3,949 research outputs found
Transient dynamics of a colloidal particle driven through a viscoelastic fluid
We experimentally study the transient motion of a colloidal particle actively
dragged by an optical trap through different viscoelastic fluids (wormlike
micelles, polymer solutions, and entangled -phage DNA). We observe
that, after sudden removal of the moving trap, the particle recoils due to the
recovery of the deformed fluid microstructure. We find that the transient
dynamics of the particle proceeds via a double exponential relaxation, whose
relaxation times remain independent of the initial particle velocity whereas
their amplitudes strongly depend on it. While the fastest relaxation mirrors
the viscous damping of the particle by the solvent, the slow relaxation results
from the recovery of the strained viscoelastic matrix. We show that this
transient information, which has no counterpart in Newtonian fluids, can be
exploited to investigate linear and nonlinear rheological properties of the
embedding fluid, thus providing a novel method to perform transient rheology at
the micron-scale.Comment: 19 pages, 7 Figures, submitted to New Journal of Physic
Probing linear and nonlinear microrheology of viscoelastic fluids
Bulk rheological properties of viscoelastic fluids have been extensively
studied in macroscopic shearing geometries. However, little is known when an
active microscopic probe is used to locally perturb them far from the
linear-response regime. Using a colloidal particle dragged periodically by
scanning optical tweezers through a viscoelastic fluid, we investigate both,
its linear and nonlinear microrheological response. With increasing particle
velocity, we observe a transition from constant viscosity to a thinning regime,
where the drag force on the probe becomes a nonlinear function of the particle
velocity. We demonstrate that this transition is only determined by the ratio
of the fluid's equilibrium relaxation time and the period of the driving.Comment: 6 pages, 5 figure
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
Dynamics of self-propelled Janus particles in viscoelastic fluids
We experimentally investigate active motion of spherical Janus colloidal
particles in a viscoelastic fluid. Self-propulsion is achieved by a local
concentration gradient of a critical polymer mixture which is imposed by laser
illumination. Even in the regime where the fluid's viscosity is independent
from the deformation rate induced by the particle, we find a remarkable
increase of up to two orders of magnitude of the rotational diffusion with
increasing particle velocity, which can be phenomenologically described by an
effective rotational diffusion coefficient dependent on the Weissenberg number.
We show that this effect gives rise to a highly anisotropic response of
microswimmers in viscoelastic media to external forces depending on its
orientation.Comment: 5 pages, 4 figures, Physical Review Letters (accepted
Fluctuations in an aging system: absence of effective temperature in the sol-gel transition of a quenched gelatin sample
We study the fluctuations of a Brownian micro particle trapped with optical
tweezers in a gelatin solution undergoing a fast local temperature quench below
the sol-gel transition. Contrary to what was previously reported, we observe no
anomalous fluctuations in the particle's position that could be interpreted in
terms of an effective temperature. A careful analysis with ensemble averages
shows only equilibrium-like properties for the fluctuations, even though the
system is clearly aging. We also provide a detailed discussion on possible
artifacts that could have been interpreted as an effective temperature, such as
the presence of a drift or a mixing in time and ensemble averages in data
analysis. These considerations are of general interest when dealing with
non-ergodic or non-stationary systems
Nuevos arreglos institucionales en el subsistema lácteo de la región norte de Perú : un estudio exploratorio
El Subsistema de Agronegocios (SSAG) lácteo de la región norte del Perú ha desarrollado un alentador crecimiento durante los últimos años, tanto por el impulso del mercado interno como de los nuevos nichos de mercado cada vez más exigentes. Sin embargo, las transacciones entre productores-intermediarios e intermediarios-industriales vienen incurriendo en fallas de coordinación, con problemas de asimetría de información, oportunismo y costos elevados de transacción. El objetivo del estudio es realizar un diagnóstico de la innovación organizacional generada en este sistema, a partir de los nuevos arreglos organizacionales generados entre la industria láctea y el productor primario. La presente investigación tipo cuali-cuantitativa tuvo como enfoque teórico a la Nueva Economía Institucional (NEI) , partiendo de fuentes de información primaria y secundaria. Bajo este análisis, se identificaron los principales puntos conflictivos y la ocurrencia de altos costos de transacción, que permitieron el surgimiento de un nuevo diseño organizacional productor primario-industria láctea con el fin de remediar estos acontecimientos turbulentos presentes en el SSAG. Este nuevo arreglo organizacional se caracteriza por una mayor frecuencia de transacción, elevada especificidad de los activos involucrados -mejora en la calidad-, y una disminución de la incertidumbre por el precio transado, principalmente debido a una estructura de gobernancia del tipo contratos informales ("guía de especificación del producto") , minimizando los costos de la transacción. La nueva transacción productor primario - industria láctea ha conllevado a crear ventajas: relación directa entre los actores, mejor manejo de la información, incentivos en la retribución/pago por el producto entregado, implementación de mejoras tecnológicas en los sistemas de producción primaria, etc., generando una mayor transparencia y rentabilidad hacia todos los niveles del SSA
Bolivia landlocked; a long search for solutions
La falta de salida al mar de Bolivia, supone un inmenso desafío y una desventaja notable para esta nación en diferentes sentidos. A más de un siglo de perder su soberanía al mar, en la Guerra del Pacífico, se siguen presentando polémicas entre las naciones latinoamericanas en torno a este tema, que ponen en riesgo la estabilidad y el equilibrio regional que se ha mantenido durante años. El actual momento de esta situación está marcado por la demanda de Bolivia ante la Corte Internacional de Justicia (CIJ) de la Haya, lo cual ha tensionado sus relaciones con Chile y nos lleva a plantear la hipótesis que en un corto plazo Bolivia no obtendrá salida al mar producto de un tratado bilateral que le conceda esta posibilidad, por razones que se explicarán más adelante.Lack of landlocked Bolivia is a huge challenge and a significant disadvantage to this nation in different ways. More than a century to lose this power in the Pacific War, there continue to be controversial among Latin American nations on this issue, which endanger the stability and regional balance that has been maintained for years at risk. The current time of this situation is marked by the demand for Bolivia before the International Court of Justice (ICJ) in The Haya, which has strained relations with Chile and leads us to hypothesize that in a short term Bolivia will not get out sea product of a bilateral treaty to provide this opportunity for reasons that will be explained later.Pregrad
Detecting Distributed SQL Injection Attacks in a Eucalyptus Cloud Environment
The cloud computing environment offers malicious users the ability to spawn multiple instances of cloud nodes that are similar to virtual machines, except that they can have separate external IP addresses. In this paper we demonstrate how this ability can be exploited by an attacker to distribute his/her attack, in particular SQL injection attacks, in such a way that an intrusion detection system (IDS) could fail to identify this attack. To demonstrate this, we set up a small private cloud, established a vulnerable website in one instance, and placed an IDS within the cloud to monitor the network traffic. We found that an attacker could quite easily defeat the IDS by periodically altering its IP address. To detect such an attacker, we propose to use multi-agent plan recognition, where the multiple source IPs are considered as different agents who are mounting a collaborative attack. We show that such a formulation of this problem yields a more sophisticated approach to detecting SQL injection attacks within a cloud computing environment
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