shear history on the transient extensional viscosity in a uniaxial stretching flow for a model filled viscoelastic suspension (consisting of inert rigid non-Brownian spheres dispersed in a dilute polymer solution).
- This experiment will generate additional scientific data for dilute viscoelastic polymer solutions in a broad subclass of transient uniaxial extensional flows.
- Understanding extensional rheology is key in understanding how to process thermoplastic elastomers (flexible elastic polymeric materials), which are very resilient and can be made very thin, and hence, lightweight.
- Understanding the rheological properties of highly viscoelastic suspensions may be of paramount importance for lunar in-situ resource utilization and for the future construction of a permanent lunar base.
- SHERE can provide data for engineering design tools that are part of computer-assisted manufacturing (CAM) systems to ensure that the rheological properties of polymeric parts have not been impacted in a variable gravity environment.
Unlike Newtonian fluids, complex fluids such as polymers cannot be characterized by a single material parameter such as the Newtonian viscosity, ?. Instead, they exhibit non-linear and time-dependent responses to imposed deformations. Constitutive models have shown the extensional function of non-Newtonian fluids are not constant but depend on both the rate of deformation and the total strain experienced by a fluid element. A class of dilute polymer solutions collectively referred to as ?Boger fluids,? have become a popular choice for fundamental rheological studies of non-Newtonian fluids and will be used in this experiment.
Gravitational body forces cause appreciable sagging of thin fluid filaments. This sagging is most notable for low deformation rates where strain-hardening is not significant. Removing these perturbative forces will allow one to probe a wider parametric range of strain rates while simultaneously measuring the total stress and velocity field (shape and diameter) in the deforming fluid element. These measurements will provide an idealized data set for comparison with theoretical models and will serve as a gold standard for ground-based extensional rheometry.
Filled polymeric suspensions form the bulk of the engineering plastic market. By replacing a fraction of the relatively expensive synthetic polymer in a plastic product with a cheap mineral filler (e.g. titania (TiO2), silica (SiO2) or carbon black), it is possible to lower the cost, and enhance properties such as environmental sensitivity to UV degradation, waterswelling, etc. We will formulate an enhanced fluid consisting of a dilute suspension (5 vol%) of rigid inert polymethylmethacrylate (PMMA or ?Plexiglas?) microspheres (diameter d = 6 or 15 Ám) dispersed in the same Polystyrene Boger fluid used for Increment 17 and 18 tests (consisting of a dilute solution (0.025 wt.%) of a narrow polydispersity high molecular weight polystyrene dissolved in oligomeric styrene oil).
The goal of the SHERE II experiment will be to investigate the effects of a pre-shear history on the transient extensional viscosity in a uniaxial stretching flow for a model filled viscoelastic suspension (consisting of inert rigid non-Brownian spheres dispersed in a dilute polymer solution). The role of internal stresses produced by shearing between the rigid filler and the viscoelastic matrix can be explored systematically using the pre-shear capabilities of the SHERE platform. Access to extended microgravity also allows the subsequent relaxation behavior to be measured after cessation of the extensional deformation during the extended range of time scales that can be accessed because of the absence of gravitational sagging.
As with the original SHERE experiment, SHERE II will be operated inside the Microgravity Science Glovebox (MSG) aboard the International Space Station (ISS). On-orbit operations will consist of crew installation, hardware turn-on and checkout, fluid sample installation, experiment execution, and fluid sample removal. During experiment execution a test point is selected (with specified preshear and strain rates), and the experiment will then automatically execute. The fluid is presheared and stretched according to a pre-programmed exponential velocity profile. The stretch is stopped abruptly at 194 mm in length, and the tensile stress in the fluid thread is allowed to relax. Each experiment lasts approximately 5 minutes, most of which is spent waiting for the fluid column to drain to the end plates, and the fluid filament eventually breaks in the middle. Several key measurements will be made during the experiment. They include measuring the axial force induced due to shearing and stretching of the elastic fluid, axial displacement history of the translation stage, axial midplane diameter of the fluid filament, temperature of the fluid, and fluid filament shape and evolution. Afterwards, the translation slider is repositioned to the starting position, and the fluid column is reconstituted. If it is reusable (based on a criteria of temperature, bubble contamination, and previous strain encountered), then another test can be performed with the same fluid sample. Otherwise, the fluid sample is removed, and the next one is installed or the hardware is powered down.
