Shear History Extensional Rheology Experiment (SHERE) is designed to investigate the effect of preshearing (rotation) on the stress and strain response of a polymer fluid (a complex fluid containing long chains of polymer molecules) being stretched in microgravity. The fundamental understanding and measurement of the extensional rheology of complex fluids is important for understanding containerless processing, an important operation for fabrication of parts (such as adhesives or fillers) using elastomeric materials on future exploration missions. This knowledge can be applied to controlling and improving Earth-based manufacturing processes, as well.Principal Investigator(s)
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:
April 2008 - April 2009Expeditions Assigned
17,18Previous ISS Missions
SHERE is a unique investigation which has not been performed in microgravity.
The main objective of Shear History Extensional Rheology Experiment (SHERE) is to study the effect of rotational preshear on the extensional behavior of a fluid. The resistance of a fluid to an imposed flow is termed "viscosity", and it is a fundamental material parameter by which manufacturers characterize a material. Unlike common Newtonian fluids, like water for example, complex fluids such as polymers cannot be characterized by a single material parameter such as the Newtonian (shear) viscosity, Polymers are made up of long chains and when exposed to a rotational shearing flow, will align 45 degrees to the flow direction and flip over and over again to coil the polymer chain. When exposed to an extensional (stretching) flow, the coil extends in the direction the fluid is being stretched and can be tightly drawn if the flow is strong enough. Because polymers act like springs, more stress is required to stretch them to higher strains. This relationship between stress and extensional deformation rate (i.e., strain rate) is expressed as an extensional viscosity and is a fundamental material parameter independent of shear viscosity. Due to the coiling effect of rotational shear flow on the polymer chains, shearing on the fluid immediately before extension will have an effect on the extensional behavior of the fluid.
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. These ideal elastic fluids exhibit a nearly constant shear viscosity, which allows a direct comparison of Boger fluids with Newtonian fluids having similar viscosities. The high viscosity of the suspending solvent results in long relaxation times and substantial normal stresses, and the low concentration of high molecular weight polymers facilitates modeling analysis.
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.
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 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 while on the Moon, Mars, and other extraterrestrial bodies.
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.
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 force induced due to shearing and stretching of the elastic fluid, recording the position of the moving plate to verify the imposed velocity profile, recording the fluid diameter at its midpoint , measuring the temperature of the fluid, and measuring the 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.
The SHERE hardware consists of the Rheometer, Interface Box, Camera Arm, Keyboard, Toolbox, Fluid Module Stowage Tray, and cabling.
The crew will install the SHERE hardware into the MSG, power up the hardware, and perform hardware checkout using calibration tools. 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 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 degrees 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.
Twenty-five tests will be performed in groups of five tests. Each test will nominally use one Fluid Module. Data downlink will be performed after each group of five tests, and the results will be analyzed before the next group is run. 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.
Hall NR, McKinley GH, Erni P, Soulages J, Magee KS. Preliminary Findings from the SHERE ISS Experiment. 47th Aerospace Sciences Meeting and Exhibit, Orlando, FL; 2009
Soulages J, McKinley GH, Hall NR, Magee KS, Chamitoff GE, Fincke EM. Extensional Properties of a Dilute Polymer Solution Following Preshear in Microgravity. 48th Aerospace Sciences Meeting and Exhibit, Orlando, FL; 2010
Hall NR, Logsdon KA, Magee KS. Shear History Extensional Rheology Experiment: A Proposed ISS Experiment. NASA Technical Memorandum; 2007.
McKinley GH, Sridhar T. Filament Stretching Rheometry of Complex Fluids. Annual Review of Fluid Mechanics. 2002; 34: 375-415. DOI: 10.1146/annurev.fluid.34.083001.125207.