Mads Rode

We show how to construct and apply a setup to acoustically tether and enable behavioral observations of individual microorganisms using simple laboratory equipment and a standard light microscope. We explore the capability of the setup with the freely swimming dinoflagellate Alexandrium minutum as the study organism. The setup allows us to tether cells in focus in the mid-plane of the sample chamber and make observations of individual organisms at high magnification without affecting their flagellar beat frequencies. We discuss the prospect of the method to explore appendage motion and swimming kinematics of other flagellates and ciliates, and we argue that the method will be applicable to a broad range of cell sizes and shapes.

Ciliates are ubiquitous in the marine environment and important consumers of phytoplankton and flagellates. The feeding on suspended food particles in ciliates is complex and relies typically on coordinated motion in bands of transversal rows of cilia known as membranelles. Many ciliates are upstream collectors that use a single membranelle band to generate feeding flow, retain food particles, and transport them to the mouth region. To explore a representative upstream collector, we consider the ciliate Euplotes vannus. We determine the clearance rate using particle tracking and estimate the flow-generating force by fitting a point force model to the observed flow. To obtain a mechanistic understanding of feeding flow and particle retention, we use high-speed video microscopy. The cilia move parallel to the membranelle band towards the mouth region in the power strokes, and a metachronal wave propagates away from the mouth region parallel to the band and outwards along the membranelles. A gap, therefore, opens between neighboring membranelles from the inner side of the band where the mouth region is located, and while food particles are retained, water is drawn in and pushed outwards across the membranelle band as the gap closes from the inner side. We suggest a model that rationalizes our observations and quantifies the mechanism by which the membranelle band can both generate feeding flow and retain food particles. The model compares favorably with our observations and suggests a trade-off between the clearance rate and the membranelle gap that determines the lower cutoff in the prey size spectrum. We quantify the trade-off and discuss the ecological significance of our findings.

Many aquatic microorganisms are attached to solid surfaces while creating feeding flows that bring prey particles to them. To explore the effects of surface proximity and orientation of the flow-generating force, we analyze the low-Reynolds-number flow due to a point force above a plane no-slip surface. The presence of the surface reduces the feeding flow relative to the unbounded situation. We show that the reduction of the flow rate through a circular clearance disk perpendicular to the force and centered at its position is twice as large when the force is perpendicular to the surface as when it is parallel. When the force is perpendicular to the surface, the flow forms a toroidal eddy with closed streamlines, and the resulting flow recirculation may lead to refiltration of water that has already been cleared for prey. We prove that due to the nature of the far-field flow, the shortest recirculation time along a streamline through a circular clearance disk is inversely proportional to the flow rate to the power four. Finally, we discuss the biological advantages and disadvantages of perpendicular and parallel force orientation and the effects of prey diffusion and ambient flow, and we argue that recirculation is irrelevant in the typical perpendicular feeding flow since the recirculation time is long compared to the biologically relevant timescales.

We present an experimental study of surface wave fields in single- and double-slit experiments with single wave-driven droplets of silicone oil that bounce on a vertically vibrated bath of the same liquid. During slit passage, we observe significant reduction of the central wave crest behind the droplet and disruption of the horseshoe-shaped wave field that is characteristic of a droplet over flat topography. In the double-slit experiments, we find a weak wave field in and through the slit that is not visited by the droplet, and there are only minute differences between the single- and double-slit experiments in the wave field along the center line of the slit that is visited by the droplet. We discuss our observations in relation to the slit experiments by Couder and Fort [ Phys. Rev. Lett. 97, 154101 (2006)] and Pucci et al. [ J. Fluid Mech. 835, 1136 (2018)].