Li, Run
(2019).
Hydrodynamics of colloidal ellipsoids and helices under shear flow and active deformation.
PhD thesis, Universität zu Köln.
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Abstract
Colloidal suspensions widely exit in our daily life, in the form of food, medicaments, biological materials, or cosmetics, among other examples. Hydrodynamic interactions in colloids suspensions at non-equilibrium give rise to many intriguing phenomena, where the shape of colloidal particles plays an important role. For instance, colloidal ellipsoids tumble and kayak in shear flow. Flexible polymers deform, or multicomponent systems in various phases. Besides the tumbling of elongated colloids, further characteristics are expected in shear flow when the particles are chiral, like a helical structures which display a drift in the vorticity direction. Furthermore, the helix may generate locomotion, when external actuation is able to deform its shape in non-reciprocal ways. Artificial systems have been recently synthesized showing that it is possible to generate net motion by smoothly deforming their helical body, as a response to changing temperature. We combine a theoretical approach based on the Smoluchowski equation and flow- dichroism experiments to measure hydrodynamic aspect ratios and polydispersity of nano particles, which is not feasible with standard methods similar to light scattering. A mesoscale hydrodynamics simulations are used to study the transport of helix in shear flow and swimming of deformable helix at low Reynolds numbers. The particle-based approach of multi-particle collision dynamics also enables simulations of colloids at non- equilibrium where thermal fluctuations are not negligible. The first part of tis thesis investigates the validity of flow dichroism as a characteriza- tion tool by employing dispersions of prolate and oblate quantum dots (QDs). Flow dichroism quantifies the tumbling motion of QDs in shear flow by optical means, which provides a characteristic signature of the particle shape, hydrodynamic friction, and size distribution. Particle size, shape, polydispersity, and shear rate have an important ef- fect on the temporal evolution of the flow-induced alignment which we discuss in detail on the basis of numerical solutions of the Smoluchowski equation describing the QDs motion in the basis of the probability of the orientation of colloids in shear flow. This combination of flow dichroism and the Smoluchowski equation approach is not only use- ful for determining shape and anisotropic of colloidal QDs, but also for other nanoscale systems. The second part of this thesis discusses the transport of a deformable helical polymer in a uniform shear flow. The rigidity of the helix essentially affects its configuration under shear stress. The deformable structure still keep helicity while it is compressed and stretched periodically (breathing), and tumble simultaneously in the shear flow below certain shear stress, above which the helix performs noticeable chaotic motion. The tumbling motion follows Jefferys theory for rigid rod-like particles, although with an effective aspect ratio value which depends on flexibility and chirality parameters. The lateral drift shows to be a hydrodynamic effect with a maximum impact between rod and tube limits, obtained by changing the geometry of rigid helix. The flexibility also plays an important role on the lateral drift because its geometry and chirality keep changing. Finally, we investigate the transport of a perfectly deforming helix interacting with a viscous fluid by hydrodynamic simulations. Maintaining the helical structure and a single handedness along its entire length, we first discuss how the deformation periods, helix configuration, and fluid viscosity impact the net rotational swimming stroke and identify its principle direction of motion. We then explore how the presence of confinement in a planar slit influences the rotation speed, trajectory, and position of the deformable helix. Interestingly the active helix shows to consistently migrate to the channel center while passive helix and helix with reciprocal deformation do not show significant migration in any direction. These results including second part provide important criteria to consider in the design and optimization of helical machines at the nanoscale, and in the understanding of some biological functions, for example of flagellated structures. In summary, non-equilibrium dynamics of anisotropic colloids are studied, which include rods and helices in shear flow, and actuated helices. These results show the importance of shape anisotropy in hydrodynamics of colloidal suspensions, which has significant potential in practical applications and solution of some fundamental questions.
| Item Type: | Thesis (PhD thesis) | ||||||||
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| URN: | urn:nbn:de:hbz:38-96139 | ||||||||
| Date: | 10 March 2019 | ||||||||
| Language: | English | ||||||||
| Faculty: | Faculty of Mathematics and Natural Sciences | ||||||||
| Divisions: | Faculty of Mathematics and Natural Sciences > Department of Physics > Institute for Theoretical Physics | ||||||||
| Subjects: | Physics | ||||||||
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| Date of oral exam: | 6 May 2019 | ||||||||
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| Refereed: | Yes | ||||||||
| URI: | http://kups.ub.uni-koeln.de/id/eprint/9613 |
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