4,165 research outputs found
Active colloids at fluid interfaces
If an active Janus particle is trapped at the interface between a liquid and
a fluid, its self-propelled motion along the interface is affected by a net
torque on the particle due to the viscosity contrast between the two adjacent
fluid phases. For a simple model of an active, spherical Janus colloid we
analyze the conditions under which translation occurs along the interface and
we provide estimates of the corresponding persistence length. We show that
under certain conditions the persistence length of such a particle is
significantly larger than the corresponding one in the bulk liquid, which is in
line with the trends observed in recent experimental studies
Diffusive spreading and mixing of fluid monolayers
The use of ultra-thin, i.e., monolayer films plays an important role for the
emerging field of nano-fluidics. Since the dynamics of such films is governed
by the interplay between substrate-fluid and fluid-fluid interactions, the
transport of matter in nanoscale devices may be eventually efficiently
controlled by substrate engineering. For such films, the dynamics is expected
to be captured by two-dimensional lattice-gas models with interacting
particles. Using a lattice gas model and the non-linear diffusion equation
derived from the microscopic dynamics in the continuum limit, we study two
problems of relevance in the context of nano-fluidics. The first one is the
case in which along the spreading direction of a monolayer a mesoscopic-sized
obstacle is present, with a particular focus on the relaxation of the fluid
density profile upon encountering and passing the obstacle. The second one is
the mixing of two monolayers of different particle species which spread side by
side following the merger of two chemical lanes, here defined as domains of
high affinity for fluid adsorption surrounded by domains of low affinity for
fluid adsorption.Comment: 12 pages, 3 figure
Collective dynamics of chemically active particles trapped at a fluid interface
Chemically active colloids generate changes in the chemical composition of
their surrounding solution and thereby induce flows in the ambient fluid which
affect their dynamical evolution. Here we study the many-body dynamics of a
monolayer of active particles trapped at a fluid-fluid interface. To this end
we consider a mean-field model which incorporates the direct pair interaction
(including also the capillary interaction which is caused specifically by the
interfacial trapping) as well as the effect of hydrodynamic interactions
(including the Marangoni flow induced by the response of the interface to the
chemical activity). The values of the relevant physical parameters for typical
experimental realizations of such systems are estimated and various scenarios,
which are predicted by our approach for the dynamics of the monolayer, are
discussed. In particular, we show that the chemically-induced Marangoni flow
can prevent the clustering instability driven by the capillary attraction.Comment: 8 pages, 2 figure
Spectral properties of the largest asteroids associated with Taurid Complex
We obtained spectra of six of the largest asteroids (2201, 4183, 4486, 5143,
6063, and 269690) associated with Taurid complex. The observations were made
with the IRTF telescope equipped with the spectro-imager SpeX. Their taxonomic
classification is made using Bus-DeMeo taxonomy. The asteroid spectra are
compared with the meteorite spectra from the Relab database. Mineralogical
models were applied to determine their surface composition. All the spectral
analysis is made in the context of the already published physical data.
Five of the objects studied in this paper present spectral characteristics
similar to the S taxonomic complex. The spectra of ordinary chondrites
(spanning H, L, and LL subtypes) are the best matches for these asteroid
spectra. {\bf The asteroid} (269690) 1996 RG3 presents a flat featureless
spectrum which could be associated to a primitive C-type object. The increased
reflectance above 2.1 microns constrains its geometrical albedo to a value
around 0.03.
While there is an important dynamical grouping among the Taurid Complex
asteroids, the spectral data of the largest objects do not support a common
cometary origin. Furthermore, there are significant variations between the
spectra acquired until now.Comment: Accepted for publication in A&
On quantum non-signalling boxes
A classical non-signalling (or causal) box is an operation on classical
bipartite input with classical bipartite output such that no signal can be sent
from a party to the other through the use of the box. The quantum counterpart
of such boxes, i.e. completely positive trace-preserving maps on bipartite
states, though studied in literature, have been investigated less intensively
than classical boxes. We present here some results and remarks about such maps.
