305 research outputs found
Slow Kinetics of Capillary Condensation in Confined Geometry: Experiment and Theory
When two solid surfaces are brought in contact, water vapor present in the
ambient air may condense in the region of the contact to form a liquid bridge
connecting the two surfaces : this is the so-called capillary condensation.
This phenomenon has drastic consequences on the contact between solids,
modifying the macroscopic adhesion and friction properties. In this paper, we
present a survey of the work we have performed both experimentally and
theoretically to understand the microscopic foundations of the kinetics of
capillary condensation. From the theoretical point of view, we have computed
the free energy barrier associated with the condensation of the liquid from the
gas in a confined system. These calculations allow to understand the existence
of very large hysteresis, which is often associated with capillary
condensation. This results are compatible with experimental results obtained
with a surface forces apparatus in a vapor atmosphere, showing a large hysteris
of the surface energy of two parallel planes as a function of their distance.
In the second part, we present some experiments on the influence of humidity on
the avalanche angle of granular media. We show that the ageing in time of this
avalanche angle can be explained by the slow kinetics of capillary condensation
in a random confined geometry.Comment: Special Volume of Colloids and Surfaces A,Proceedings of
Nanocapillarity: Wetting of Heterogeneous Surfaces and Porous Solids,June
25-27, 2001, TRI/Princeton International Workshop, Editor: Alexander V.
Neimar
Thermally Activated Dynamics of the Capillary Condensation
This paper is devoted to the thermally activated dynamics of the capillary
condensation. We present a simple model which enables us to identify the
critical nucleus involved in the transition mechanism. This simple model is
then applied to calculate the nucleation barrier from which we can obtain
informations on the nucleation time. We present a simple estimation of the
nucleation barrier in slab geometry both in the two dimensional case and in the
three dimensional case. We extend the model in the case of rough surfaces which
is closer to the experimental case and allows comparison with experimental
datas.Comment: 6 pages, 3 figures, Submitted to J. Phys. : Condens. Matter,
Proceedings of the IV Liquid Matter Conference - Grenada(Spain) july 199
Mixing and condensation in a wet granular medium
We have studied the effect of small amounts of added liquid on the dynamic
behavior of a granular system consisting of a mixture of glass beads of two
different sizes. Segregation of the large beads to the top of the sample is
found to depend in a nontrivial way on the liquid content. A transition to
viscoplastic behavior occurs at a critical liquid content, which depends upon
the bead size. We show that this transition can be interpreted as a
condensation due to the hysteretic liquid bridge forces connecting the beads,
and provide the corresponding phase diagram.Comment: submitted to PR
Modelling microgels with controlled structure across the volume phase transition
Thermoresponsive microgels are soft colloids that find widespread use as
model systems for soft matter physics. Their complex internal architecture,
made of a disordered and heterogeneous polymer network, has been so far a major
challenge for computer simulations. In this work we put forward a
coarse-grained model of microgels whose structural properties are in
quantitative agreement with results obtained with small-angle X-ray scattering
experiments across a wide range of temperatures, encompassing the volume phase
transition. These results bridge the gap between experiments and simulations of
individual microgel particles, paving the way to theoretically address open
questions about their bulk properties with unprecedented nano and microscale
resolution
Quantifying the Reversible Association of Thermosensitive Nanoparticles
Under many conditions, biomolecules and nanoparticles associate by means of
attractive bonds, due to hydrophobic attraction. Extracting the microscopic
association or dissociation rates from experimental data is complicated by the
dissociation events and by the sensitivity of the binding force to temperature
(T). Here we introduce a theoretical model that combined with light-scattering
experiments allows us to quantify these rates and the reversible binding energy
as a function of T. We apply this method to the reversible aggregation of
thermoresponsive polystyrene/poly(N-isopropylacrylamide) core-shell
nanoparticles, as a model system for biomolecules. We find that the binding
energy changes sharply with T, and relate this remarkable switchable behavior
to the hydrophobic-hydrophilic transition of the thermosensitive nanoparticles
Flow curves of colloidal dispersions close to the glass transition: Asymptotic scaling laws in a schematic model of mode coupling theory
The flow curves, viz. the curves of stationary stress under steady shearing,
are obtained close to the glass transition in dense colloidal dispersions using
asymptotic expansions in a schematic model of mode coupling theory. The shear
thinning of the viscosity in fluid states and the yielding of glassy states is
discussed. At the transition between fluid and shear-molten glass, simple and
generalized Herschel-Bulkley laws are derived with power law exponents that can
be computed for different particle interactions from the equilibrium structure
factor.Comment: 14 pages, 14 figures, 4 tables, Eur. Phys. J. E (submitted
Acid/base-triggered switching of circularly polarized luminescence and electronic circular dichroism in organic and organometallic helicenes.
Electronic circular dichroism and circularly polarized luminescence acid/base switching activity has been demonstrated in helicene-bipyridine proligand 1 a and in its “rollover” cycloplatinated derivative 2 a. Whereas proligand 1 a displays a strong bathochromic shift (>160 nm) of the nonpolarized and circularly polarized luminescence upon protonation, complex 2 a displays slightly stronger emission. This strikingly different behavior between singlet emission in the organic helicene and triplet emission in the organometallic derivative has been rationalized by using quantum-chemical calculations. The very large bathochromic shift of the emission observed upon protonation of azahelicene-bipyridine 1 a has been attributed to the decrease in aromaticity (promoting a charge-transfer-type transition rather than a π–π* transition) as well as an increase in the HOMO–LUMO character of the transition and stabilization of the LUMO level upon protonation
Aging in humid granular media
Aging behavior is an important effect in the friction properties of solid
surfaces. In this paper we investigate the temporal evolution of the static
properties of a granular medium by studying the aging over time of the maximum
stability angle of submillimetric glass beads. We report the effect of several
parameters on these aging properties, such as the wear on the beads, the stress
during the resting period, and the humidity content of the atmosphere. Aging
effects in an ethanol atmosphere are also studied. These experimental results
are discussed at the end of the paper.Comment: 7 pages, 9 figure
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