La Hécuba de Eurípides: la perra que ladraba a la libertad

Este artículo se centra fundamentalmente en el episodio final de la tragedia Hécuba de Eurípides. A partir de la transformación de la protagonista, tema original del tragediógrafo, se intenta explicar la causa por la cual Eurípides elige la forma canina para su caracterización. Para ello, se define la imagen ambigua del animal, teniendo en cuenta los aspectos positivos y negativos evidenciados por los textos literarios que conservamos. Además se plantea la posible influencia de la doctrina de la metempsicosis, a través del análisis gramatical del texto. Finalmente, para sustentar la idea de la metempsicosis de la protagonista, se realiza una comparación con la versión del poeta latino Ovidio.This article focuses on the final episode of Euripides’ Hecabe. Looking at the transformation of the heroine, which is to be considered an original subject in the tragedian, we aim to explain the reasons for Euripides’ choice for such a dog-like characterization at the end of the play. In order to do so, one must define the ambiguous portrait of that animal, considering its positive and negative characteristics in the preserved literary texts. Furthermore, and by means of the grammatical analysis of the text, we suggest the possible influence of the metempsychosis theory. Finally, a comparison with Ovid’s Roman version of the myth helps to sustain the theory of the protagonist’s metempsychosis

Run-and-Tumble-Like Motion of Active Colloids in Viscoelastic Media

Run-and-tumble (RNT) motion is a prominent locomotion strategy employed by many living microorganisms. It is characterized by straight swimming intervals (runs), which are interrupted by sudden reorientation events (tumbles). In contrast, directional changes of synthetic microswimmers (active particles, APs) are caused by rotational diffusion, which is superimposed with their translational motion and thus leads to rather continuous and slow particle reorientations. Here we demonstrate that active particles can also perform a swimming motion where translational and orientational changes are disentangled, similar to RNT. In our system, such motion is realized by a viscoelastic solvent and a periodic modulation of the self-propulsion velocity. Experimentally, this is achieved using light-activated Janus colloids, which are illuminated by a time-dependent laser field. We observe a strong enhancement of the effective translational and rotational motion when the modulation time is comparable to the relaxation time of the viscoelastic fluid. Our findings are explained by the relaxation of the elastic stress, which builds up during the self-propulsion, and is suddenly released when the activity is turned off. In addition to a better understanding of active motion in viscoelastic surroundings, our results may suggest novel steering strategies for synthetic microswimmers in complex environments.Comment: 6 figures, New Journal of Physics accepte

Phototaxis of synthetic microswimmers in optical landscapes

Many microorganisms, with phytoplankton and zooplankton as prominent examples, display phototactic behaviour, that is, the ability to perform directed motion within a light gradient. Here we experimentally demonstrate that sensing of light gradients can also be achieved in a system of synthetic photo-activated microparticles being exposed to an inhomogeneous laser field. We observe a strong orientational response of the particles because of diffusiophoretic torques, which in combination with an intensity-dependent particle motility eventually leads to phototaxis. Since the aligning torques saturate at high gradients, a strongly rectified particle motion is found even in periodic asymmetric intensity landscapes. Our results are in excellent agreement with numerical simulations of a minimal model and should similarly apply to other particle propulsion mechanisms. Because light fields can be easily adjusted in space and time, this also allows to extend our approach to dynamical environments.Comment: 10 pages, 7 figure

Colloidal Brazil nut effect in microswimmer mixtures induced by motility contrast

We numerically and experimentally study the segregation dynamics in a binary mixture of microswimmers which move on a two-dimensional substrate in a static periodic triangular-like light intensity field. The motility of the active particles is proportional to the imposed light intensity and they possess a motility contrast, i.e., the prefactor depends on the species. In addition, the active particles also experience a torque aligning their motion towards the direction of the negative intensity gradient. We find a segregation of active particles near the intensity minima where typically one species is localized close to the minimum and the other one is centered around in an outer shell. For a very strong aligning torque, there is an exact mapping onto an equilibrium system in an effective external potential that is minimal at the intensity minima. This external potential is similar to (height-dependent) gravity, such that one can define effective `heaviness' of the self-propelled particles. In analogy to shaken granular matter in gravity, we define a `colloidal Brazil nut effect' if the heavier particles are floating on top of the lighter ones. Using extensive Brownian dynamics simulations, we identify system parameters for the active colloidal Brazil nut effect to occur and explain it based on a generalized Archimedes' principle within the effective equilibrium model: heavy particles are levitated in a dense fluid of lighter particles if their effective mass density is lower than that of the surrounding fluid. We also perform real-space experiments on light-activated self-propelled colloidal mixtures which confirm the theoretical predictions.Comment: 10 pages, 5 figures, JCP Special Topic on Chemical Physics of Active Matte

