Bulletin of the American Physical Society
APS March Meeting 2021
Volume 66, Number 1
Monday–Friday, March 15–19, 2021; Virtual; Time Zone: Central Daylight Time, USA
Session P24: Active Colloids IFocus Live
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Sponsoring Units: DFD DSOFT DBIO Chair: Becca Thomases, University of California, Davis |
Wednesday, March 17, 2021 3:00PM - 3:36PM Live |
P24.00001: Swimming in Elastic Liquids: Faster or Slower than their Newtonian Counterparts? Invited Speaker: Eric S Shaqfeh Many of the biological fluids in which microorganisms swim have a complex rheological behavior since they contain large biomolecules that create a rich underlying microstructure. Heretofore, there has been a great deal of work demonstrating that micro-swimmers will move either slower or faster in an elastic fluid depending on the particular swimming gait, fluid rheology, and material properties of the swimming body. In this talk, I will review the field including how large scale numerical simulation can predict mechanisms of speed enhancement or retardation. In particular we will demonstrate mechanisms of speed retardation in elastic swimming for undulatory and amoeboid swimmers such as C. Elegans. We will then demonstrate that swimming with swirl, such as that which characterizes E. Coli, can lead to speed enhancement and this is found even in a model as simple as the swirling squirmer model. However, interestingly the mechanism of speed enhancement depends on the rheological model. In the latter case, we show that propulsive forces associated with elastic normal forces or, alternatively, fluid pressure can either be the driving mechanism for speed enhancement depending on the assumed elastic response of the surrounding fluid. The results above are all in the context of steady swimming. Of course, micro-swimmers constantly change their direction of motion; for example, flagellated bacteria like E. coli perform a characteristic “run-and-tumble” motion. Again using numerical simulations (with comparison to experiments--Patteson et al., 2015) in viscoelastic fluids, these tumbling events lead to momentary overshoots in the swim speed above its steady-state value. We will also examine how fluid elasticity leads to an increase in the translational diffusivity of these micro-swimmers. |
Wednesday, March 17, 2021 3:36PM - 4:12PM Live |
P24.00002: Microorganism locomotion in viscoelastic fluids Invited Speaker: Becca Thomases Many microorganisms and cells function in complex (non-Newtonian) fluids, which are mixtures of different materials and exhibit both viscous and elastic stresses. For example, mammalian sperm swim through cervical mucus on their journey through the female reproductive tract, and they must penetrate the viscoelastic gel outside the ovum to fertilize. In micro-scale swimming the dynamics emerge from the coupled interactions between the complex rheology of the surrounding media and the passive and active body dynamics of the swimmer. We use computational models of swimmers in viscoelastic fluids to investigate and provide mechanistic explanations for emergent swimming behaviors. I will discuss how flexible filaments (such as flagella) can store energy from a viscoelastic fluid to gain stroke boosts due to fluid elasticity. I will also describe 3D simulations of model organisms such as C. Reinhardtii and mammalian sperm, where we use experimentally measured stroke data to separate naturally coupled stroke and fluid effects. We explore why strokes that are adapted to Newtonian fluid environments might not do well in viscoelastic environments. |
Wednesday, March 17, 2021 4:12PM - 4:24PM Live |
P24.00003: Pairwise and collective behavior of dumbbell swimmers at intermediate Reynolds numbers Thomas Dombrowski, Hong Nguyen, Daphne Klotsa We computationally investigate the pairwise and collective behavior of simple, reciprocal dumbbell swimmers composed of two unequally sized spheres at intermediate Reynolds numbers (0.1 < Re < 100). For nonzero Re, a single dumbbell swims small-sphere-leading and interestingly, as Re increases, switches its swimming direction to large-sphere leading. A pair of identical dumbbells experience a rich phase space of interactions dependent on their initial configuration, Re, and individual swimming parameters. Several stable configurations are found including in-line, in-tandem V-shape, and orbiting pairs. Averaged flow fields are analyzed to further understand these stable configurations. Pair interactions are then used to develop a framework for the collective behavior of dumbbell swimmers. |
Wednesday, March 17, 2021 4:24PM - 4:36PM Live |