In particular, we analyze: the relations among properties as causality,
non-locality and entanglement; the connection between causal and entanglement
breaking maps; the characterization of causal maps in terms of the
classification of states with fixed reductions. We also provide new proofs of
the fact that every non-product unitary transformation is not causal, as well
as for the equivalence of the so-called semicausality and semilocalizability
properties.Comment: 18 pages, 7 figures, revtex
The smallest refrigerators can reach maximal efficiency
We investigate whether size imposes a fundamental constraint on the
efficiency of small thermal machines. We analyse in detail a model of a small
self-contained refrigerator consisting of three qubits. We show analytically
that this system can reach the Carnot efficiency, thus demonstrating that there
exists no complementarity between size and efficiency.Comment: 9 pages, 1 figure. v2: published versio
Compositional characterisation of the Themis family
Context. It has recently been proposed that the surface composition of icy
main-belt asteroids (B-,C-,Cb-,Cg-,P-,and D-types) may be consistent with that
of chondritic porous interplanetary dust particles (CPIDPs). Aims. In the light
of this new association, we re-examine the surface composition of a sample of
asteroids belonging to the Themis family in order to place new constraints on
the formation and evolution of its parent body. Methods. We acquired NIR
spectral data for 15 members of the Themis family and complemented this dataset
with existing spectra in the visible and mid-infrared ranges to perform a
thorough analysis of the composition of the family. Assuming end-member
minerals and particle sizes (<2\mum) similar to those found in CPIDPs, we used
a radiative transfer code adapted for light scattering by small particles to
model the spectral properties of these asteroids. Results. Our best-matching
models indicate that most objects in our sample possess a surface composition
that is consistent with the composition of CP IDPs.We find ultra-fine grained
Fe-bearing olivine glasses to be among the dominant constituents. We further
detect the presence of minor fractions of Mg-rich crystalline silicates. The
few unsuccessfully matched asteroids may indicate the presence of interlopers
in the family or objects sampling a distinct compositional layer of the parent
body. Conclusions. The composition inferred for the Themis family members
suggests that the parent body accreted from a mixture of ice and anhydrous
silicates (mainly amorphous) and subsequently underwent limited heating. By
comparison with existing thermal models that assume a 400km diameter
progenitor, the accretion process of the Themis parent body must have occurred
relatively late (>4Myr after CAIs) so that only moderate internal heating
occurred in its interior, preventing aqueous alteration of the outer shell.Comment: 9 pages, 5 figures, accepted for publication in A&
Scalar and tensorial topological matter coupled to (2+1)-dimensional gravity:A.Classical theory and global charges
We consider the coupling of scalar topological matter to (2+1)-dimensional
gravity. The matter fields consist of a 0-form scalar field and a 2-form tensor
field. We carry out a canonical analysis of the classical theory, investigating
its sectors and solutions. We show that the model admits both BTZ-like
black-hole solutions and homogeneous/inhomogeneous FRW cosmological
solutions.We also investigate the global charges associated with the model and
show that the algebra of charges is the extension of the Kac-Moody algebra for
the field-rigid gauge charges, and the Virasoro algebrafor the diffeomorphism
charges. Finally, we show that the model can be written as a generalized
Chern-Simons theory, opening the perspective for its formulation as a
generalized higher gauge theory.Comment: 40 page
Anisotropic Diffusion Limited Aggregation
Using stochastic conformal mappings we study the effects of anisotropic
perturbations on diffusion limited aggregation (DLA) in two dimensions. The
harmonic measure of the growth probability for DLA can be conformally mapped
onto a constant measure on a unit circle. Here we map preferred directions
for growth of angular width to a distribution on the unit circle which
is a periodic function with peaks in such that the width
of each peak scales as , where defines the
``strength'' of anisotropy along any of the chosen directions. The two
parameters map out a parameter space of perturbations that allows a
continuous transition from DLA (for or ) to needle-like fingers
as . We show that at fixed the effective fractal dimension of
the clusters obtained from mass-radius scaling decreases with
increasing from to a value bounded from below by
. Scaling arguments suggest a specific form for the dependence
of the fractal dimension on for large , form which compares
favorably with numerical results.Comment: 6 pages, 4 figures, submitted to Phys. Rev.
Progress in noncommutative function theory
In this expository paper we describe the study of certain non-self-adjoint
operator algebras, the Hardy algebras, and their representation theory. We view
these algebras as algebras of (operator valued) functions on their spaces of
representations. We will show that these spaces of representations can be
parameterized as unit balls of certain -correspondences and the
functions can be viewed as Schur class operator functions on these balls. We
will provide evidence to show that the elements in these (non commutative)
Hardy algebras behave very much like bounded analytic functions and the study
of these algebras should be viewed as noncommutative function theory
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