Pabellón de Ventas y Gestión

La luz es color. El movimiento perceptual se fomenta, principalmente, con la forma, el ritmo y el color

Análisis del proceso de descarga de un silo con un obstáculo cerca del orificio

Jamming is an important problem in numerous industrial processes, and in other situation such as traffic and evacuation. Some reports show that an obstacle placed before the exit may prevent jamming a pedestrian flow. However, this is a general hypothesis and there are still related questions that have not been fully addressed, mainly the dynamics of the system or the optimal position of the obstacle. The present work aims at shedding some more light on these phenomena. We present an experimental work where we analyze systematically and under well controlled conditions, the macroscopic and microscopic processes involved during the discharge of a silo by gravity with an obstacle placed before an orifice. We fixed at the size of the orifice and change the position of the insert. In order to do that, we have designed a 2D silo with transparent walls which allowed visualization of the particles. The first conclusion of this work is the existence of an optimal position of the obstacle where the jamming probability is drastically reduced. If the obstacle is far away from the orifice, it does not have any effect. When the obstacle is close to the orifice, the avalanche size is higher and the probability that a particle clogs the outlet decreases. We find that, if the insert position is properly selected, the probability that the granular flow gets jammed can be decreased by a factor of 100. This dramatic effect occurs without any remarkable modification of the flow rate or the packing fraction above the outlet. However, for low positions of the insert we saw that some particles in the region of arch formation can be displaced upwards. This phenomenon is less evident when the insert is at high positions. This effect could be related with the reduction of the clogging probability. So, we propose that the mechanism by which the insert prevents clogging is a reduction of the pressure exerted to the particles in the region of arch formation

Breaking arches with vibrations: the role of defects

We present experimental results about the stability of arches against external vibrations. Two dimensional strings of mutually stabilizing grains are geometrically analyzed and subsequently submitted to a periodic forcing at fixed frequency and increasing amplitude. The main factor that determines the granular arch resistance against vibrations is the maximum angle among those formed between any particle of the arch and its two neighbors: the higher the maximum angle is, the easier to break the arch. Based in an analysis of the forces, a simple explanation is given for this dependence. From this, interesting information can be extracted about the expected magnitudes of normal forces and friction coefficients of the particles conforming the arches

Tuning the motility and directionality of self-propelled colloids

Microorganisms are able to overcome the thermal randomness of their surroundings by harvesting energy to navigate in viscous fluid environments. In a similar manner, synthetic colloidal microswimmers are capable of mimicking complex biolocomotion by means of simple self-propulsion mechanisms. Although experimentally the speed of active particles can be controlled by e.g. self-generated chemical and thermal gradients, an in-situ change of swimming direction remains a challenge. In this work, we study self-propulsion of half-coated spherical colloids in critical binary mixtures and show that the coupling of local body forces, induced by laser illumination, and the wetting properties of the colloid, can be used to finely tune both the colloid's swimming speed and its directionality. We experimentally and numerically demonstrate that the direction of motion can be reversibly switched by means of the size and shape of the droplet(s) nucleated around the colloid, depending on the particle radius and the fluid's ambient temperature. Moreover, the aforementioned features enable the possibility to realize both negative and positive phototaxis in light intensity gradients. Our results can be extended to other types of half-coated microswimmers, provided that both of their hemispheres are selectively made active but with distinct physical properties.Comment: 12 pages, 5 figures. Scientific Reports (Received: 04 August 2017, accepted: 04 October 2017, published online: 02 November 2017