Vorticity Alignment of rigid fibers in an oscillatory shear flow: Role of confinement
By: Robert Matias
Faculty Advisors: Jason E. Butler, Elisabeth Guazzelli
Abstract
There is great practical interest in generating improved models for the dynamics and properties of suspensions due to their widespread occurrence in nature and many chemical processes. Important examples of suspensions can be found in everyday items, including foods, personal care products, and pharmaceuticals. Suspensions are often encountered in many chemical processes, including in petroleum production and processing as well as fiber suspensions found in the production of paper. The dynamics of suspension materials are heavily influenced by multiple factors, including particle shape and orientation. This understanding can then be applied to ensure and promote sustainability in the processes. Thus, an experiment was conducted to investigate the effects of confinement and shear flow on the rheology and dynamics of concentrated suspensions of rigid rods.
The focus of the experiment was to analyze the orientation distribution of rigid fibers suspended in a viscous, Newtonian fluid. The fibers can be aligned in the direction perpendicular to the flow-gradient plane (vorticity direction) by applying a low oscillatory shear flow. Analysis of this phenomena was performed by fluorescing a suspending fluid (Triton X-100) with PMMA (acrylic glass) fibers suspended. The suspension is index-matched with the exception of a Rhodamine dye which fluoresces the fluid and not the particle when exposed to a green laser. This method ensures clear visibility of all fibers . An images is taken with every oscillation and then, using Python, an image analyzing code can determine the flow-vorticity angles of fibers. The orientation distribution can now be calculated and be used to determine its impact on the suspension’s properties.
Overall, preliminary conclusions support vorticity alignment in specific strain amplitudes but were difficult to reproduce. Experimental issues such as a slippery band and motor jams during runs caused results to be inconsistent. Results that did not have said issues favored vorticity alignment. Further work for the project is set to continue and solutions to previous experimental problems are being pursued.
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Objectives
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Investigate alignment of fibers.
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Calculate order parameters.
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Compare empirical and simulated data.
Materials & Method
The phenomenon was investigated by suspending PMMA particles in a mixture of Triton X-100, zinc chloride, and deionized water. This particular mixture was ideal because of its similar density and refractive index to PMMA. This suspension was loaded into a cell were the suspensions experienced oscillatory shear flow from a transparent band. Figure 1-4 illustrates the cell design. After each oscillation an image of the suspension was taken using a Nikon camera. Rhodamine dye was also added to the suspension because of its ability to fluoresce under a green laser. This helped make the particles visible. After a set of oscillation - usually between 400 and 1000 – the images would be processed by an image analysis code developed by Dr. Elisabeth Guazzelli that would calculate each particle’s angle with respects to the flow-gradient direction – referred to as α. Figure 5 is an illustration of the a particle’s orientation.
Over 40 runs were performed during an 8 week period at the University of Aix-Marseille. Each run investigated the same phenomenon under different parameters such as volume fractions, aspect ratios, and shear rates.
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Figure 1-4: Cell Design
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Figure 5: Particle Orientation
Results and Discussions
Throughout the runs a wide range of order parameters were observed. Volume fractions of .20 with a shear rate amplitude of 3 revolutions per minute yielded a higher order parameter of .5 compared to other runs. However, shear rate amplitudes of 3.4 and 2.5 did not show high aspect ratios, which could imply a narrow margin for vorticity alignment. Figure 6 illustrates the average order parameter with each oscillations. As the number of oscillation increased the system reached steady state. But, due to issues with the band slipping and an inconsistence motor the system would sometimes not reach steady state which caused significant errors in the calculations. Slips and jolts during oscillation would occur due to the motor not having enough horse power to counter the drag forces from the band. Thus, different ideas such as replacing the band and implementing a gear system was tested but did not fix the issue. This issue only occurred with low and high shear rates of 1, 2, 5 and 6 revolutions per minute. When compared to simulated data the empirical results differed slightly. Under the same conditions there was 20% difference. Figure 7 illustrates simulated results.
Further work for the project is set to continue and solutions to previous experimental problems are being pursued. The results will be used to increase the efficiency and lower the energy consumption needed for different chemical processes.
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Figure 6: Empirical Results Figure 7: Simulated Results
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Figure 9: Order Parameter of .5
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