Session M23: Microscale Flows: Oscillation and Locomotion

Micro-capsule swimmer controlled by flow oscillation

Recently, many kinds of micro-swimmer have been proposed for medical and engineering applications. The flow field around a micro-swimmer can be regarded as Stokes flow, in which the swimmer cannot migrate by reciprocal motion. We propose a novel micro-capsule model that propel itself by nonreciprocal deformation under flow oscillation conditions in Stokes flow regime. To the best of our knowledge, this is the first study on the propulsion principle using an elastic capsule, and one of only a few studies using flow oscillation. The micro-capsule consists of a thin elastic membrane containing fluid and a rigid sphere. A micro-capsule in liquid is oscillated by a vertical excitation force. The flow oscillation generates opposing forces on a capsule and a rigid sphere, because the densities of the internal fluid and sphere are different. The opposing forces induce nonreciprocal body deformation. Thus, our micro-capsule locomotes upward overcoming gravity. Using numerical simulations, we found that the micro-swimmer propel itself by effective and recovery strokes. We discuss the feasibility of the proposed micro-swimmer and show that the most efficient swimmer can migrate tens of micro-meters per second. These new findings pave the way for future artificial micro-swimmer designs.   

Flagellated Magnetic Particle Swimming

Swimming bacteria have been the main inspiration behind development of microscale robots for drug delivery applications. Many bacteria, like Salmonella typhimurium, swim by rotating and bundling helical flagella extremities, allowing them to propagate on the microscale. The flagellin proteins which makeup the flagellum are highly dynamic materials, which can not only self-assemble, but also optimize their helical forms in response to physical and chemical property changes within the surrounding fluidic environment. Harvesting bacterial flagella and chemically bonding them to rigid magnetic particles allows for simplistic and reliable bottom up manufacturing to create environmentally responsive microswimmers. In this work, we present research into biohybrid flagellated magnetic particles for microswimmer applications. Flagellated magnetic particles of different diameters are actuated using rotating magnetic fields. The pH of the fluid environment is adjusted to compare different helical forms and their effects on swimming motion. Finally, we examine how multiflagellated particles swim in comparison to their living bacterial counterparts. These microswimmers are promising for diagnosing and treating diseases in a minimally invasive manner.   

Pumping, mixing, and swimming with oscillating arms

At microscopic scales, it is not trivial to generate and manipulate flow due to the dominant effects of viscosity. Here we demonstrate in table-top experiments how a simple oscillatory motion of rigid arms with phase delay can produce complex flow patterns at low Reynolds number.  The experiments are in good agreement with simulations performed using the regularized Stokeslets method. The resultant flow enables mixing of the surrounding fluid, manipulation of large objects at a distance, and locomotion of mobile bodies. This could inspire new designs for pumps, mixers, and robotic swimmers at the microscopic scale.   

Enhancement of mass transport and separations of species in a electroosmotic flow by distinct oscillatory signals.

The mass transport driven by an oscillatory electroosmotic flow (OEOF) through a parallel flat plate microchannel connecting two reservoirs with different concentrations of an electro-neutral solute is studied. The OEOF is caused by the simultaneous effect of zeta potentials at its walls with an oscillatory external electric field, analyzing  three different periodic signals. The governing equations are given by the Poisson-Boltzmann equation, the modified Navier-Stokes equations and the mass transport equation of the solute. The non-dimensionalized equations are solved and four dimensionless parameters appear which control the transport of the solute: an angular Reynolds number, the Schmidt and Peclet numbers and the ratio of the microchannel height to the Debye length. The mass transport of the solute depends on the type of the used periodic electrical signal for driving the electroosmotic flow and is numerically determined. It is observed that there are values of the angular Reynolds number where the total mass transport of species is the same, independently of the molecular diffusion coefficient.   

Interactions of phoretic micro-swimmers: beyond far-field models

Phoretic particles exploit local gradients in a self-generated solute concentration for propulsion. The dynamics of each particle is influenced by the solute concentration and flow fields generated by the surrounding particles. Far-field modelling which retains only the dominant coupling induced by the chemical source and hydrodynamic stresslet, is unable to capture the interactions at closer-ranges. Here, we propose a unified framework based on the method of reflections for Laplace and Stokes equations to model these higher-order interactions. We further validate this method by comparing against numerical simulations using Boundary Element Methods and present a few cases of its application in understanding collective dynamics of phoretic particles.   

Flow field around a swimming droplet close to a wall

Micro-swimmers rarely evolve in a 3D infinite and unbounded medium. Instead, they are confined by external geometries which strongly modify their behavior. There is however no exact theoretical knowledge of the flow fields in this context and experimental data are scarce.   Here we consider a swimming water droplet [1], denser than the continuous phase, which performs a linear steady motion parallel to the bottom wall. The flow field around the droplet, measured using 3D confocal PIV, is quantitatively characterized.  Besides we propose an analytical formulation derived from a simplified description of the swimmer as the superposition of dipolar and quadrupolar singularities and an exact treatment of the wall [2]. The effect of the wall on quadrupole terms is very well captured. Although lower terms suffer from the approximation, they show that the long-range interaction between droplets can not be reduced to a simple description in terms of pushers or pullers.

[1] Izri et al. PRL 113, 248302 (2014).
[2] Blake et al. JEM 8 (1) (1974).
  

Self-learning how to self-propel at low Reynolds numbers

Synthetic swimmers capable of moving at the microscopic scale offer exciting opportunities for biomedical applications. However, existing swimmers are typically designed with fixed swimming gaits for a particular type of medium or environmental conditions. Their locomotion performance therefore may not be robust to environmental changes. In this talk, we will present a machine learning framework that enables the design of a new class of self-learning, adaptive (or "smart") swimmers at low Reynolds numbers. Unlike the traditional approach, we do not specify the swimming gaits a priori but allow the swimmer to self-learn swimming policies via machine learning algorithms.  This framework can serve as a useful tool for designing smart micro-robots with robust locomotive capabilities for biomedical applications.
  

Enhanced microorganism swimming in active matter

We study a swimming undulating sheet in the isotropic phase of an active nematic liquid crystal. Activity changes the effective shear viscosity, reducing it to zero at a critical value of activity. Expanding in the sheet amplitude, we find that the correction to the swimming speed due to activity is inversely proportional to the effective shear viscosity. Our perturbative calculation becomes invalid near the critical value of activity; using numerical methods to probe this regime, we find that activity enhances the swimming speed by an order of magnitude compared to the passive case.