Harnessing low Reynolds number flow for net migration: Locomotion of a deformable microcapsule by random fluid forces

Random noise in low Reynolds number flow has rarely been used to obtain net migration of microscale objects. In this paper, we show that net migration of a microscale object can be extracted from random directional fluid forces, by introducing deformability and inhomogeneous density into the object. As a model system, we considered an elastic microcapsule containing fluid and a rigid sphere with different densities. The numerical results showed that the microcapsule could migrate vertically downward when random forces were applied using a migration mechanism based on non-reciprocal body deformation in Stokes flow. We also developed a mathematical framework to describe the deformation-induced migration caused by noise. The proposed theory showed good agreement with the simulation results, and illustrated that drag asymmetry acts like a rachet to generate net downward motion under noise. These results provide a basis for understanding the noise-induced migration of a micro-swimmer and are useful for harnessing energy from low Reynolds number flow.