Drag force on micron-sized objects with different surface morphologies in a flow with a small Reynolds number

The lack of a suitable experimental setup and a limited number of in vitro investigations have impeded understanding of the factors that have important roles in altering the drag force acting on tiny microobjects in a fluidic medium. Here we investigated how the surface morphology affects the drag force acting on microparticles. The drag coefficient for micron-sized polymer-grafted polystyrene (PS) and negatively charged hard (silica) and soft (hydrogel) spheres were evaluated and compared with that of a PS particle. The drag coefficient of the PS particle was found to be comparable to that predicted according to the Stokes' formula for a hard sphere. By contrast, polymer-grafted PS and negatively charged hard and soft spheres showed a considerable deviation from the Stokes' formula. The drag coefficient decreased by ∼60 and ∼86% for the negatively charged hard and soft spheres, respectively, whereas it increased by ∼20% for the polymer-grafted PS particles with respect to that of the PS particle. This work reveals the role of the surface morphology of small objects in altering the drag force acting on them; such knowledge may help elucidate the mechanism by which the surface morphology of aquatic microorganisms helps them to control the drag force, which consequently might facilitate the invention of new technologies for micromachines or microdevices.