Physics of microswimmers—single particle motion and collective behavior: a review

Abstract

Locomotion and transport of microorganisms in fluids is an essential aspect of life. Search for food, orientation toward light, spreading of off-spring, and the formation of colonies are only possible due to locomotion. Swimming at the microscale occurs at low Reynolds numbers, where fluid friction and viscosity dominates over inertia. Here, evolution achieved propulsion mechanisms, which overcome and even exploit drag. Prominent propulsion mechanisms are rotating helical flagella, exploited by many bacteria, and snake-like or whip-like motion of eukaryotic flagella, utilized by sperm and algae. For artificial microswimmers, alternative concepts to convert chemical energy or heat into directed motion can be employed, which are potentially more efficient. The dynamics of microswimmers comprises many facets, which are all required to achieve locomotion. In this article, we review the physics of locomotion of biological and synthetic microswimmers, and the collective behavior of their assemblies. Starting from individual microswimmers, we describe the various propulsion mechanism of biological and synthetic systems and address the hydrodynamic aspects of swimming. This comprises synchronization and the concerted beating of flagella and cilia. In addition, the swimming behavior next to surfaces is examined. Finally, collective and cooperate phenomena of various types of isotropic and anisotropic swimmers with and without hydrodynamic interactions are discussed.

Guidance and Self‐Sorting of Active Swimmers: 3D Periodic Arrays Increase Persistence Length of Human Sperm Selecting for the Fittest

“…Transport behavior of MPCD was well characterized by this study and others,59, 60, 61, 62, 63, 64, 65 and applied to a diverse set of problems 66, 67, 68, 69. including sperm motility and cooperation 46, 70, 71. In the MPCD simulations, the solvent dynamics was consisted of a streaming and a collision step, during which particles exchange momentum within a collision cell of linear dimension a (Figure S8, Supporting Information) 58, 62, 63.…”

“…In addition, our results have implications beyond human fertility including biodiversity and conservation of endangered or rare species. Moreover, the similarities in fluid physics of swimmers across different species,46 from bacteria to parasites, make these findings broadly applicable to other motile microorganisms. In fact, many such active swimmers across diverse biological systems have been an inspiration for the design of artificial micrometer‐scale flagellar systems including biohybrid microrobots,47, 48, 49, 50, 51, 52 and we also envision broad applications of our findings in areas such as medical diagnosis,53 biosensing,54 and targeted drug delivery 55…”

Chemically Propelled Motors Navigate Chemical Patterns

“…Often collections of active particles are observed to form dynamic clusters, segregate into low and high density domains, and evolve into other inhomogeneous states not seen in equilibrium systems 20, 21, 22, 23, 24. Simple active Brownian particle models are able to capture some aspects of this dynamics 50, 51, 52, 53, 54…”

“…Alternatively, external fields8, 11, 12, 13, 14, 15 and interactions with surfaces16, 17, 18, 19 have also been proposed as ways to control motor motion. When many motors interact with each other, phenomena such as dynamic clustering, swarming and active phase segregation are observed 20, 21, 22, 23, 24. These features have stimulated research on the design and properties of motors that is aimed at potential applications 5, 12, 25, 26, 27, 28, 29, 30, 31…”

A Pickering Emulsion Route to Swimming Active Janus Colloids

“…We focus specifically on spherical Janus colloids of the type shown in Figure 1 . This type of swimming device has been found to display a wide range of interesting colloidal phenomena, including autonomous guidance effects (gravitaxis,16 chemotaxis,17 and boundary steering),18 and has also been the subject of many currently untested theoretical proposals for high volume fraction collective phenomena 19. An advantage of Janus spheres as a system to explore emergent behavior is that they move without issuing bubbles and so their interactions via chemical “wakes” and hydrodynamics are amenable to being analyzed and experimentally observed, whereas nanotubes produce considerable convective flow due to bubble release 20.…”