The role of fluid flow on the behavior of swimming microorganisms

Abstract

Swimming microorganisms have been observed to accumulate near surfaces in both experimental and computational studies. In the presence of an external flow, we observed that the accumulation of the organisms at the surfaces reduced. We found that this accumulation at the surfaces was higher when hydrodynamic interactions (HIs) between the organism and surfaces were included. To understand the distribution and dynamics of the organisms across the channel, the effect of flow on the orientations of the organisms was quantified and compared to previous work on upstream swimming of organisms and the alignment of passive rods in flow. The transport of the organisms from one surface to another was also affected by the alignment of the organisms due to the flow.

We have investigated the interplay of a fluid flow and the motility of the microorganisms in this thesis. Our findings may provide insight into the formation of biofilms where surface accumulation and subsequent adhesion are critical steps in these processes. Further, our results may be used to explore how other flows and geometries could enable efficient sorting of cells based on size, shape and motility for a range of applications, including diagnostics or quantifying components of complex microbial communities.

Preliminary studies were also performed to study the effect of an external flow on the surface attachment of a community of microorganisms that differ in motility. A simple probability based model was used to model surface attachment of the organisms. For a dilute population of motile microorganisms, we showed that high flow rates reduce the attachment due to the flow-induced alignment that reduce the swimming from one surface to another. In a community of motile and non-motile organisms, the surface attachment of non-motile organisms due to the disturbances generated by the motile organisms observed was at a faster rate than in a population solely of non-motile organisms without an external flow. The presence of an external flow was found to reduce the surface attachment of non-motile organisms.

We also examined the dynamics of the organisms in the direction of the external flow. The dispersion of swimming microorganisms was shown to be different from that of passive particles. We find a non-monotonic trend in the dispersivity of the organisms that was not due to the non-uniform distribution of the cells across the confinement as in previous studies, but due to the alignment of the cells by the external flow. A scaling analysis was described to capture the dispersion of the organisms at both low and high flow rates. Explicit formulas were presented to

Swimming microorganisms are of widespread interest to scientists in the fields of biology, medicine and industry. Cell motility in microorganisms has an impact on important biological phenomena like chemotaxis, reproduction and infection. These phenomena have been extensively studied from the biological perspective. The examination of swimming microorganisms as active colloidal suspensions has garnered the attention of researchers in the fields of physics and applied mathematics. Experimental and theoretical studies on swimming microorganisms have reported collective behavior, enhanced transport, reduced viscosity and surface accumulation. The key challenge in this area is to tie our understanding of these observations across different scales, from the level of individual cells to large cell populations, to biological behavior. Microorganisms are often found in environments subject to different flow velocities. In this thesis, we have investigated the role of fluid flow on the properties of swimming microorganisms.

We have performed Brownian dynamics (BD) simulations of swimming microorganisms, represented by a collection of beads joined by stiff springs. We examined two situations based on the environment microorganisms exist in and the nature of flow that influences their motion. They are the impact of self-driven flows in bulk suspensions of microorganisms and the effect of an externally applied flow on microorganisms in confinements.

Large scale disturbances generated by hydrodynamically interacting swimming microorganisms enhance the diffusivity of tracer particles in their neighborhood. We have investigated the effect of these self-driven flows on the distribution of particles ranging from the size of small molecules to peptides. We found that the flows caused by a concentrated suspension of swimming microorganisms enhanced the diffusivity of small molecules. The trends observed for enhancement in the diffusivity of tracers from our discrete simulations was compared to the predictions from continuum theories. These results highlight the need for inclusion of short-range excluded volume interactions in the continuum theories described by mean-field assumptions.

calculate the dispersivity at the extreme flow conditions using an analytical theory.