624 research outputs found
Wetting, Spreading, and Adsorption on Randomly Rough Surfaces
The wetting properties of solid substrates with customary (i.e., macroscopic)
random roughness are considered as a function of the microscopic contact angle
of the wetting liquid and its partial pressure in the surrounding gas phase.
Analytic expressions are derived which allow for any given lateral correlation
function and height distribution of the roughness to calculate the wetting
phase diagram, the adsorption isotherms, and to locate the percolation
transition in the adsorbed liquid film. Most features turn out to depend only
on a few key parameters of the roughness, which can be clearly identified. It
is shown that a first order transition in the adsorbed film thickness, which we
term 'Wenzel prewetting', occurs generically on typical roughness topographies,
but is absent on purely Gaussian roughness. It is thereby shown that even
subtle deviations from Gaussian roughness characteristics may be essential for
correctly predicting even qualitative aspects of wetting
Equation of State of Wet Granular Matter
A theory is derived for the nonequilibrium probability currents of the
capillary interaction which determines the pair correlation function near
contact. This yields an analytic expression for the equation of state, P =
P(N/V,T), of wet granular matter for D=2 dimensions, valid in the complete
density range from gas to jamming. Driven wet granular matter exhibits a
van-der-Waals-like unstable branch at granular temperatures T<T_c corresponding
to a first order segregation transition of clusters. For the realistic rupture
length of the liquid bridge, s_crit=0.07 d, the critical point is located at
T_c = 0.274 E_cb. While the critical temperature weakly depends on the rupture
length, the critical density phi_c is shown to scale with s_crit according to
s_crit = 4d (sqrt(phi_J / phi_c) -1). The segregation transition is closely
related to the precipitation of granular droplets reported for the free cooling
of one-dimensional wet granular matter [Phys. Rev. Lett. 97, 078001 (2006)],
and extends the effect to higher dimensional systems. Since the limiting case
of sticky bonds, E_cb >> T, is of relevance for aggregation in general,
simulations have been performed which show very good agreement with the
theoretically predicted coordination K of capillary bonds as a function of the
bond length s_crit. This result implies that particles that stick at the
surface, s_crit=0, form isostatic clusters.Comment: 29 pages, 20 figure
Generic morphologies of viscoelastic dewetting fronts
A simple model is put forward which accounts for the occurrence of certain
generic dewetting morphologies in thin liquid coatings. It demonstrates that by
taking into account the elastic properties of the coating, a morphological
phase diagram may be derived which describes the observed structures of
dewetting fronts. It is demonstrated that dewetting morphologies may also serve
to determine nanoscale rheological properties of liquids.Comment: 4 pages, 2 figure
Chaoticity of the Wet Granular Gas
In this work we derive an analytic expression for the Kolmogorov-Sinai
entropy of dilute wet granular matter, valid for any spatial dimension. The
grains are modelled as hard spheres and the influence of the wetting liquid is
described according to the Capillary Model, in which dissipation is due to the
hysteretic cohesion force of capillary bridges. The Kolmogorov-Sinai entropy is
expanded in a series with respect to density. We find a rapid increase of the
leading term when liquid is added. This demonstrates the sensitivity of the
granular dynamics to humidity, and shows that the liquid significantly
increases the chaoticity of the granular gas.Comment: 13 pages, 10 figures, Physical Review
Nucleation Induced Undulative Instability in Thin Films of nCB Liquid Crystals
A surface instability is reported in thin nematic films of 5CB and 8CB,
occurring near the nematic--isotropic phase transition.
Although this instability leads to patterns reminiscent of spinodal
dewetting, we show that it is actually based on a nucleation mechanism. Its
characteristic wavelength does not depend markedly on film thickness, but
strongly on the heating rate.Comment: 4 pages, 5 figure
Self-assembled granular walkers
Mechanisms of locomotion in microscopic systems are of great interest not
only for technological applications, but also for the sake of understanding,
and potentially harnessing, processes far from thermal equilibrium.
Down-scaling is a particular challenge, and has led to a number of interesting
concepts including thermal ratchet systems and asymmetric swimmers. Here we
present a system which is particularly intriguing, as it is self-assembling and
uses a robust mechanism which can be implemented in various settings. It
consists of small spheres of different size which adhere to each other, and are
subject to an oscillating (zero average) external force eld. An inherent
nonlinearity in the mutual force network leads to force rectication and hence
to locomotion. We present a model that accounts for the observed behaviour and
demonstrates the wide applicability and potential scalability of the concept.Comment: 17 pages, 4 figure
Fluidization of granular media wetted by liquid He
We explore experimentally the fluidization of vertically agitated PMMA
spheres wetted by liquid He. By controlling the temperature around the
point we change the properties of the wetting liquid from a normal
fluid (helium I) to a superfluid (helium II). For wetting by helium I, the
critical acceleration for fluidization () shows a steep increase
close to the saturation of the vapor pressure in the sample cell. For helium II
wetting, starts to increase at about 75% saturation, indicating that
capillary bridges are enhanced by the superflow of unsaturated helium film.
Above saturation, enters a plateau regime where the capillary force
between particles is independent of the bridge volume. The plateau value is
found to vary with temperature and shows a peak at 2.1 K, which we attribute to
the influence of the specific heat of liquid helium.Comment: 4 pages, 3 figures, Accepted by Phys. Rev. E as a rapid communicatio
Dynamics of Chainlike Molecules on Surfaces
We consider the diffusion and spreading of chainlike molecules on solid
surfaces. We first show that the steep spherical cap shape density profiles,
observed in some submonolayer experiments on spreading polymer films, imply
that the collective diffusion coefficient must be an increasing
function of the surface coverage for small and intermediate coverages.
Through simulations of a discrete model of interacting chainlike molecules, we
demonstrate that this is caused by an entropy-induced repulsive interaction.
Excellent agreement is found between experimental and numerically obtained
density profiles in this case, demonstrating that steep submonolayer film edges
naturally arise due to the diffusive properties of chainlike molecules. When
the entropic repulsion dominates over interchain attractions,
first increases as a function of but then eventually approaches zero
for . The maximum value of decreases for increasing
attractive interactions, leading to density profiles that are in between
spherical cap and Gaussian shapes. We also develop an analytic mean field
approach to explain the diffusive behavior of chainlike molecules. The
thermodynamic factor in is evaluated using effective free energy
arguments, and the chain mobility is calculated numerically using the recently
developed dynamic mean field theory. Good agreement is obtained between theory
and simulations.Comment: 16 pages, 13 Postscript figure
Shape of a liquid front upon dewetting
We examine the profile of a liquid front of a film that is dewetting a solid
substrate. Since volume is conserved, the material that once covered the
substrate is accumulated in a rim close to the three phase contact line.
Theoretically, such a profile of a Newtonian liquid resembles an exponentially
decaying harmonic oscillation that relaxes into the prepared film thickness.
For the first time, we were able to observe this behavior experimentally. A
non-Newtonian liquid - a polymer melt - however, behaves differently. Here,
viscoelastic properties come into play. We will demonstrate that by analyzing
the shape of the rim profile. On a nm scale, we gain access to the rheology of
a non-Newtonian liquid.Comment: 4 pages, 4 figure
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