4,458 research outputs found
A surface force apparatus for nanorheology under large shear strain
We describe a surface force apparatus designed to probe the rheology of a
nanoconfined medium under large shear amplitudes (up to 500 m). The
instrument can be operated in closed-loop, controlling either the applied
normal load or the thickness of the medium during shear experiments. Feedback
control allows to greatly extend the range of confinement/shear strain
attainable with the surface force apparatus. The performances of the instrument
are illustrated using hexadecane as the confined medium
Colloidal aggregation and critical Casimir forces
A recent Letter [Phys. Rev. Lett. 103, 156101 (2009)] reports the
experimental observation of aggregation of colloidal particles dispersed in a
liquid mixture of heavy water and 3-methylpyridine. The experimental data are
interpreted in terms of a model which accounts solely for the competing effects
of the interparticle electrostatic repulsion and of the attractive critical
Casimir force. Here we show, however, that the reported aggregation actually
occurs within ranges of values of the correlation length and of the Debye
screening length ruled out by the proposed model and that a significant part of
the experimental data presented in the Letter cannot be consistently
interpreted in terms of such a model.Comment: 1 page, 1 figure; For the reply see arXiv:1007.077
Heterogeneous critical nucleation on a completely-wettable substrate
Heterogeneous nucleation of a new bulk phase on a flat substrate can be
associated with the surface phase transition called wetting transition. When
this bulk heterogeneous nucleation occurs on a completely-wettable flat
substrate with a zero contact angle, the classical nucleation theory predicts
that the free energy barrier of nucleation vanishes. In fact, there always
exist a critical nucleus and a free energy barrier as the first-order
pre-wetting transition will occur even when the contact angle is zero.
Furthermore, the critical nucleus changes its character from the critical
nucleus of surface phase transition below bulk coexistence (undersaturation) to
the critical nucleus of bulk heterogeneous nucleation above the coexistence
(oversaturation) when it crosses the coexistence. Recently, Sear [J.Chem.Phys
{\bf 129}, 164510 (2008)] has shown by a direct numerical calculation of
nucleation rate that the nucleus does not notice this change when it crosses
the coexistence. In our work the morphology and the work of formation of
critical nucleus on a completely-wettable substrate are re-examined across the
coexistence using the interface-displacement model. Indeed, the morphology and
the work of formation changes continuously at the coexistence. Our results
support the prediction of Sear and will rekindle the interest on heterogeneous
nucleation on a completely-wettable substrate.Comment: 11pages, 9 figures, Journal of Chemical Physics to be publishe
Layering Transitions and Solvation Forces in an Asymmetrically Confined Fluid
We consider a simple fluid confined between two parallel walls (substrates),
separated by a distance L. The walls exert competing surface fields so that one
wall is attractive and may be completely wet by liquid (it is solvophilic)
while the other is solvophobic. Such asymmetric confinement is sometimes termed
a `Janus Interface'. The second wall is: (i) purely repulsive and therefore
completely dry (contact angle 180 degrees) or (ii) weakly attractive and
partially dry (the contact angle is typically in the range 160-170 degrees). At
low temperatures, but above the bulk triple point, we find using classical
density functional theory (DFT) that the fluid is highly structured in the
liquid part of the density profile. In case (i) a sequence of layering
transitions occurs: as L is increased at fixed chemical potential (mu) close to
bulk gas--liquid coexistence, new layers of liquid-like density develop
discontinuously. In contrast to confinement between identical walls, the
solvation force is repulsive for all wall separations and jumps discontinuously
at each layering transition and the excess grand potential exhibits many
metastable minima as a function of the adsorption. For a fixed temperature
T=0.56Tc, where Tc is the bulk critical temperature, we determine the
transition lines in the L, mu plane. In case (ii) we do not find layering
transitions and the solvation force oscillates about zero. We discuss how our
mean-field DFT results might be altered by including effects of fluctuations
and comment on how the phenomenology we have revealed might be relevant for
experimental and simulation studies of water confined between hydrophilic and
hydrophobic substrates, emphasizing it is important to distinguish between
cases (i) and (ii).Comment: 16 pages, 13 figure
Premicellar aggregation of amphiphilic molecules: Aggregate lifetime and polydispersity
A recently introduced thermodynamic model of amphiphilic molecules in
solution has yielded, under certain realistic conditions, a significant
presence of metastable aggregates well below the critical micelle concentration
-- a phenomenon that has been reported also experimentally. The theory is
extended in two directions pertaining to the experimental and technological
relevance of such premicellar aggregates. (a) Combining the thermodynamic model
with reaction rate theory, we calculate the lifetime of the metastable
aggregates. (b) Aggregation number fluctuations are examined. We demonstrate
that, over most of the metastable concentration range, the premicellar
aggregates should have macroscopic lifetimes and small polydispersity.Comment: 7 pages, 2 figure
Liquid transport generated by a flashing field-induced wettability ratchet
We develop and analyze a model for ratchet-driven macroscopic transport of a
continuous phase. The transport relies on a field-induced dewetting-spreading
cycle of a liquid film with a free surface based on a switchable, spatially
asymmetric, periodic interaction of the liquid-gas interface and the substrate.
