69 research outputs found
Influence of surface roughness on superhydrophobicity
Superhydrophobic surfaces, with liquid contact angle theta greater than 150
degree, have important practical applications ranging from self-cleaning window
glasses, paints, and fabrics to low-friction surfaces. Many biological
surfaces, such as the lotus leaf, have hierarchically structured surface
roughness which is optimized for superhydrophobicity through natural selection.
Here we present a molecular dynamics study of liquid droplets in contact with
self-affine fractal surfaces. Our results indicate that the contact angle for
nanodroplets depends strongly on the root-mean-square surface roughness
amplitude but is nearly independent of the fractal dimension D_f of the
surface.Comment: 5 Pages, 6 figures. Minimal changes with respect to the previous
versio
Why Are Alkali Halide Solid Surfaces Not Wetted By Their Own Melt?
Alkali halide (100) crystal surfaces are anomalous, being very poorly wetted
by their own melt at the triple point. We present extensive simulations for
NaCl, followed by calculations of the solid-vapor, solid-liquid, and
liquid-vapor free energies showing that solid NaCl(100) is a nonmelting
surface, and that its full behavior can quantitatively be accounted for within
a simple Born-Meyer-Huggins-Fumi-Tosi model potential. The incomplete wetting
is traced to the conspiracy of three factors: surface anharmonicities
stabilizing the solid surface; a large density jump causing bad liquid-solid
adhesion; incipient NaCl molecular correlations destabilizing the liquid
surface. The latter is pursued in detail, and it is shown that surface
short-range charge order acts to raise the surface tension because incipient
NaCl molecular formation anomalously reduces the surface entropy of liquid NaCl
much below that of solid NaCl(100).Comment: 4 pages, 3 figure
Ectoplasm & Superspace Integration Measure for 2D Supergravity with Four Spinorial Supercurrents
Building on a previous derivation of the local chiral projector for a two
dimensional superspace with eight real supercharges, we provide the complete
density projection formula required for locally supersymmetrical theories in
this context. The derivation of this result is shown to be very efficient using
techniques based on the Ectoplasmic construction of local measures in
superspace.Comment: 18 pages, LaTeX; V2: minor changes, typos corrected, references
added; V3: version to appear in J. Phys. A: Math. Theor., some comments and
references added to address a referee reques
Feedback Loops Between Fields and Underlying Space Curvature: an Augmented Lagrangian Approach
We demonstrate a systematic implementation of coupling between a scalar field
and the geometry of the space (curve, surface, etc.) which carries the field.
This naturally gives rise to a feedback mechanism between the field and the
geometry. We develop a systematic model for the feedback in a general form,
inspired by a specific implementation in the context of molecular dynamics (the
so-called Rahman-Parrinello molecular dynamics, or RP-MD). We use a generalized
Lagrangian that allows for the coupling of the space's metric tensor (the first
fundamental form) to the scalar field, and add terms motivated by RP-MD. We
present two implementations of the scheme: one in which the metric is only
time-dependent [which gives rise to ordinary differential equation (ODE) for
its temporal evolution], and one with spatio-temporal dependence [wherein the
metric's evolution is governed by a partial differential equation (PDE)].
Numerical results are reported for the (1+1)-dimensional model with a
nonlinearity of the sine-Gordon type.Comment: 5 pages, 3 figures, Phys. Rev. E in pres
Six-dimensional Supergravity and Projective Superfields
We propose a superspace formulation of N=(1,0) conformal supergravity in six
dimensions. The corresponding superspace constraints are invariant under
super-Weyl transformations generated by a real scalar parameter. The known
variant Weyl super-multiplet is recovered by coupling the geometry to a
super-3-form tensor multiplet. Isotwistor variables are introduced and used to
define projective superfields. We formulate a locally supersymmetric and
super-Weyl invariant action principle in projective superspace. Some families
of dynamical supergravity-matter systems are presented.Comment: 39 pages; v3: some modifications in section 2; equations (2.3),
(2.14b), (2.16) and (2.17) correcte
Three-dimensional (p,q) AdS superspaces and matter couplings
We introduce N-extended (p,q) AdS superspaces in three space-time dimensions,
with p+q=N and p>=q, and analyse their geometry. We show that all (p,q) AdS
superspaces with X^{IJKL}=0 are conformally flat. Nonlinear sigma-models with
(p,q) AdS supersymmetry exist for p+q4 the target space geometries
are highly restricted). Here we concentrate on studying off-shell N=3
supersymmetric sigma-models in AdS_3. For each of the cases (3,0) and (2,1), we
give three different realisations of the supersymmetric action. We show that
(3,0) AdS supersymmetry requires the sigma-model to be superconformal, and
hence the corresponding target space is a hyperkahler cone. In the case of
(2,1) AdS supersymmetry, the sigma-model target space must be a non-compact
hyperkahler manifold endowed with a Killing vector field which generates an
SO(2) group of rotations of the two-sphere of complex structures.Comment: 52 pages; V3: minor corrections, version published in JHE
Ballistic nanofriction
Sliding parts in nanosystems such as Nano ElectroMechanical Systems (NEMS)
and nanomotors, increasingly involve large speeds, and rotations as well as
translations of the moving surfaces; yet, the physics of high speed nanoscale
friction is so far unexplored. Here, by simulating the motion of drifting and
of kicked Au clusters on graphite - a workhorse system of experimental
relevance -- we demonstrate and characterize a novel "ballistic" friction
regime at high speed, separate from drift at low speed. The temperature
dependence of the cluster slip distance and time, measuring friction, is
opposite in these two regimes, consistent with theory. Crucial to both regimes
is the interplay of rotations and translations, shown to be correlated in slow
drift but anticorrelated in fast sliding. Despite these differences, we find
the velocity dependence of ballistic friction to be, like drift, viscous
Off-shell supergravity-matter couplings in three dimensions
We develop the superspace geometry of N-extended conformal supergravity in
three space-time dimensions. General off-shell supergravity-matter couplings
are constructed in the cases N=1,2,3,4.Comment: 73 pages; V5: typos in eqs. (3.4b), (3.17) and (4.24) correcte
On 2D N=(4,4) superspace supergravity
We review some recent results obtained in studying superspace formulations of
2D N=(4,4) matter-coupled supergravity. For a superspace geometry described by
the minimal supergravity multiplet, we first describe how to reduce to
components the chiral integral by using ``ectoplasm'' superform techniques as
in arXiv:0907.5264 and then we review the bi-projective superspace formalism
introduced in arXiv:0911.2546. After that, we elaborate on the curved
bi-projective formalism providing a new result: the solution of the covariant
type-I twisted multiplet constraints in terms of a weight-(-1,-1) bi-projective
superfield.Comment: 18 pages, LaTeX, Contribution to the proceedings of the International
Workshop "Supersymmetries and Quantum Symmetries" (SQS'09), Dubna, July
29-August 3 200
On the nature of surface roughness with application to contact mechanics, sealing, rubber friction and adhesion
Surface roughness has a huge impact on many important phenomena. The most
important property of rough surfaces is the surface roughness power spectrum
C(q). We present surface roughness power spectra of many surfaces of practical
importance, obtained from the surface height profile measured using optical
methods and the Atomic Force Microscope. We show how the power spectrum
determines the contact area between two solids. We also present applications to
sealing, rubber friction and adhesion for rough surfaces, where the power
spectrum enters as an important input.Comment: Topical review; 82 pages, 61 figures; Format: Latex (iopart). Some
figures are in Postscript Level
- …