Understanding the extensional rheology of a complex fluid such as a liquid polymer is key for containerless processing because the absence of the bounding walls of a container or vessel removes the shearing component of the deformation which typically dominates earth-based processing operations. The resulting flow is thus shear-free or extensional in character. Containerless processing is a central component in the development of in-situ fabrication technology, such as a means of producing new parts on demand or replacing existing parts or tools. This represents a critical element in the evolution of an autonomous exploration capability. In-situ fabricated parts, which may include both new and recycled materials, will be composed of plastics, filled polymers, metals, ceramics and composites. SHERE plays a role in SHERE plays a role in this area by measuring, in microgravity conditions, a material property that has a direct connection to in-situ manufacturing and fabrication of polymeric parts. In-situ manufacturing operations can occur in microgravity or reduced gravity levels (e.g. on the Moon or Mars) and may include for example, the extrusion and processing of thermoplastic elastomer films, which are very resilient and can be made thin and lightweight. These elastomeric materials may form the basis of adhesives and fillers utilized in a wide variety of repair applications, especially under a reduced gravity environment, such as the repair of space suits or other similar materials. Understanding and exploiting the ability to fabricate new parts in-situ from a limited number of precursor components is critical in future space missions where weight plays a critical role in the overall cost of a mission. Additionally, in-situ repair provides a means of maintaining systems during transport and during future exploration missions.Earth Applications
A fundamental understanding and measurement of the extensional rheology of complex fluids also allows Earth-based manufacturing processes to be controlled and improved. Ground-based work using variants of the Filament Stretching Rheometer includes studies of spinnability, and the investigations of cohesive and adhesive instabilities which manifests themselves in adhesion and tackiness of materials. It has lead to the development of a Resin-spinning technology that allows the formation of ultra-fine elastic threads analogous to spider-silks. Control of the fluid shear history and extensional rheology of test fluid is essential to optimizing the ultimate web properties. Extensional rheology is of critical importance in optimization of polymer processing operations that involve complex flows, i.e. flows that contain both shearing (rotation) and elongation (stretching) components. Suspensions of particles in viscoelastic liquids are used in many terrestrial processing operations: polymer melts with fillers, ceramic pastes, biomedical materials, food, cosmetics or detergents. The final properties of the suspensions are greatly determined by the shape, concentration and size of the filler. In particular, the fillers can range from nanoscopic to microscopic characteristic dimensions, and this leads to very different types of flow behaviors, filler/matrix interactions and dynamics.
SHERE will be conducted inside the MSG work volume. Fluid samples need to be kept at 20 deg C for 24 hours prior to the start of testing. The crew will place the Fluid Module Stowage Tray inside the Commercial Generic Bioprocessing Apparatus (CGBA) testing order to control their temperature. Twenty-five (25) tests will be performed in groups of 5 tests per group. Each test will nominally use one Fluid Module. Data downlink will be performed after each group of 5 tests, and the results will be analyzed before the next group is run. There is no time limit between tests or groups, but testing once a Fluid Module has been removed from the CGBA should be as expedient as possible to prevent heating of the fluid sample above the cutoff temperature of 25 deg C. Sample return of the Fluid Modules is desired but not required. Data will also be stored on up to 25 digital video tapes for later return to the ground for more complete analysis.Operational Protocols
The crew will install the hardware into the MSG, power up the hardware, and perform hardware check-out using calibration tools from the Tool Box. A Fluid Module is then installed into the Rheometer by attaching its two ends to the force transducer and preshear motor/slider respectively. The crew then deploys the fluid by removing the outer shell and sliding back the inner shroud. The experiment?s execution automatically occurs once the crew has selected a test point on the Keyboard. The fluid is presheared at a steady rotation rate and then stretched according to an exponentially-increasing velocity profile. Following the completion of a test, the fluid column is recombined by bringing the Fluid Module halves back together again. If possible, another test can be performed using the same sample (provided no bubbles are present within the fluid and the sample temperature remains below 25 deg C). If not, the Fluid Module is sealed, removed, and a new Fluid Sample is prepared for testing or the hardware is powered down. Upon completion of testing, the hardware is removed from the MSG.
During Increment 17, a total of 28 experiments were performed, including 20 test points and 8 repeat test points using reconstituted fluid samples from previously deployed fluid modules. Twenty five new redesigned FM (to correct original design deficiencies) were launched on Shuttle STS-126 (Mission ULF2). They were used in 28 experiments during Increment 18 that consisted of 25 test points and 3 special test points whose purpose was to troubleshoot the electronics of the force transducer. Data analysis continues for the test points performed during Increment 17 and 18. This analysis involves the computation of the time evolution in the crosssectional area of the filament from the radius data, a temperature correction for the relaxation time and the zero-shear rate viscosity to correct for thermal fluctuations in the ISS Glovebox environment, and the computation of the extensional viscosity together with the Trouton ratio. For the follow-on experiment we seek to extend our knowledge by augmenting the existing polystyrene flight fluid with a dilute concentration (3.5 vol%) of rigid inert filler.
NASA Image: ISS017E012296 View of the Laser Micrometer,Deployment Tool,Preshear Motor,and Force Transducer on the Shear History Extensional Rheology Experiment (SHERE) rheometer within the Microgravity Science Glovebox (MSG). Photo was taken in the European Laboratory/Columbus during Expedition 17.
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SHERE hardware. Image courtesy of Glenn Research Center.
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