Stroke patterns for Purcell's three-link swimmer are optimized. We model the swimmer as a jointed chain of three slender rods moving in an inertialess flow. The swimmer is optimized for efficiency and speed. We were able to attain swimmer designs significantly more efficient than those previously suggested by authors who only consider geometric design rather than kinematic criteria. The influence of slenderness on optimality is considered as well.

We consider a small droplet of water sitting on top of a heated superhydrophobic surface. A toroidal convection pattern develops in which fluid is observed to rise along the surface of the spherical droplet and to accelerate downwards in the interior towards the liquid/solid contact point. The internal dynamics arise due to the presence of a vertical temperature gradient; this leads to a gradient in surface tension which in turn drives fluid away from the contact point along the interface. We develop a solution to this thermocapillary-driven Marangoni flow analytically in terms of streamfunctions. Quantitative comparisons between analytical and experimental results are presented as well as effective heat transfer coefficients.

The influence of hydrodynamic forces on eukaryotic flagella synchronization is investigated by triggering phase locking between a controlled external flow and the flagella of C. reinhardtii. Hydrodynamic forces required for synchronization are over an order of magnitude larger than hydrodynamic forces experienced in physiological conditions. Our results suggest that synchronization is due instead to coupling through cell internal fibers connecting the flagella. This conclusion is confirmed by observations of the vfl3 mutant, with impaired mechanical connection between the flagella.

Locomotion is widely observed in life at micrometric scales and is exhibited by many eukaryotic unicellular organisms. Motility of such organisms can be achieved through periodic deformations of a tail-like projection called the eukaryotic flagellum. Although the mechanism allowing the flagellum to deform is largely understood, questions related to the functional significance of the observed beating patterns remain unresolved. Here, we focus our attention on the stroke patterns of biflagellated phytoplanktons resembling the green alga Chlamydomonas. Such organisms have been widely observed to beat their flagella in two different ways-a breaststroke and an undulatory stroke-both of which are prototypical of general beating patterns observed in eukaryotes. We develop a general optimization procedure to determine the existence of optimal swimming gaits and investigate their functional significance with respect to locomotion and nutrient uptake. Both the undulatory and the breaststroke represent local optima for efficient swimming. With respect to the generation of feeding currents, we found the breaststroke to be optimal and to enhance nutrient uptake significantly, particularly when the organism is immersed in a gradient of nutrients.optimization | stroke kinematics | low Reynolds number | efficiency T he vast majority of living organisms are found in an astonishing diversity at micrometric scales. Such seemingly simple unicellular organisms interact with their surroundings and exhibit complex dynamic behaviors in order to access resource-rich environments. The ability to locate and take advantage of such favorable environments relies on a rapid chemotactic response (1, 2). In this paper, we investigate the motility of specific microorganisms to address questions related to the functional adaptation, efficiency, and optimization of biolocomotion.A dominant trait shared by many microorganisms is the prevalence of flagella as motility appendages. The structure of the axoneme, which constitutes the core of the eukaryotic flagellum, is extremely well preserved across the eukaryotic domain. This same structure is found to propel sperm cells and, when several of them beat collectively as cilia, to expel mucus from human lungs. The eukaryotic flagellum has a complex internal "9 þ 2" microtubule structure (3), whose elaborate molecular machinery allows the flagellum to induce local bending moments and actively deform its shape. It is noteworthy that these flagella are widely observed to beat in two significantly different ways: undulatory traveling waves that propagate down the flagellum (as exhibited by sperm cells), and so-called "effective recovery" strokes commonly observed in cilia. Because it is possible for organisms to alter and control the waveform along the flagellum, it is of interest to investigate the functional significance of these specific flagellar stroke patterns and their utility with respect to swimming and feeding.Previous investigations of nutrient transport and hydrodynamics around microorganisms (4...