Locomotion of small organisms in complex fluids at low Reynolds number

Complex fluids are a broad class of materials that are usually homogeneous at macroscopic scales but possess internal structures at an intermediate scale. Such fluids are ubiquitous in nature, e.g. colloidal suspensions, polymeric gels, and human mucus. Many organisms like bacteria live in complex fluids but our understanding of the main mechanisms governing their motility in such fluids is limited. In this thesis, I investigate the motility behavior of a biological model in both Newtonian and complex fluids. The organism is the nematode Caenorhabditis (C.) elegans that is widely used in biomedical and genetic research. I will focus on two main topics: (i) swimming in viscoelastic fluids and (ii) undulatory motility on wet surfaces. In the study of the effect of viscoelasticity on swimming, particle velocimetry and in-house developed nematode tracking software is used to characterize the flow fields and motility behaviors of C. elegans. I find that compared to Newtonian solutions, fluid elasticity leads to up to 35% slower propulsion speed. In the study of the locomotion of C. elegans on wet surfaces, the kinematics of C. elegans moving on wet agar surfaces are characterized and a model based on lubrication theory is developed to understand such kinematic behaviors. Results show that the spatial patterns and bending force generated during the locomotion of C. elegans correlate well with previously described gait-specific features of calcium signals in muscle. I also find that one may be able to control the motility gaits of C. elegans by adjusting the magnitude of the surface drag coefficients. The final chapter of this thesis is dedicated to applying the acquired knowledge on the motility kinematics and dynamics for genetic analysis of C. elegans. I integrate image analysis algorithms and fluid and solid mechanics principles to describe wild-type and mutant C. elegans motility gaits. Quantification of body shapes and hydrodynamics, and newly developed model-based estimates of biomechanics reveal that mutants affecting similar biological processes exhibit related patterns of biomechanical differences. The biomechanical profiling using C. elegans locomotion could be useful for predicting the function of previously unstudied motility genes