Looking forward, building on the findings in this dissertation, I think the following few topics disserves further investigations in the future:
Pattern Formation in Active Suspensions
In Chapter 2, I investigated the collective behavior of active undulatory microswimmers. Toward the goal of understanding and utilizing the unique properties of these active systems, a lot more needs to be done. There is a large parameter space that has not been explored. For example, one can add passive particles into the active systems and investigate how the active particles interact with the passive particles and how the dynamic patterns formed by the active particles are altered by the addition of passive particles. The ratio of active and passive particles, the shape and size of the particles can be varied. Also,
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one can study the effect of confinements on the pattern formation in these systems. An accurate understanding of the pattern formations in active suspensions as functions of all these various perturbations will provide us with additional ways of controlling the behavior of active suspensions which could be very useful in many engineering applications.
Transport of Active Matter in Porous Media
In Chapter 4, I investigated the rheotaxis behavior of undulatory microswimmers. The experiment was performed in a liquid filled microfluidic conduit. However, in the wild, many undulatory microswimmers such as nematodes lives in water-saturated soil. It has been reported that active nematodes can resist fluid flow in soils (108-110). Since the typical pore sizes in soils used in these experiments were smaller than the length of the animals, the rheotaxis mechanism that I presented in Chapter 4 most likely are suppressed in these experiments. A different mechanism is needed to explain the flow resistance behavior. Partially due to the lack of direct observation of the transport of microswimmers in porous media in the presence of fluid flow, it remains unclear how the animals managed to resist the fluid flow in the soils. Interestingly, inactive animals does not exhibit this flow resistance behavior. An accurate understanding of the flow resistance mechanism in soils will help designing new interventions to perturb the life cycles of these animals in the field and high-throughput motility-based sorting methods.
Fluid Mechanical Sensation of C. elegans
In Chapter 5, I investigated the swimming gait of C. elegans as a function of flow intensity and found out that the swimming gait is not affected by the flow intensity.
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However, these experiments were performed at the time scale of a few minutes which is relatively short compared to the life span of these animals (a few weeks). It would be interesting to see if long term exposure to fluid flow or fluid mechanical forces will alter the swimming behavior of the animals. One hypothesis is that due to the presence of fluid mechanical forces, they would be more active than the ones that are not exposure to fluid mechanical forces. This question by itself is scientifically interesting because it may help us understand, at the molecular level, how animals sense fluid mechanical forces. In addition, if the animals were indeed more active when exposed to fluid mechanical forces, one can then use fluid mechanical forces to control the activity level (exercise amount) of the animals which will enable the investigation of effect of exercise on various aspect of animal physiology at the molecular level.
Application/optimization of the High-throughput Sorters for other Microswimmers
In Chapter 6, I presented the designs of a few motility-based sorters and their applications in C. elegans related research. However, in principle, the sorting methods should also work for other types of microswimmers such as sperms. It would be interesting to test whether this is indeed the case. There is currently a need for high-throughput selection of motile sperm for many clinical procedures such as in intro fertilization.
Harvesting Energy from Biological Microswimmers
In Chapter 7, I demonstrate the use of 3D printed structures to direct the motion of microswimmers. The method is simple and its operation cost no energy at all. It would be interesting to see whether one can harvest useful energy in the forms of, for example,
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electricity from microswimmers using this control method. Many biological microswimmers can obtain energy from sources that are otherwise considered as waste.
Probing Surface Topography with Microswimmers
In Chapter 7, I demonstrate the feasibility of using microswimmers as moving probes to collect information about the topography of surfaces. In the study, I used relatively large microswimmers (C. elegans that are about 1 mm long) and has only performed preliminary studies on one type of surface. It would be interesting to perform experiment using microswimmers with different sizes on different shaped surfaces to systematically study the swimming behavior of microswimmers on structured surfaces. Such studies will generate a data base which can be used to help infer topography of unknown surfaces based on the behavior of microswimmers on these surfaces.
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