The concept is exemplified using an evolution equation for a dielectric liquid
film under an inhomogeneous voltage. We analyse the influence of the various
phases of the ratchet cycle on the transport properties. Conditions for maximal
transport and the efficiency of transport under load are discussed.Comment: 10 pages, 5 figure
Solvent-mediated interactions between nanoparticles at fluid interfaces
We investigate the solvent mediated interactions between nanoparticles
adsorbed at a liquid-vapor interface in comparison to the solvent mediated
interactions in the bulk liquid and vapor phases of a Lennard-Jones solvent.
Molecular dynamics simulation data for the latter are in good agreement with
results from integral equations in the reference functional approximation and a
simple geometric approximation. Simulation results for the solvent mediated
interactions at the interface differ markedly from the interactions of the
particles in the corresponding bulk phases. We find that at short interparticle
distances the interactions are considerably more repulsive than those in either
bulk phase. At long interparticle distances we find evidence for a long-ranged
attraction. We discuss these observations in terms of interfacial interactions,
namely, the three-phase line tension that would operate at short distances, and
capillary wave interactions for longer interparticle distances.Comment: 22 pages, 6 figure
Myelin figures: the buckling and flow of wet soap
Myelin figures are interfacial structures formed when certain surfactants
swell in excess water. Here, I present data and model calculations suggesting
myelin formation and growth is due to the fluid flow of surfactant, driven by
the hydration gradient at the dry surfactant/water interface; a simple model
based on this idea qualitatively reproduces the various myelin growth behaviors
observed in different experiments. From a detailed experimental observation of
how myelins develop from a planar precursor structure, I identify a mechanical
instability that may underlie myelin formation. These results indicate the
mixed mechanical character of the surfactant lamellar structure, where fluid
and elastic properties coexist, is what enables the formation and growth of
myelins.Comment: 11 pages, 10 figures, to appear in Phys. Rev. E. Corrected
figures/typo
Surface tension of electrolytes: Hydrophilic and hydrophobic ions near an interface
We calculate the ion distributions around an interface in fluid mixtures of
highly polar and less polar fluids (water and oil) for two and three ion
species. We take into account the solvation and image interactions between ions
and solvent. We show that hydrophilic and hydrophobic ions tend to undergo a
microphase separation at an interface, giving rise to an enlarged electric
double layer. We also derive a general expression for the surface tension of
electrolyte systems, which contains a negative electrostatic contribution
proportional to the square root of the bulk salt density. The amplitude of this
square-root term is small for hydrophilic ion pairs, but is much increased for
hydrophilic and hydrophobic ion pairs. For three ion species including
hydrophilic and hydrophobic ions, we calculate the ion distributions to explain
those obtained by x-ray reflectivity measurements.Comment: 8 figure
Manning condensation in two dimensions
We consider a macroion confined to a cylindrical cell and neutralized by
oppositely charged counterions. Exact results are obtained for the
two-dimensional version of this problem, in which ion-ion and ion-macroion
interactions are logarithmic. In particular, the threshold for counterion
condensation is found to be the same as predicted by mean-field theory. With
further increase of the macroion charge, a series of single-ion condensation
transitions takes place. Our analytical results are expected to be exact in the
vicinity of these transitions and are in very good agreement with recent
Monte-Carlo simulation data.Comment: 4 pages, 4 figure
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