Dynamics of a helical swimmer crossing viscosity gradients

Abstract

We experimentally and theoretically study the dynamics of a low-Reynolds number helical swimmer moving across viscosity gradients. Experimentally, a double-layer viscosity is generated by superposing two miscible fluids with similar densities but different dynamic viscosities. A synthetic helical magnetically-driven swimmer is then made to move across the viscosity gradients along four different configurations: either head-first (pusher swimmer) or tail-first (puller), and through either positive (i.e. going from low to high viscosity) or negative viscosity gradients. We observe qualitative differences in the penetration dynamics for each case. We find that the swimming speed can either increase or decrease while swimming across the viscosity interface, which results from the fact that the head and the tail of the swimmer can be in environments in which the local viscosity leads to different relative amounts of drag and thrust. In order to rationalize the experimental measurements, we next develop a theoretical hydrodynamic model. We assume that the classical resistive-force theory of slender filaments is locally valid along the helical propeller and use it to calculate the swimming speed as a function of the position of the swimmer relative to the fluid-fluid interface. The predictions of the model agree well with experiments for the case of positive viscosity gradients. When crossing across a negative gradient, gravitational forces in the experiment become important, and we modify the model to include buoyancy, which agrees with experiments. In general our results show that it is harder for a pusher swimmer to cross from low to high viscosity, whereas for a puller swimmer it is the opposite. Our model is also extended to the case of a swimmer crossing a continuous viscosity gradient.

References in corpus (24)

• Active Particles in Complex and Crowded Environments
• Physics of Microswimmers - Single Particle Motion and Collective Behavior
• Active Brownian Particles. From Individual to Collective Stochastic Dynamics
• Bacterial hydrodynamics
• Phototaxis of synthetic microswimmers in optical landscapes
• Clusters, asters and collective oscillations in chemotactic colloids
• Gravitaxis of asymmetric self-propelled colloidal particles
• 'Fuelled' motion: phoretic motility and collective behaviour of active colloids
• Dynamic clustering and chemotactic collapse of self-phoretic active particles
• Clustering and pattern formation in chemorepulsive active colloids
• Phoretic Interactions Generically Induce Dynamic Clusters and Wave Patterns in Active Colloids
• Polarization of active Janus particles
• Viscotaxis: microswimmer navigation in viscosity gradients
• A steering mechanism for phototaxis in Chlamydomonas
• Active particles in viscosity gradients
• Pattern Formation in Chemically Interacting Active Rotors with Self-Propulsion
• Helical propulsion in shear-thinning fluids
• Destabilization of a flow focused suspension of magnetotactic bacteria
• Helical paths, gravitaxis, and separation phenomena for mass-anisotropic self-propelling colloids: experiment versus theory
• Axisymmetric Squirmers in Stokes Fluid with Nonuniform Viscosity
• Active Brownian agents with concentration-dependent chemotactic sensitivity
• Velocity condensation for magnetotactic bacteria
• Modelling of Dictyostelium Discoideum Movement in Linear Gradient of Chemoattractant
• Focusing and splitting of particle streams in microflows via viscosity gradients

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