Passive swimming of a microcapsule in vertical fluid oscillation

Abstract

The artificial microswimmer is a cutting-edge technology with applications in drug delivery and micro-total-analysis systems. The flow field around a microswimmer can be regarded as Stokes flow, in which reciprocal body deformation cannot induce migration. In this study, we propose a microcapsule swimmer that undergoes amoeboidlike shape deformations under fluid oscillation conditions. This is a study on the propulsion principle using a capsule with a solid membrane, and one of only a few studies using fluid oscillation. The microswimmer consists of an elastic capsule containing fluid and a rigid sphere. Opposing forces are generated when fluid oscillations are applied, because the densities of the internal fluid and sphere are different. The opposing forces induce nonreciprocal body deformation, which leads to migration of the microswimmer under Stokes flow conditions. Using numerical simulations, we found that the microswimmer propels itself in one of two modes, i.e., stroke swimming or drag swimming. We discuss the feasibility of the proposed microswimmer and show that the most efficient swimmer can migrate tens of micrometers per second. These findings pave the way for future artificial microswimmer designs.

Engineering passive swimmers by shaking liquids

“…This type of passive swimming and the theoretical model of a brake controlled triangle [19] are similar to cross-stream migration of droplets and soft particles in stationary low Reynolds number Poiseuille flows [20][21][22][23][24]. Other recent studies identified the finite inertia of soft particles in oscillatory homogeneous liquid motion as a crucial property for passive swimming [11,12]. The non-reciprocal body shape and therefore the different Stokes drag in both half periods of the periodic liquid motion is the driving force of these novel locomotion mechanism.…”

“…Previous studies focused on the propulsion of intrinsically asymmetric soft particles in sinusoidal liquid motion [11,12]. With our extension to soft bead-spring tetrahedrons and to asymmetric, soft Janus capsules, we show that the inertia driven propulsion mechanism can even outweigh gravity.…”

“…The first inertia driven particle locomotion at low Reynolds number was demonstrated for a soft, asymmetric, Λ-shaped particle in a shaken liquid [11]. This was extended to an internally structured capsule with an inhomogeneous mass distribution in a gravitation field [12].…”

“…Their exploration and understanding inspires, among others, passive microswimmers that are indirectly driven by a time-dependent liquid motion. An example is a recently identified inertia-driven, passive microswimmer: a non-buoyant asymmetric soft microparticle in oscillatory liquid motion of zero mean displacement was studied in [11,12]. Here we show how this inertia-driven locomotion mechanism can be generalized to the much wider class of homogeneous, soft particles, such as capsules, by engineering an appropriate time-dependent liquid motion.…”

“…If all beads of a tetrahedron have the same mass density, the upward oriented tetrahedron () and the downward oriented one () are both stable. By changing the mass density of one of the four beads then one of both orientations with respect to the gravitational field is preferred, similar as in reference [12]. For example, if the tetrahedron sinks (without liquid shaking) the lighter bead points upwards after a certain time.…”