Fluid mixing by an electromagnetically driven floating rotor.

We analyze the mixing properties of a floating stirrer driven electromagnetically in a thin layer of electrolyte, consisting of two free-floating magnets with opposite polarities connected by a rigid coupling. The magnetic rotor is set in circular motion using Lorentz forces created due to the interaction of the magnetic field of the rotor with dc currents actuated in logic sequence. We identify a coherent structure similar to a tripolar vortex whose central vortex rotates in the same direction of the rotor promoting chaotic mixing of the fluid in the laminar regime (Re=45). Dyed water visualization and particle image velocimetry were performed to characterize experimentally the mixing and flow dynamics at the surface of the electrolyte layer. A quasitwo-dimensional numerical simulation based on the immersed boundary method, which incorporates the fluid-solid interaction and reproduces the experimental observations satisfactorily, was carried out. Optimal mixing conditions are determined through the exponential growth of the material interfaces, which are established mainly by varying the distance separating the magnets of the rotor.

Substrate-mediated leg interactions play a key role in insect stability on granular slopes.

Locomotion on granular inclines is a subject of high relevance in ecological physics as well as in biomimmetics and robotics. Enhancing stability on granular materials represents a huge challenge due to the fluidization transition when inclination approaches the avalanche angle. Our motivating example is the predator-prey system made of the antlion, its pit, and its prey. Recent studies have demonstrated that stability on granular inclines strongly depends on the pressure exerted on the substrate. In this work we show that for multilegged locomotion, along with pressure, the distance between the leg contacts on the substrate also plays a major role in the determination of the stability threshold. Through a set of model experiments using artificial sliders, we determine a critical distance below which stability is importantly affected by the interactions between the perturbed regions generated by each contact point. A simple model based on the Coulomb method of wedges allows us to estimate a stability criterion based on pressure, interleg distance, and substrate characteristics. Our work suggests that mass to leg-length allometric relationships, as the ones observed in ants, may be an important key in determining the locomotion success of multilegged locomotion on granular inclines.

Assessing learning in mosquito larvae using video-tracking.

Mosquito larvae display a stereotyped escape response when they rest attached to the water surface. It consists in detaching from the surface and diving, to return to the surface after a brief time. It has been shown that this response can be evoked several times, by repeatedly presenting a moving shadow. Diving triggered by a potential danger revealed as a simple bioassay for investigating behavioural responses in mosquito larvae, in particular their ability to learn. In the present work, we describe an automated system, based on video-tracking individuals, and extracting quantitative data of their movements. We validated our system, by reinvestigating the habituation response of larvae of Aedes aegypti reared in the laboratory, and providing original data on field-collected larvae of genera Culex and Anopheles. Habituation could be demonstrated to occur in all the species, even though it was not possible to induce dishabituation in Culex and Anopheles mosquitoes. In addition to non-associative learning, we characterised motor activity in the studied species, thanks to the possibility offered by the tracking system to extract multiple variables. The here-described system and algorithms can be easily adapted to multiple experimental situations and variables of interest.

Vibratory behaviour produces different vibrations patterns in presence of reproductives in a subterranean termite species.

Vibratory behaviours are widespread in social insects, but the produced vibrations remain poorly explored. Communication using vibrations is an efficient way to transmit information in subterranean environments where visual and odorant signals are less efficient. In termites, different vibratory behaviours are performed in different contexts like reproductive regulation and alarm signalling, but only few studies explored the structure of the produced vibrations (i.e., duration, number of pulses, amplitude). Here, we described several types of vibrations produced by a vibratory behaviour widespread in termites (body-shaking), which can be transmitted through the substrate and detected by other colony members. We analysed the structures of the emitted vibrations and the occurrence of the body-shaking events in presence/absence of reproductives and/or in presence/absence of a stress stimuli (flashlight) in the subterranean termite Reticulitermes flavipes. Interestingly, only the presence of the reproductives did influence the number of pulses and the duration of the emitted vibrations. Moreover, the first part of the emitted vibrations seems to be enough to encode reproductive information, but other parts might hold other type of information. Body-shaking occurrence did increase in presence of reproductives but only briefly under a flashlight. These results show that vibratory cues are complex in termites and their diversity might encode a plurality of social cues.

Wrinkling hierarchy in constrained thin sheets from suspended graphene to curtains.

We show that thin sheets under boundary confinement spontaneously generate a universal self-similar hierarchy of wrinkles. From simple geometry arguments and energy scalings, we develop a formalism based on wrinklons, the localized transition zone in the merging of two wrinkles, as building blocks of the global pattern. Contrary to the case of crumpled paper where elastic energy is focused, this transition is described as smooth in agreement with a recent numerical work [R. D. Schroll, E. Katifori, and B. Davidovitch, Phys. Rev. Lett. 106, 074301 (2011)]. This formalism is validated from hundreds of nanometers for graphene sheets to meters for ordinary curtains, which shows the universality of our description. We finally describe the effect of an external tension to the distribution of the wrinkles.