Shear History Extensional Rheology Experiment-II (SHERE-II)
Following the successful completion of the SHERE Increment 18 operations, the newly redesigned 25 FM was returned to earth. These Fluid Modules will be emptied, cleaned and inspected. This will allow them to be refilled to permit reflight and re-use of the extensional rheometer that is currently in storage aboard ISS.
The natural next step is to use the SHERE hardware to probe the extensional rheology of a different non-Newtonian fluid. In principle this is straightforward, however stringent ISS safety requirements would require ground based testing of seal compatibility, sample cleanup, offgassing if a completely new fluid was selected. However, we propose a much simpler approach: we seek to extend our knowledge by augmenting the existing polystyrene flight fluid with a dilute concentration (5%) of rigid inert filler.
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, water-swelling etc. However, as we describe in detail below, the systematic understanding of such fluids is in its infancy because of the nonlinear interactions between the matrix viscoelasticity and the rigid filler. The no-slip boundary condition on the surface of the filler results in additional shear stresses, in all processing flows and enhances the total viscoelasticity. In the past four years, theoretical advances have led to a proper asymptotic constitutive theory for dilute suspensions of rigid spherical fillers dispersed in dilute polymer solutions [3,4]. This theory was validated in shear by comparison with ground-based shear rheology experiments performed in the P.I.’s lab , and makes explicit predictions (See Eq. (1) below) of the change in the extensional viscosity induced by the additional shearing kinematics induced by the rigid particles. The SHERE rheometer provides a perfect platform for testing this new theory because of the ability to independently control the shear rate and the extension rate in the rheometer.
We thus propose to formulate an enhanced fluid consisting of a dilute suspension (5 vol%) of rigid inert polymethylmethacrylate (PMMA or ‘Plexiglas’) microspheres (diameter d = 6 or 15 µm) dispersed in the same Polystyrene Boger fluid used for Increment 17 and 18 (consisting of a dilute solution (0.025 wt.%) of a narrow polydispersity high molecular weight polystyrene dissolved in oligomeric styrene oil). The specific formulation is discussed in the next section; however it is important to note that this fluid can be rapidly formulated and readily characterized on the ground in the P.I.’s laboratory because (i) an excess of base fluid was originally formulated for the SHERE experiments, and (ii) the inert filler (spherical plexiglass beads) are also already available in the P.I.’s laboratory. The base fluid that has been stored in cold dark conditions will comprise 95% of the new test fluid and is ready to use immediately.
We propose to clean and fill the 25 FM that have been returned following Increment 18 with this new test fluid. Because the rheometer hardware is already in orbit, it is only necessary to launch new fluid samples, and the resulting upmass is minimal. The redesigned fluid modules used in Increment 18 performed flawlessly and there is no necessary additional redesign work for the hardware or the fluid modules. The new design enables a better deployment of the fluid column using a new chamfered lip design that prevents the fluid from wetting the force transducer endplate. As a result, a nice cylindrical fluid column pinned at the edges of the transducer plate is obtained, which guarantees well-defined initial conditions for subsequent numerical modeling. The only ground-based testing required before reflight will be a deployment test to ensure that the dilute concentration of inert microspheres do not interfere with the performance of the seals.
Theoretical Background and Objectives
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).
Rapid Turn-Around: The major advantage of the proposed SHERE II experiment is that it will only require us to clean and refill the existing (redesigned) 25 fluid modules from Increment 18 once they have been returned to earth. No additional engineering and optimization will be required for these fluid modules that already proved their performance during Increment 18.
In addition, the base fluid (or ‘matrix fluid’ in the language of suspension mechanics) will remain unchanged, namely a dilute solution (0.025 wt.%) of a narrow polydispersity high molecular weight polystyrene dissolved in oligomeric styrene oil. This fluid has already been fully characterized in our lab and sufficient fluid remains to form the matrix fluid for the proposed SHERE II test matrix. The ISS experiments already performed without suspended microspheres correspond to a particle volume fraction f = 0 and thus serve as a reference or baseline; these tests will not have to be repeated for SHERE II. This is another benefit of using the same base Boger fluid.
Also, the hard microspheres of diameter 15 µm that will be used as filler particles do not have to be synthesized from scratch but are now readily commercially available under the brand name “spheromer”  from Microbeads SA (http://www.micro-beads.com). A range of sizes are available from 1 – 50 µm. The smallest sizes d ≤ 1 µm cannot be used because the existing theory is for non-Brownian spheres (ie Brownian motion is negligible); the larger sizes are increasing prone to sedimentation effects (under 1 g) which become worse as ~ d2. These sedimentation effects can be mitigated using an available rotisserie system, but preliminary calculations show they can also be designed to be of minimal importance by reducing the particle size to d = 6 or 15 µm. A SEM image of the 15 µm microbeads is shown in Fig. 2 (c). According to the supplier, the bead size distribution is very narrow and the microspheres are almost monodisperse, which is suitable for subsequent numerical modeling.
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. Indeed, the lunar soil, referred to as regolith, is composed of loose dust and broken rocks as shown in Fig. 3 that could be used as a high volume fraction filler when mixed with a polymeric binder or matrix for building foundation or roadbed materials. The viscoelastic dilute suspension used for SHERE II thus serves as a highly idealized model system amenable to theoretical analysis which can shed light on future development of lunar regolith-based construction materials.
Suspensions of particles in viscoelastic liquids are used in many earth 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, which leads to very different types of flow behaviors, filler/matrix interactions and dynamics.
Relevance to NASA Research Mission
The SHERE II experiment will allow us to measure, for the first time, the transient extensional viscosity in a uniaxial stretching flow for dilute polymer suspensions. In addition, SHERE II will enable us to study the subsequent fluid relaxation behavior after cessation of the extensional deformation, which is not possible in a 1-g environment because of sagging issues . This research has direct applications in many industrial processes involving filled viscoelastic systems, ceramic pastes, biomedical materials, food, cosmetics or detergents.
Secondly, as noted above, the possibility of using lunar regolith as high volume fraction filler when mixed with a polymeric binder or matrix has application for future lunar exploration; e.g. the regolith may be blended with a viscoelastic binder (e.g. bitumen or other resin-like binder) for augmenting the volume and compressive strength of building foundation or roadbed materials. We have recently become aware of a regolith simulant (JSC-1A) that could also be used in future rheometric investigations. The additional rheological challenges associated with this material relate to the broad polydispersity of sizes and the high aspect ratio/angularity of the filler. In parallel with preparation of the SHERE II flight fluid (0.025wt%PS/5% PMMA spheres/94.975% oligomer), the PI will also begin ground-based experiments using a viscoelastic regolith suspension.
Project Scientist/Project Manager: Nancy R. Hall, NASA Glenn Research Center
Principal Investigator: Prof. Gareth McKinley, MIT
Engineering Team: ZIN Technologies, Inc.