P24.00004: Sedimenting chiral swimmers Federico Fadda, John Jairo Molina, Ryoichi Yamamoto The squirmer model is a theoretical model introduced to study microorganisms like algae and bacteria [1]. |
Wednesday, March 17, 2021 4:36PM - 4:48PM Live |
P24.00005: Chiral propulsion: the method of effective boundary conditions Leonid Korneev, Dmitri E Kharzeev, Alexandre G Abanov We revisit a problem of computing the chiral propulsion for bodies with helical symmetry in a fluid at low Reynolds numbers. Using the method of effective partial slip boundary conditions (PSBC) we compute the propulsion coefficient in the limit of small pitch of the helix. We check the method by comparing the results with the Lighthill solution for an infinite flagellum. The universality of the chiral propulsion in the limit of small pitch is discussed. |
Wednesday, March 17, 2021 4:48PM - 5:00PM Live |
P24.00006: Effect of inertia on the collective dynamics of an active suspension of mesoscale model swimmers Hong Nguyen, Thomas Dombrowski, Daphne Klotsa We computationally examine how the collective behaviors emerge in a collection of simple model swimmers immersed in a viscous incompressible fluid as the impact of inertia, characterized by the Reynolds number (Re), gradually increases. While most studies on the collective dynamics of swimming organisms have focused either on the Stokes regime at low Re or the Eulerian regime at high Re wherein viscous and inertial forces respectively dominate, less is known about such behaviors in the intermediate Re range where the two forces play a role. We show that our model can exhibit a wide range of nontrivial swimming patterns dictated by the degree of introduced inertia. At low Re, we observe a stable network-like arrangement in which swimmers tend to follow the swimming axis of their neighbors. As Re increases, a side-by-side swimming arrangement sets in, as swimmers favor alignment with their partner, leading to the formation of small stable moving clusters. The clusters become unstable at higher Re as the frequency of collision between the clusters increases, resulting in a rapid change of swimming partners. We relate these changes in swimming patterns to hydrodynamic signatures found in the swimmer pairwise interactions, as well as the resulting averaged flow field. |
Wednesday, March 17, 2021 5:00PM - 5:12PM Live |
P24.00007: Enhanced hydrodynamic performance of biomimetic tapered propulsor Ersan Demirer, Oluwafikayo Abisola Oshinowo, Alexander Alexeev Through millions of years of evolution, fish have achieved unmatched swimming velocities and efficiencies. Different species of fish use different parts and sections of their flexible bodies to pass down waves from their head to their fins that propel the fish forward. On one hand of the spectrum anguilliform fish (eel-like) use travelling waves generated through their entire body while thunniform fish generate standing waves using their caudal fins for propulsion. Different swimming types can be characterized by examining the nature of the wave propagation in terms of the standing wave ratio (SWR). Standing wave type of propulsion generally yields higher swimming velocities at the cost of efficiency. Although travelling wave based propulsion is generally more efficient, it is not trivial to generate travelling waves within a relatively short elastic structures. Using three-dimensional fully coupled fluid-structure interaction simulations, we showed that fin tapering is an effective approach to generate travelling waves in underwater propulsors. Through different tapering shapes, we demonstrate that SWR is a robust metric characterizing to the hydrodynamic efficiency of tapered plates. |
Wednesday, March 17, 2021 5:12PM - 5:24PM Live |
P24.00008: Command of active droplets in a nematic liquid crystal by an electric field Mojtaba Rajabi, O Lavrentovich A water droplet placed in a thermotropic nematic liquid crystal creates a topological point-defect in its vicinity, a so-called hyperbolic hedgehog, when the director is anchored normally to the interface. When such a droplet contains active bacteria, the dipolar configuration of the director field enables its unidirectional propulsion by rectifying the flows caused by the bacteria. Here we report real-time control of both the propulsion direction and the speed of active droplets in a nematic by an ac electric field. The dielectric torque on the director leads to two effects: (i) realignment of the overall director outside the droplet and (ii) transformation of the point-defect into a disclination ring. The first effect redirects the trajectory of the active droplet when the direction of the electric field changes. The second effect decreases the speed since the disclination ring reduces the asymmetry of director field around the droplet. When the disclination expands to an equatorial ring with a quadrupolar symmetry, the droplet stops. The real-time control of the direction and speed of active colloids is a step forward as compared to the current approaches in which the trajectories are predesigned, and no control over speed is present. |
