A trio of algal biophysics talks

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The 70th Annual Meeting of the American Physical Society Division of Fluid Dynamics, November 19–21, 2017 in Denver, Colorado, will include a few talks about Volvox and Chlamydomonas motility. Timothy Pedley from Cambridge will present “An improved squirmer model for Volvox locomotion“:

We recently used the Lighthill-Blake envelope (or ‘squirmer’) model for ciliary propulsion to predict the mean swimming speed U and angular velocity Ω of spherical Volvox colonies [1]. Input was the measured flagellar beating patterns (a symplectic metachronal wave) of Volvox carteri colonies with different radii a [2]. The predictions were compared with independent measurements of U and Ω as functions of a, and proved to be substantial underestimates of both U and Ω, by about 80%, probably because the envelope model ignores the fact that, during the recovery stroke, most of a flagellum is much closer to the no-slip colony surface than during the power stroke. In consequence U and Ω will be proportional to the beating amplitude ϵ not to ϵ2 as in the Lighthill-Blake theory. A new model is proposed, based on a shear-stress (not velocity) distribution (cf [4]) that is applied at a smaller radius in the recovery stroke than in the power stroke. Agreement with experiment is greatly improved. [1] Pedley et al, JFM 798:165,2016. [2] Brumley et al, PRL 109:268102,2012. [3] Drescher et al,PRL 102:168101,2009. [4]Short et al, PNAS 103:8315,2006.

Kyle Welch from University of Minnesota, Twin Cities will present “Imaging the 3D flow around swimming Chlamydomonas reinhardtii using digital inline holographic microscopy“:

Understanding the 3D flow induced by microswimmers is paramount to revealing how they interact with each other and their environment. While many studies have measured 2D projections of flow fields around single microorganisms, reliable 3D measurement remains elusive due to the difficulty in imaging fast 3D fluid flows at submicron spatial and millisecond temporal scales. Here, we present a precision measurement of the 3D flow field induced by motile planktonic algae cells, Chlamydomonas reinhardtii. We manually capture and hold stationary a single alga using a micropipette, while still allowing it to beat its flagella in the breastroke pattern characteristic to C. reinhardtii. The 3D flow field around the alga is then tracked by employing fast holographic imaging on 1 um tracer particles, which leads to a spatial resolution of ~100 nm along the optical axis and ~40 nm in the imaging plane normal to the optical axis. We image the flow around a single alga continuously through thousands of flagellar beat cycles and aggregate that data into a complete 3D flow field. Our study demonstrates the power of holography in imaging fast complex microscopic flow structures and provides crucial information for understanding the detailed locomotion of swimming microorganisms.

Mehdi Mirzakhanloo from University of California, Berkeley will present “Dynamic equilibrium of microswimmers“:

Here we show that two propelling microswimmers may fall into an equilibrium state at which they both remain stagnant indefinitely. This so-called “Dynamic Equilibrium” is a result of hydrodynamic interactions between the two swimmers, and is obtained through the formation of a nested saddle-shaped flow field near swimmers. We use, as a benchmark, a newly proposed artificial microswimmer named Quadroar which consists of two axles (with rotating disks at each end) connected by a reciprocating linear actuator. Quadroar induces an oscillatory flow field which closely resembles that of Chlamydomonas Reinhardtii (a single-cell green alga). Dynamic equilibrium has not been observed at large Reynolds number regimes, and therefore this finding may have unique and important implications in the collective behavior at low Reynolds numbers. Specifically, if our finding can be generalized to many microswimmers, that is, if a dynamic equilibrium can be found between multiple microswimmers, then it means that a flock of microswimmers may come to an absolute halt in which they will be trapped forever.

“Trapped forever”? Terrifying!

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