Torque and Drag on Spherical Bacteria Moving Near Rough Boundaries

Computational models of bacterial swimming are often compared to biological experiments that have large uncertainties, so precise calibration of the numerical models is difficult. The Trinity-Centre Collaboration, instead, seeks to precisely calibrate numerical models and use them as a noninvasive probe to calculate forces and torques on swimming bacteria imaged using total internal reflection fluorescence microscopy. Our previous macroscopic low Reynolds number experiments measured the resistive drag and torque on cylinders, spheres, and helices moving near smooth boundaries, and verified the theories of Jeffrey and Onishi (1981) and Lee and Leal (1980). However, these theories and experiments considered smooth boundaries, whereas bacteria generally swim near complex surfaces. Our work uses similar experimental methods to measure the drag and resistive torque on spheres moving near 3D printed boundaries with different properties. We use silicone oil that is 60,000 times more viscous than water to ensure that the Reynolds number is much less than unity. These data provide an additional calibration resource for numerical simulations where there is no theory for reference.