Session A14: Biofluids: Low Re Swimming I

Calibrating numerical models of spherical bacteria using macroscopic experiments

Biological experiments and numerical simulations are the most common methods of studying the swimming of microorganisms. However, biological measurements used to calibrate numerical simulations generally have large uncertainties. The Trinity-Centre Collaboration uses dynamically similar, macroscopic low Reynolds Number experiments to precisely calibrate the method of regularized Stokeslets (MRS) and the method of images for regularized Stokeslets (MIRS) to extract quantitatively accurate values for the forces and torques on a bacterial model moving near a boundary. We previously produced calibration data and optimal computational parameters for cylinders and helices (Shindell et al., Fluids, 2021). Our latest experiments measure the drag and torque on spheres moving parallel and perpendicular to boundaries. These data confirm, for the first time, the theory of Lee and Leal (1980) for the forces and torques present on a rotating and translating sphere near a plane wall. We have also developed an open-source MATLAB APP that calculates theoretical values for other researchers. The comparison among experimental measurements, theory, and optimized MIRS simulations shows excellent agreement.   

Finding optimal computational parameters for the method of images for regularized Stokeslet

Many numerical simulations in Stokes flow require modeling a sphere in motion near a boundary. Since the method of images for regularized Stokeslets (MIRS) has been widely used for this purpose, we develop a systematic way to calibrate the MIRS with theoretical formulations (when they exist) and dynamically similar macroscopic experiments. Our work in 2021 calibrated the discretization sizes and regularization parameters in the MIRS using torques on cylinders and helices of different wavelengths as they rotated in a viscous fluid at various distances from a boundary. Using the same approach, our present work focuses on forces and torques of spheres moving near a boundary.

We discover that the surface discretization called spherical centroidal Voronoi tessellations (SCVT) is the most accurate and robust for all motions when we compare SCVT with discretizations whose point distributions on the sphere’s surface are symmetric with respect to the boundary. Depending on which regularization function is used in the MIRS, we find a constant ratio, for all motions, of the optimal regularization parameter in free space to the average distance used in the SCVT discretization. Our study reveals how the discretization type and size, optimal regularization parameter, and regularization function affect the accuracy and robustness of sphere-motion simulations.   

Using experimental data and numerical models to compute the dynamics of Pseudomonas aeruginosa

The Trinity-Centre Collaboration uses dynamically similar table-top experiments to calibrate the Method of Images for Regularized Stokeslets (MIRS) for use as a noninvasive probe of bacterial swimming dynamics. We study the motile bacterium Pseudomonas aeruginosa, which consists of a rod-shaped body and a single polar flagellum. A molecular motor and a helical flagellar filament propel the cell through its low Reynolds number environment, in which nearby boundaries significantly increase fluid forces and torques. Bacterial trajectories are measured using TIRF microscopy, and our new tracking scheme yields both orientation and boundary distance. The trajectories are inputs into the MIRS from which forces and torques are computed to better understand how P. aeruginosa and other bacteria swim using a helical flagellum. See the accompanying talks by Kate Brown and Hoa Nguyen to see how the experiments are conducted and the MIRS is calibrated.   

Spheroidal swimmers in viscosity gradients

Swimming microorganisms and active particles demonstrate taxis, a unique behavior that facilitates their navigation within intricate and variable environments by adjusting their orientation in response to changes in chemical or material gradients. Past research has shown that spherical active (squirmer) particles tend to reorient to swim down viscosity gradients, i.e., display negative viscotaxis. In talk, we discuss the role of shape on viscotaxis, namely we studied the hydrodynamics of prolate spheroidal squirmers in linear viscosity gradients, offering a more accurate model for non-spherical swimmers like paramecium. Our findings suggest that spheroidal squirmers, irrespective of being classified as pushers, pullers, or neutral swimmers, display behaviors analogous to those of their spherical counterparts; however, slenderness tends to diminish the impact of the viscosity gradient on the dynamics.   

Dynamics and Synchronization of Surface Active Flow Driven Droplets

Active droplets which can autonomously locomote are of both fundamental interest and practical importance. Due to the action of the active agents in the droplets, hydrodynamic flows may arise on its interface, which can drive the droplets to propel. However, our current understanding of how surface active flows determine the dynamical modes of the droplets remains elusive to date. We investigate the dynamics of droplets above a no-slip bottom wall, driven by neutral swimmers on their surfaces. Specifically, we use the rigid multiblob method to simulate the dynamic trajectory of spherical droplets. Interestingly, we find that for a single droplet, its trajectories exhibit petal-like circular motions. Synchronization phenomena are observed in the presence of multiple droplets, due to translation-rotation hydrodynamic coupling. The dynamical behaviors and synchronizations are further quantitatively analyzed and explained. Our work sheds light on understanding the surface flow mediated autonomous motion of active droplets.