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1. Dynamics of rigid achiral magnetic microswimmers in shear-thinning fluids

   https://doi.org/10.1063/5.0167307  

   Quashie, David; Wang, Qi; Jermyn, Sophie; Katuri, Jaideep; Ali, Jamel (September 2023, Physics of Fluids)

   Here, we use magnetically driven self-assembled achiral swimmers made of two to four superparamagnetic micro-particles to provide insight into how swimming kinematics develop in complex, shear-thinning fluids. Two model shear-thinning polymer fluids are explored, where measurements of swimming dynamics reveal contrasting propulsion kinematics in shear-thinning fluids vs a Newtonian fluid. When comparing the velocity of achiral swimmers in polymer fluids to their dynamics in water, we observe kinematics dependent on (1) no shear-thinning, (2) shear-thinning with negligible elasticity, and (3) shear-thinning with elasticity. At the step-out frequency, the fluidic environment's viscoelastic properties allow swimmers to propel faster than their Newtonian swimming speed, although their swimming gait remains similar. Micro-particle image velocimetry is also implemented to provide insight into how shear-thinning viscosity fluids with elasticity can modify the flow fields of the self-assembled magnetic swimmers. Our findings reveal that flow asymmetry can be created for symmetric swimmers through either the confinement effect or the Weissenberg effect. For pseudo-chiral swimmers in shear-thinning fluids, only three bead swimmers show swimming enhancement, while four bead swimmers always have a decreased step-out frequency velocity compared to their dynamics in water. 

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2. Reinforcement learning of a multi-link swimmer at low Reynolds numbers

   https://doi.org/10.1063/5.0140662  

   Qin, Ke; Zou, Zonghao; Zhu, Lailai; Pak, On Shun (March 2023, Physics of Fluids)

   The use of machine learning techniques in the development of microscopic swimmers has drawn considerable attention in recent years. In particular, reinforcement learning has been shown useful in enabling swimmers to learn effective propulsion strategies through its interactions with the surroundings. In this work, we apply a reinforcement learning approach to identify swimming gaits of a multi-link model swimmer. The swimmer consists of multiple rigid links connected serially with hinges, which can rotate freely to change the relative angles between neighboring links. Purcell [“Life at low Reynolds number,” Am. J. Phys. 45, 3 (1977)] demonstrated how the particular case of a three-link swimmer (now known as Purcell's swimmer) can perform a prescribed sequence of hinge rotation to generate self-propulsion in the absence of inertia. Here, without relying on any prior knowledge of low-Reynolds-number locomotion, we first demonstrate the use of reinforcement learning in identifying the classical swimming gaits of Purcell's swimmer for case of three links. We next examine the new swimming gaits acquired by the learning process as the number of links increases. We also consider the scenarios when only a single hinge is allowed to rotate at a time and when simultaneous rotation of multiple hinges is allowed. We contrast the difference in the locomotory gaits learned by the swimmers in these scenarios and discuss their propulsion performance. Taken together, our results demonstrate how a simple reinforcement learning technique can be applied to identify both classical and new swimming gaits at low Reynolds numbers. 

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3. Reconfigurable paramagnetic microswimmers: Brownian motion affects non-reciprocal actuation

   https://doi.org/10.1039/c8sm00069g  

   Du, Di; Hilou, Elaa; Biswal, Sibani Lisa (January 2018, Soft Matter)

   Swimming at low Reynolds number is typically dominated by a large viscous drag, therefore microscale swimmers require non-reciprocal body deformation to generate locomotion. Purcell described a simple mechanical swimmer at the microscale consisting of three rigid components connected together with two hinges. Here we present a simple microswimmer consisting of two rigid paramagnetic particles with different sizes. When placed in an eccentric magnetic field, this simple microswimmer exhibits non-reciprocal body motion and its swimming locomotion can be directed in a controllable manner. Additional components can be added to create a multibody microswimmer, whereby the particles act cooperatively and translate in a given direction. For some multibody swimmers, the stochastic thermal forces fragment the arm, which therefore modifies the swimming strokes and changes the locomotive speed. This work offers insight into directing the motion of active systems with novel time-varying magnetic fields. It also reveals that Brownian motion not only affects the locomotion of reciprocal swimmers that are subject to the Scallop theorem, but also affects that of non-reciprocal swimmers. 

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4. The role of suction thrust in the metachronal paddles of swimming invertebrates

   https://doi.org/10.1038/s41598-020-74745-y  

   Colin, Sean P.; Costello, John H.; Sutherland, Kelly R.; Gemmell, Brad J.; Dabiri, John O.; Du Clos, Kevin T. (October 2020, Scientific Reports)

   Abstract An abundance of swimming animals have converged upon a common swimming strategy using multiple propulsors coordinated as metachronal waves. The shared kinematics suggest that even morphologically and systematically diverse animals use similar fluid dynamic relationships to generate swimming thrust. We quantified the kinematics and hydrodynamics of a diverse group of small swimming animals who use multiple propulsors, e.g. limbs or ctenes, which move with antiplectic metachronal waves to generate thrust. Here we show that even at these relatively small scales the bending movements of limbs and ctenes conform to the patterns observed for much larger swimming animals. We show that, like other swimming animals, the propulsors of these metachronal swimmers rely on generating negative pressure along their surfaces to generate forward thrust (i.e., suction thrust). Relying on negative pressure, as opposed to high pushing pressure, facilitates metachronal waves and enables these swimmers to exploit readily produced hydrodynamic structures. Understanding the role of negative pressure fields in metachronal swimmers may provide clues about the hydrodynamic traits shared by swimming and flying animals. 

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5. Mantis Shrimp Locomotion: Coordination and Variation of Hybrid Metachronal Swimming

   https://doi.org/10.1093/iob/obad019  

   Hanson, S. E.; Ray, W. J.; Santhanakrishnan, A.; Patek, S. N. (June 2023, Integrative Organismal Biology)

   Synopsis Across countless marine invertebrates, coordination of closely spaced swimming appendages is key to producing diverse locomotory behaviors. Using a widespread mechanism termed hybrid metachronal propulsion, mantis shrimp swim by moving five paddle-like pleopods along their abdomen in a posterior to anterior sequence during the power stroke and a near-synchronous motion during the recovery stroke. Despite the ubiquity of this mechanism, it is not clear how hybrid metachronal swimmers coordinate and modify individual appendage movements to achieve a range of swimming capabilities. Using high-speed imaging, we measured pleopod kinematics of mantis shrimp (Neogonodactylus bredini), while they performed two swimming behaviors: burst swimming and taking off from the substrate. By tracking each of the five pleopods, we tested how stroke kinematics vary across swimming speeds and the two swimming behaviors. We found that mantis shrimp achieve faster swimming speeds through a combination of higher beat frequencies, smaller stroke durations, and partially via larger stroke angles. The five pleopods exhibit non-uniform kinematics that contribute to the coordination and forward propulsion of the whole system. Micro-hook structures (retinacula) connect each of the five pleopod pairs and differ in their attachment across pleopods—possibly contributing to passive kinematic control. We compare our findings in N. bredini to previous studies to identify commonalities across hybrid metachronal swimmers at high Reynolds numbers and centimeter scales. Through our large experimental dataset and by tracking each pleopod's movements, our study reveals key parameters by which mantis shrimp adjust and control their swimming, yielding diverse locomotor abilities. 

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