274 research outputs found
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
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