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Title: Undulatory microswimmers in complex environments : the effects of fluid composition, swimming gait, and swimmer interactions
Author: Kamal, Arshad Ahmed
ISNI:       0000 0004 9357 028X
Awarding Body: Imperial College London
Current Institution: Imperial College London
Date of Award: 2020
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Whether they be spermatozoa in cervical mucus, or {\it C. elegans} in wet soil, swimming cells often negotiate through immersed microstructures that are comparable in size to themselves. Using detailed simulation and data-driven stochastic models, we explore finite-size, planar undulatory swimmers in heterogeneous environments composed of spherical obstacles tethered via linear springs to random points in the plane of locomotion. We examine the impact of spring stiffness, obstacle density and swimmer gait on swimmer motion. Depending on both the environmental and swimmer parameters, we find that interactions with obstacles can enhance swimming speeds, or prevent the swimmer moving at all. We find that swimmers actuated closer to the front can exhibit significant enhanced swimming speeds, while those with rear-actuation have primarily hindered values. We also find that the discrete interactions between the swimmers and the obstacles lead to significant fluctuations in the swimming translational and angular velocity. These fluctuations over time can lead to diffusive behaviour primarily due to a coupling between swimming and rotational diffusion. Using a stochastic model, we quantify the diffusion coefficient resulting from the changes in swimming speed and velocity fluctuations for the different gait types. We observe cases where the swimmer is indefinitely trapped by the environment. We quantify the trapping times for these cases. We find that trapping is most likely at high tether stiffness and high obstacle density, with rear-actuated swimmers producing the shortest trapping times. Where trapping occurs, the mean-trapping times is comparable to the correlation times, suggesting the swimmers, on average, are trapped by the environment before motion is governed by diffusion alone. Finally, in addition to examining the motion of single swimmers in complex environments, we also perform simulations demonstrating how collective dynamics and the flow generated by the swimmer are affected by the presence of the obstacles.
Supervisor: Keaveny, Eric Sponsor: Imperial College London
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral