Implications of flow field modes for transport by swimming microalgae

Flagellar and ciliary propulsion are ubiquitous among swimming microorganisms, whose resultant flow fields dictate the organisms’ interactions with the physical environment, including resource uptake, predation, and interactions with particulates. While theoretical models of microbial swimming abound, compact comparisons with experiments are lacking and the essential flow field features that determine transport remain unresolved. Here, we use proper orthogonal decomposition (POD), in conjunction with micro-PIV measurements, to examine flagellar-generated flow fields of individual somatic cells and colonies of the model microalga Volvox carteri. Hierarchical POD modes of flow fields from individual cells reveal a striking similarity to a multipole expansion of the Stokes equations. For cell colonies, modal analysis provides a compact description of the complex spatio-temporal flow fields resulting from metachronal waves. Flow fields reconstituted from POD modes are used to computationally quantify uptake efficiency, including the implications of swimming modes and contrasting symplectic and antipletic waves.