Effective swimming strategies in low Reynolds number flows

The optimal strategy for a microscopic swimmer to migrate across a linear shear flow is discussed. The two cases, in which the swimmer is located at large distance, and in the proximity of a solid wall, are taken into account. It is shown that migration can be achieved by means of a combination of sailing through the flow and swimming, where the swimming strokes are induced by the external flow without need of internal energy sources or external drives. The structural dynamics required for the swimmer to move in the desired direction is discussed and two simple models, based respectively on the presence of an elastic structure, and on an orientation dependent friction, to control the deformations induced by the external flow, are analyzed. In all cases, the deformation sequence is a generalization of the tank-treading motion regimes observed in vesicles in shear flows. Analytic expressions for the migration velocity as a function of the deformation pattern and amplitude are provided. The effects of thermal fluctuations on propulsion have been discussed and the possibility that noise be exploited to overcome the limitations imposed on the microswimmer by the scallop theorem have been discussed.Comment: 14 pages, 5 figure

A circle swimmer at low Reynolds number

Swimming in circles occurs in a variety of situations at low Reynolds number. Here we propose a simple model for a swimmer that undergoes circular motion, generalising the model of a linear swimmer proposed by Najafi and Golestanian (Phys. Rev. E 69, 062901 (2004)). Our model consists of three solid spheres arranged in a triangular configuration, joined by two links of time-dependent length. For small strokes, we discuss the motion of the swimmer as a function of the separation angle between its links. We find that swimmers describe either clockwise or anticlockwise circular motion depending on the tilting angle in a non-trivial manner. The symmetry of the swimmer leads to a quadrupolar decay of the far flow field. We discuss the potential extensions and experimental realisation of our model.Comment: 9 pages, 9 Figure

Controlling Marangoni flow directionality: patterning nano-materials using sessile and sliding volatile droplets

Controlling the droplet shape and the corresponding deposition patterns is pivotal in a wide range of processes and applications based on surface phenomena, such as self-assembly of different types of nanomaterials and fabrication of functional electronic devices. In this paper we study different flow regimes and deposition patterns from volatile sessile droplets and droplets sliding over inclined solid substrates. The directionality and intensity of the Marangoni flow was controlled by vapor composition in a sealed chamber enclosing the evaporating droplets. Two types of volatile droplets are investigated: single component droplets and binary solution droplets. Binary solution droplets can exhibit either inward or outward Marangoni soluto-capillary flow, depending on a surface tension dependence on the concentration of the fast evaporating component. We carried out a detailed experimental study of the micro-rivulet (μ-R) regime in different binary solutions. The μ-R formation in a certain range of Ca proved to be a universal phenomenon subject to the occurrence of inward Marangoni flow. We propose a simplified mathematical model for the shape of μ-R based on the lubrication approximation. The resulting μ-R profile shows a good agreement with the experimental results

Optimal strokes for axisymmetric microswimmers

We present a theory for low-Reynolds-number axisymmetric swimmers and a general strategy for the computation of strokes of maximal efficiency. An explicit equation characterizing optimal strokes is derived, and numerical strategies to obtain solutions are discussed. The merits of this approach are demonstrated by applying it to two concrete examples: the three linked spheres of Najafi and Golestanian and the pushmepullyou of Avron, Kenneth, and Oakmin