2,601 research outputs found
Stochastic Hydrodynamic Synchronization in Rotating Energy Landscapes
Hydrodynamic synchronization provides a general mechanism for the spontaneous
emergence of coherent beating states in independently driven mesoscopic
oscillators. A complete physical picture of those phenomena is of definite
importance to the understanding of biological cooperative motions of cilia and
flagella. Moreover, it can potentially suggest novel routes to exploit
synchronization in technological applications of soft matter. We demonstrate
that driving colloidal particles in rotating energy landscapes results in a
strong tendency towards synchronization, favouring states where all beads
rotate in phase. The resulting dynamics can be described in terms of activated
jumps with transition rates that are strongly affected by hydrodynamics leading
to an increased probability and lifetime of the synchronous states. Using
holographic optical tweezers we quantitatively verify our predictions in a
variety of spatial configurations of rotors.Comment: Copyright (2013) by the American Physical Societ
Motility fractionation of bacteria by centrifugation
Centrifugation is a widespread laboratory technique used to separate mixtures
into fractions characterized by a specific size, weight or density. We
demonstrate that centrifugation can be also used to separate swimming cells
having different motility. To do this we study self-propelled bacteria under
the influence of an external centrifugal field. Using dynamic image correlation
spectroscopy we measure the spatially resolved motility of bacteria after
centrifugation. A significant gradient in swimming-speeds is observed for
increasing centrifugal speeds. Our results can be reproduced by a model that
treats bacteria as "hot" colloidal particles having a diffusion coefficient
that depends on the swimming speed.Comment: 7 pages, 5 figures (in press
Multidimensional Stationary Probability Distribution for Interacting Active Particles
We derive the stationary probability distribution for a non-equilibrium
system composed by an arbitrary number of degrees of freedom that are subject
to Gaussian colored noise and a conservative potential. This is based on a
multidimensional version of the Unified Colored Noise Approximation. By
comparing theory with numerical simulations we demonstrate that the theoretical
probability density quantitatively describes the accumulation of active
particles around repulsive obstacles. In particular, for two particles with
repulsive interactions, the probability of close contact decreases when one of
the two particle is pinned. Moreover, in the case of isotropic confining
potentials, the radial density profile shows a non trivial scaling with radius.
Finally we show that the theory well approximates the "pressure" generated by
the active particles allowing to derive an equation of state for a system of
non-interacting colored noise-driven particles.Comment: 5 pages, 2 figure
SAFE: Self-Attentive Function Embeddings for Binary Similarity
The binary similarity problem consists in determining if two functions are
similar by only considering their compiled form. Advanced techniques for binary
similarity recently gained momentum as they can be applied in several fields,
such as copyright disputes, malware analysis, vulnerability detection, etc.,
and thus have an immediate practical impact. Current solutions compare
functions by first transforming their binary code in multi-dimensional vector
representations (embeddings), and then comparing vectors through simple and
efficient geometric operations. However, embeddings are usually derived from
binary code using manual feature extraction, that may fail in considering
important function characteristics, or may consider features that are not
important for the binary similarity problem. In this paper we propose SAFE, a
novel architecture for the embedding of functions based on a self-attentive
neural network. SAFE works directly on disassembled binary functions, does not
require manual feature extraction, is computationally more efficient than
existing solutions (i.e., it does not incur in the computational overhead of
building or manipulating control flow graphs), and is more general as it works
on stripped binaries and on multiple architectures. We report the results from
a quantitative and qualitative analysis that show how SAFE provides a
noticeable performance improvement with respect to previous solutions.
Furthermore, we show how clusters of our embedding vectors are closely related
to the semantic of the implemented algorithms, paving the way for further
interesting applications (e.g. semantic-based binary function search).Comment: Published in International Conference on Detection of Intrusions and
Malware, and Vulnerability Assessment (DIMVA) 201
Generalized energy equipartition in harmonic oscillators driven by active baths
We study experimentally and numerically the dynamics of colloidal beads
confined by a harmonic potential in a bath of swimming E. coli bacteria. The
resulting dynamics is well approximated by a Langevin equation for an
overdamped oscillator driven by the combination of a white thermal noise and an
exponentially correlated active noise. This scenario leads to a simple
generalization of the equipartition theorem resulting in the coexistence of two
different effective temperatures that govern dynamics along the flat and the
curved directions in the potential landscape.Comment: 4 pages, 3 figure
Invariance properties of bacterial random walks in complex structures
Motile cells often explore natural environments characterized by a high
degree of structural complexity. Moreover cell motility is also intrinsically
noisy due to spontaneous random reorientation and speed fluctuations. This
interplay of internal and external noise sources gives rise to a complex
dynamical behavior that can be strongly sensitive to details and hard to model
quantitatively. In striking contrast to this general picture we show that the
mean residence time of swimming bacteria inside artificial complex
microstructures, can be quantitatively predicted by a generalization of a
recently discovered invariance property of random walks. We find that
variations in geometry and structural disorder have a dramatic effect on the
distributions of path length while mean values are strictly constrained by the
sole free volume to surface ratio. Biological implications include the
possibility of predicting and controlling the colonization of complex natural
environments using only geometric informations
Partial synchronisation of stochastic oscillators through hydrodynamic coupling
Holographic optical tweezers are used to construct a static bistable optical
potential energy landscape where a Brownian particle experiences restoring
forces from two nearby optical traps and undergoes thermally activated
transitions between the two energy minima. Hydrodynamic coupling between two
such systems results in their partial synchronisation. This is interpreted as
an emergence of higher mobility pathways, along which it is easier to overcome
barriers to structural rearrangement.Comment: 4 pages, 4 figures, submitted to Phys. Rev. Let
The effect of norm-based messages on reading and understanding COVID-19 pandemic response governmental rules
The new coronavirus disease (COVID-19) threatens the lives of millions of people around the world, making it the largest health threat in recent times. Billions of people around the world are asked to adhere to strict shelter-in-place rules, finalised to slow down the spread of the virus. Appeals and messages are being used by leaders and policy-makers to promote pandemic response. Given the stakes at play, it is thus important for social scientists to explore which messages are most effective in promoting pandemic response. In fact, some papers in the last month have explored the effect of several messages on people’s intentions to engage in pandemic response behaviour. In this paper, we make two contributions. First, we explore the effect of messages on people’s actual engagement, and not on intentions. Specifically, our dependent variables are the level of understanding of official COVID-19 pandemic response governmental informative panels, measured through comprehension questions, and the time spent on reading these rules. Second, we test a novel set of appeals built through the theory of norms. One message targets the personal norm (what people think is the right thing to do), one targets the descriptive norm (what people think others are doing), and one targets the injunctive norm (what people think others approve or disapprove of). Our experiment is conducted online with a representative (with respect to gender, age, and location) sample of Italians. Norms are made salient using a flier. We find that norm-based fliers had no effect on comprehension and on time spent on the panels. These results suggest that norm-based interventions through fliers have very little impact on people’s reading and understanding of COVID-19 pandemic response governmental rules
- …