| 研究実績の概要 |
Microfluidic cylinder geometries were fabricated by selective laser induced etching with a high aspect (depth:width) ratio of 5 and low blockage (diameter:width) ratios of 0.1.
Experiments were done with well characterized viscoelastic, non-shear-thinning polymer solutions. Very different flow patterns were observed compared with typical microfluidic cylinders. In our new geometry, strong flow modification occurs in the downstream wake of the cylinder as the flow rate is increased. The flow modification is correlated with the orientation of macromolecules in regions of strong extensional flow and can persist for hundreds of cylinder radii. This downstream wake affects the flow around a second cylinder located downstream. However, no clear signs of viscoelastic flow instabilities were observed even at high flow rates.
Next we examined the flow of a shear-banding viscoelastic wormlike micellar (WLM) solution around a single cylinder. In this case, we observed a fascinating and unexpected sequence of instabilities as the flow rate was increased above a critical value. Initially the flow becomes strongly asymmetric, with fluid selecting a preferential path around one side of the cylinder. At higher flow rates the flow becomes unsteady and time dependent, with power spectra that suggest the system is approaching a chaotic (turbulent-like) state.
We are now examining a series of carefully formulated fluids (polymer and WLM) of various rheology in order to understand the origin of the flow asymmetry and time-dependence in the shear-banding WLM system.
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| 現在までの達成度 (区分) |
現在までの達成度 (区分)
2: おおむね順調に進展している
理由
The project is progressing very well and possibly more smoothly than expected. The a-priori expectation that flow instabilities would be observed in the well-characterized polymeric system and could be understood in terms of established instability criteria turned out to be incorrect. However, those experiments with polymer solutions remain of significant value and interest and serve as a good benchmark test for comparison with future work to be carried out on this project. In addition, even more intriguing, unexpected and novel phenomena were discovered by examining WLM solutions instead.
Understanding these new instability phenomena has motivated a new series of experiments currently underway involving a wide range of different polymer and WLM solutions with a range of rheological properties such as shear-thinning but inelastic, elastic but not shear-thinning, and both elastic and shear-thinning. Results so far suggest that a combination of viscoelasticity and shear-thinning is required to cause the instability.
In addition, we have already begun preliminary work on the research proposed for FY2019: A number of devices have been fabricated with flexible cylinders (both isolated and 2 aligned cylinders) that deform in the flow. We are obtaining exciting results by using high speed imaging to examine the interactions between these flexible structures and the flow instabilities seen in the shear-banding WLM system.
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| 今後の研究の推進方策 |
In the second year of this project, we intend to perform a thorough characterization of how flow instabilities around a cylinder depend on the rheological properties of the viscoelastic fluid. We aim to generate stability diagrams for the flow in terms of dimensionless fluid and geometric parameters such as the Reynolds number, Weissenberg number, shear-thinning index, and blockage ratio.
In addition, we expect to obtain and publish the first results showing fluid-structure interactions with viscoelastic fluids and microscale cantilevered posts. Further to this we will report on the interactions between two such cantilevered posts aligned on the flow axis. When the flow becomes unstable and time-dependent, we expect to see coupling between the motions of the two aligned posts, that may provide insight into biological interactions between e.g. cilia and flagella in viscoelastic biofluids.
It is expected that the original proposed research plan will be completed ahead of time, allowing the research to be broadened. A few new directions are possible: (1) examining the effect of offsetting the cylinder from the channel centerline, (2) placing 2 cylinders next to each other to provide 3 possible routes for the fluid to choose, (3) examining the collective behavior of “forests” of flexible filaments representing clusters of cilia, (4) examining flows around fixed or flexible posts of different cross-sectional shapes (e.g. square, triangular or elliptical).
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