Direct measurement of the flow field around swimming microorganisms (1008.2681v1)

Abstract: Swimming microorganisms create flows that influence their mutual interactions and modify the rheology of their suspensions. While extensively studied theoretically, these flows have not been measured in detail around any freely-swimming microorganism. We report such measurements for the microphytes Volvox carteri and Chlamydomonas reinhardtii. The minute ~0.3% density excess of V. carteri over water leads to a strongly dominant Stokeslet contribution, with the widely-assumed stresslet flow only a correction to the subleading source dipole term. This implies that suspensions of V. carteri have features similar to suspensions of sedimenting particles. The flow in the region around C. reinhardtii where significant hydrodynamic interaction is likely to occur differs qualitatively from a "puller" stresslet, and can be described by a simple three-Stokeslet model.

• The paper demonstrates that Volvox carteri's flow field is dominated by the Stokeslet component, aligning it with sedimenting particles despite a modest density excess of ~0.3%.
• The paper reveals that Chlamydomonas reinhardtii exhibits complex near-field dynamics that deviate from traditional puller-type stresslet models, supported by a three-Stokeslet model.
• The integration of tracking microscopy and particle image velocimetry provides precise measurements of flow fields, offering new insights into microorganism interactions and suspension behaviors.

Direct Measurement of the Flow Field Around Swimming Microorganisms

This paper presents a comparative paper of the flow fields generated by two different microorganisms: the colonial alga Volvox carteri and the unicellular alga Chlamydomonas reinhardtii. These insights are crucial, as the intricate fluid dynamics around microorganisms have broad implications for their ecology, physiology, and the physical behaviors of their suspensions.

The primary method employed in this paper involves an integration of tracking microscopy with fluid velocimetry. This combination facilitates precise measurements of flow fields around freely swimming V. carteri and C. reinhardtii. The results provide compelling evidence of distinct flow field topologies, asserting the dominance of the Stokeslet flow component for V. carteri due to its slight density excess over water and demonstrating a non-trivial near-field flow topology in C. reinhardtii.

Key Findings:

1. Volvox carteri:
   • The analysis shows that the flow field is strongly dominated by the Stokeslet component, even though the colony's density excess is a modest ~0.3%.
   • The high symmetry of Volvox results in the leading correction in the flow field, represented by a source doublet, and a smaller contribution from the stresslet.
   • The Stokeslet dominance aligns Volvox suspension with that found in sedimenting particles rather than typical microswimmers, impacting the understanding of suspension rheology and microorganism interactions.
2. Chlamydomonas reinhardtii:
   • The flow field is complex near the cell, with significant deviations from the expected puller-type stresslet model.
   • The paper proposes a three-Stokeslet model that accommodates the thrust exerted by the flagella and accurately describes the experimental flow pattern, indicating the role of near-field interactions in the dynamics of C. reinhardtii.
   • The research illustrates that traditional puller-type theoretical models are oversimplified for distances under 7 cell radii, suggesting the need to consider full time-dependent flow dynamics in detailed simulations of microbial suspensions.

Methodological Approach:

• For V. carteri, the paper utilized an automated tracking system with a CCD camera following the individual colonies, integrating laser-illuminated flow tracking with particle image velocimetry to determine the flow fields.
• For C. reinhardtii, an inverted microscope setting enhanced by the autofluorescence of algae chlorophyll allowed for detailed flow visualization, employing particle tracers to map flow velocities.

Implications and Future Directions:

The paper of these organism's flow fields deepens our comprehension of biological fluid dynamics and indicates potential shifts in understanding microorganism-based interactions, with broader impacts on bio-physics and soft matter. The distinct characteristics of Volvox's flow field, likening it to a sedimenting system, could inspire new models in environmental biophysics, especially in contexts mimicking natural water bodies.

Future work might extend these approaches to other microorganisms to further elucidate the diverse strategies evolved across micro-scale swimmers. Additionally, exploring the time-dependent variations in flow around Chlamydomonas could fine-tune current models toward a holistic understanding of microorganism ecology, organization, and collective behaviors such as bioconvection.

Overall, this paper contributes a nuanced view of microorganism flow fields, fostering the next stages of research in marine microbiology and biophysical simulations, while concurrently providing empirical data that challenge and refine pre-existing theoretical frameworks.