Due to their potential usefulness in engineering and medical applications, various artificial microswimmers have been proposed. In our previous paper et al. [T. Morita, T. Omori, and T. Ishikawa, Phys. Rev. E 98, 023108 (2018)10.1103/PhysRevE.98.023108], we introduced a microcapsule swimmer that underwent amoeboidlike shape deformation under vertical fluid oscillation and showed the advantages of using a solid membrane and fluid oscillation in terms of swimmer controllability. Although the microcapsule was capable of migrating in the Stokes flow regime, the propulsion direction was limited to vertically upward or downward. In this paper, therefore, we attempted to control the propulsion of a microcapsule in an arbitrary direction by imposing biaxial fluid oscillations, which is a major step toward future applications. Numerical results showed that the microcapsule could migrate in the horizontal and vertical directions by imposing biaxial fluid oscillations in the vertical and horizontal planes, respectively. The horizontal propulsion can be understood in terms of effective and recovery strokes, i.e., a stroke swimmer, whereas vertical propulsion is similar to rigid body motion induced by a torque, i.e., a torque swimmer. By sequentially imposing three types of fluid oscillations, we successfully controlled the microswimmer to draw a Π-shaped trajectory. Thus, these results illustrate that the position and trajectory of the microswimmer can be controlled arbitrarily in three dimensions. The knowledge presented in this paper is important for future artificial microswimmer designs.
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