Wednesday, March 17, 2021 5:24PM - 5:36PM Live |
P24.00009: Reconfigurable Artificial Microswimmers with Internal Feedback Laura Alvarez Frances, Migue Angel Fernandez-Rodriguez, Angel Alegria, Silvia Arrese-Igor, Kai Zhao, Martin Kröger, Lucio Isa Micron-size self-propelling particles are often proposed as synthetic models for biological microswimmers [1], yet they lack internal adaptation, which is central to the autonomy of their biological counterparts. Conversely, adaptation and autonomy can be encoded in larger-scale robotic devices [3], but transferring these capabilities to the colloidal scale remains elusive. Here, we create a new class of responsive microswimmers, powered by induced-charge electrophoresis, which can adapt their motility to external stimuli via internal feedback. We fabricate deterministic colloidal clusters comprising soft thermoresponsive microparticles, which upon spontaneous reconfiguration, induce motility changes, as the adaptation of the clusters’ propulsion velocity and reversal of its direction [5]. We rationalize the response in terms of coupling between self-propulsion and variations of particle shape and dielectric properties. Harnessing those allows for strategies to achieve local dynamical control with simple illumination patterns, with exciting opportunities for the development of new tactic active materials. |
Wednesday, March 17, 2021 5:36PM - 5:48PM Live |
P24.00010: Viscoelastic burrowing locomotion in nematodes reveal transition to complex postural dynamics Christopher Pierce, Gongchen Sun, Hang Lu, Daniel I Goldman Despite being one of the most well studied model organisms in biology, little is known about the locomotion of C. elegans in natural settings. In the wild, these animals contend with heterogenous environments like rotten fruit, soil and the backs of insects; however, the majority of behavioral studies of C. elegans consider swimming in Newtonian fluids or crawling on the surface of agarose. In these environments, C. elegans moves via regular traveling waves of body curvature, with wavelengths and frequencies that decrease with increasing fluid viscosity. We observed C. elegans burrowing within non-Newtonian gels with tunable viscoelasticity - models of conditions in rotting fruit. In these viscoelastic environments, previously identified wavelength/frequency trends persist, however the motion becomes complex and irregular. Principal component analysis reveals that similar quasi-sinusoidal component waveforms account for the majority of postural variation in each regime (swimming, crawling and viscoelastic burrowing). However, where swimming and crawling produce smooth, regular orbits in the amplitude space of the components, burrowing reveals jagged, complex trajectories, accounting for highly irregular body postures in both turning and forward locomotion. |
Wednesday, March 17, 2021 5:48PM - 6:00PM Live |
P24.00011: Quantifying the Spatiotemporal Dynamics of Locomotory Waves Tosif Ahamed, Sihui Asuka Guan, Wesley Hung, Mei Zhen Locomotor waves in animals show intricate spatiotemporal dynamics. For example, in the roundworm C. elegans, body waves can travel in opposite directions and at different speeds. Current quantification and modeling methods cannot extract information about such non-uniform wave propagation. Here, we develop methods to quantify locomotor wave dynamics at a high spatiotemporal resolution. We compute the frequency, wavenumber, and wave-velocity as a function of space and time. Additionally, we can decompose the body waves into a superposition of interpretable wave patterns. Applying these analyses to video data of C. elegans worms, we reveal that wave properties vary non-monotonically along the body, with head and tail showing higher frequency oscillations compared to the body. Analysis of transgenic worms suggests that motor neurons might be involved in maintaining the phase relationships along the body, while premotor neurons govern the frequency and amplitude of body bends. This work lays the foundation to examine and model the oscillatory neural network activities that organize C. elegans locomotion. It is also generally applicable to the locomotion of other slender-bodied animals. |
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