  Microorganisms and synthetic microswimmers often encounter complex
environments consisting of networks of obstacles embedded into viscous fluids.
Such settings include biological media, such as mucus with filamentous
networks, as well as environmental scenarios, including wet soil and aquifers.
A fundamental question in studying their locomotion is how the impermeability
of these porous media impact their propulsion performance compared with the
case that in a purely viscous fluid. Previous studies showed that the
additional resistance due to the embedded obstacles leads to an enhanced
propulsion of different types of swimmers, including undulatory swimmers,
helical swimmers, and squirmers. In this work we employ a canonical
three-sphere swimmer model to probe the impact of propulsion in porous media.
The Brinkman equation is utilized to model a sparse network of stationary
obstacles embedded into an incompressible Newtonian liquid. We present both a
far-field theory and numerical simulations to characterize the propulsion
performance of the swimmer in such porous media. In contrast to enhanced
propulsion observed in other swimmer models, our results reveal that both the
propulsion speed and efficiency of the three-sphere swimmer are largely reduced
by the impermeability of the porous medium. We attribute the substantial
reduction in propulsion performance to the screened hydrodynamic interactions
among the spheres due to the more rapid spatial decays of flows in Brinkman
media. These results highlight how enhanced or hindered propulsion in porous
media is largely dependent on individual propulsion mechanisms. The specific
example and physical insights provided here may guide the design of synthetic
microswimmers for effective locomotion in porous media in their potential
biological and environmental applications.
