Creating a noninvasive probe to study bacterial motility using macroscopic experiments and numerical models

The evolutionary pressures that cause morphological changes in bacteria are often difficult to determine because we lack precise information about fluid-structure interactions when bacteria are motile. Numerical modeling provides insights, but the models are often calibrated using biological measurements that have large uncertainties. Instead, our collaborative project (NSF POLS - 2210610) uses dynamically similar tabletop experiments to precisely calibrate the method of regularized Stokeslets (MRS) and the method of images for regularized Stokeslets (MIRS) to extract quantitatively accurate values of forces and torques on model bacteria moving near a boundary. Our experiments have provided the first experimental validation of theories for torque on cylinders (Jeffrey and Onishi 1981) and a complete test of forces and torques on spheres (Lee and Leal 1981) moving near a boundary. Calibration of the MIRS requires adjusting the value of the regularization parameter once an object is discretized. Our method uses the MRS to find the optimal ratio of the regularization parameter to the distance between grid points, which is independent of the number of grid points. However, the ratio depends strongly on the geometry of the object, e.g., cylinders versus spheres (Shindell et al. 2021 and Nguyen et al. 2025). Once optimized, the calibrated MIRS provides unique insights into how body shape affects swimming performance measures near a boundary. Our latest experiments measure the forces and torques on spheroids moving near a boundary and the free-swimming speed of a force and torque free counter-rotating bacterial model to better understand the functional dependence of the regularization parameter on a solid object's geometry. See the related posters by my students Shane Mushambi and Jack Kennedy for more details.