2015 Fiscal Year Annual Research Report
| Project/Area Number |
15F15769
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| Research Institution | The University of Tokyo |
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Principal Investigator |
田中 肇 東京大学, 生産技術研究所, 教授 (60159019)
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| Co-Investigator(Kenkyū-buntansha) |
BRUOT NICOLAS 東京大学, 生産技術研究所, 外国人特別研究員
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| Project Period (FY) |
2015-11-09 – 2018-03-31
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| Keywords | Hydrodynamics / Colloids / Viscoelasticity / Optical tweezers / Rheology |
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| Outline of Annual Research Achievements |
Hydrodynamic interactions between solid bodies are relevant to solve numerous problems. In the case of low Reynold's number flows, it plays a role in cell motility, particles sedimentation, in the flowing of complex fluids in confined environments such as blood cells in small veins, and in many colloidal systems. Colloid solutions are typically used as fundamental models of atomic systems, but also have widespread industrial applications such as lubricants and paints that require a well-tuned rheology.
While hydrodynamic interactions between colloids are well understood when they move in a Newtonian fluid, their behaviour in complex fluids such as a viscoelastic media is not well understood. In particular, the coupling forces in an active system made of a few particles driven at known velocities have never been characterized in such fluids. The aim of the project is to measure these interactions. The results are primarily expected to help in developing models of coupled active colloids that have been used to understand cilia synchronization in biological systems (such as the airways cilia that beat in viscoelastic mucus) as these are currently restricted to Newtonian fluids. The project also opens the possibility to investigate questions related to the rheology of dense colloids in viscoelastic fluids. While the rheology of such fluids has been characterized already, the link between the global properties of the fluid and the pair hydrodynamic interactions between colloids has not be elucidated.
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| Current Status of Research Progress |
Current Status of Research Progress
2: Research has progressed on the whole more than it was originally planned.
Reason
This project, started in mid-October 2015, requires optical tweezers to displace colloids at controlled speeds and directions, and to measure the resulting coupling forces between the particles. A new holographic optical tweezers setup is therefore currently being built. It is based on a previous design, but with optimizations to make it suitable to the study of hydrodynamic interactions: It is now a single objective tweezers (to improve stability of the setup) and has a large field and high resolution camera, necessary to study long range interactions, and to have enough precision in the tracking of particles. A computationally efficient C++ particle tracking program with a graphical user interface has also been written to speed up the analysis of the big amount of data that will be acquired. In parallel, some work has been dedicated to determine the viscoelastic fluids that will be used.
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| Strategy for Future Research Activity |
Most results will be obtained when the optical tweezers will be functional. First, simple configurations of two colloids will be investigated, for example by looking at the force acting on a static colloid, when the other one has an oscillating motion nearby the first particle. More complex configurations will be explored later. For Newtonian fluids, the hydrodynamic interaction can be described by a coupling tensor. The possibility of having a similar simple description of the interactions in a viscoelastic fluid under certain conditions will be investigated.
In addition, the experimental setup may allow to test other problems related to colloids in viscoelastic systems. For example, very local changes of the rheology of the fluid, because of the big size of the polymer coils, might be observed. Along the same lines, the hydrodynamic forces acting on a single colloid very close to a flat surface could also be measured to test whether the rheological properties of the fluid are perturbed in the vicinity of the surface. This might happen because the polymer chains may take different configuration close to a boundary surface compared to chains in a bulk fluid.
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