39 research outputs found
Adhesion between a viscoelastic material and a solid surface
In this paper, we present a qualitative analysis of the dissipative processes
during the failure of the interface between a viscoelastic polymer and a solid
surface. We reassess the "viscoelastic trumpet" model [P.-G. de Gennes, C. R.
Acad. Sci. Paris, 307, 1949 (1988)], and show that, for a crosslinked polymer,
the interface toughness G(V) starts from a relatively low value, G_0, due to
local processes near the fracture tip, and rises up to a maximum of order (where and stand for the elastic
modulus of the material, respectively at low and high strain frequencies). This
enhancement of fracture energy is due to far-field viscous dissipation in the
bulk material, and begins for peel-rates V much lower than previously thought.
For a polymer melt, the adhesion energy is predicted to scale as 1/V. In the
second part of this paper, we compare some of our theoretical predictions with
experimental results about the viscoelastic adhesion between a
polydimethylsiloxane polymer melt and a glass surface. In particular, the
expected dependence of the fracture energy versus separation rate is confirmed
by the experimental data, and the observed changes in the concavity of the
crack profile are in good agreement with our simple model.Comment: Revised version to appear in Macromolecule
Pattern formation during the evaporation of a colloidal nanoliter drop: a numerical and experimental study
An efficient way to precisely pattern particles on solid surfaces is to
dispense and evaporate colloidal drops, as for bioassays. The dried deposits
often exhibit complex structures exemplified by the coffee ring pattern, where
most particles have accumulated at the periphery of the deposit. In this work,
the formation of deposits during the drying of nanoliter colloidal drops on a
flat substrate is investigated numerically and experimentally. A finite-element
numerical model is developed that solves the Navier-Stokes, heat and mass
transport equations in a Lagrangian framework. The diffusion of vapor in the
atmosphere is solved numerically, providing an exact boundary condition for the
evaporative flux at the droplet-air interface. Laplace stresses and thermal
Marangoni stresses are accounted for. The particle concentration is tracked by
solving a continuum advection-diffusion equation. Wetting line motion and the
interaction of the free surface of the drop with the growing deposit are
modeled based on criteria on wetting angles. Numerical results for evaporation
times and flow field are in very good agreement with published experimental and
theoretical results. We also performed transient visualization experiments of
water and isopropanol drops loaded with polystyrene microsphere evaporating on
respectively glass and polydimethylsiloxane substrates. Measured evaporation
times, deposit shape and sizes, and flow fields are in very good agreement with
the numerical results. Different flow patterns caused by the competition of
Marangoni loops and radial flow are shown to determine the deposit shape to be
either a ring-like pattern or a homogeneous bump
Pinning of a solid--liquid--vapour interface by stripes of obstacles
We use a macroscopic Hamiltonian approach to study the pinning of a
solid--liquid--vapour contact line on an array of equidistant stripes of
obstacles perpendicular to the liquid. We propose an estimate of the density of
pinning stripes for which collective pinning of the contact line happens. This
estimate is shown to be in good agreement with Langevin equation simulation of
the macroscopic Hamiltonian. Finally we introduce a 2--dimensional mean field
theory which for small strength of the pinning stripes and for small capillary
length gives an excellent description of the averaged height of the contact
line.Comment: Plain tex, 12 pages, 3 figures available upon reques
Dissipation in Dynamics of a Moving Contact Line
The dynamics of the deformations of a moving contact line is studied assuming
two different dissipation mechanisms. It is shown that the characteristic
relaxation time for a deformation of wavelength of a contact line
moving with velocity is given as . The velocity
dependence of is shown to drastically depend on the dissipation
mechanism: we find for the case when the dynamics is governed
by microscopic jumps of single molecules at the tip (Blake mechanism), and
when viscous hydrodynamic losses inside the moving
liquid wedge dominate (de Gennes mechanism). We thus suggest that the debated
dominant dissipation mechanism can be experimentally determined using
relaxation measurements similar to the Ondarcuhu-Veyssie experiment [T.
Ondarcuhu and M. Veyssie, Nature {\bf 352}, 418 (1991)].Comment: REVTEX 8 pages, 9 PS figure
Roughening Transition in a Moving Contact Line
The dynamics of the deformations of a moving contact line on a disordered
substrate is formulated, taking into account both local and hydrodynamic
dissipation mechanisms. It is shown that both the coating transition in contact
lines receding at relatively high velocities, and the pinning transition for
slowly moving contact lines, can be understood in a unified framework as
roughening transitions in the contact line. We propose a phase diagram for the
system in which the phase boundaries corresponding to the coating transition
and the pinning transition meet at a junction point, and suggest that for
sufficiently strong disorder a receding contact line will leave a
Landau--Levich film immediately after depinning. This effect may be relevant to
a recent experimental observation in a liquid Helium contact line on a Cesium
substrate [C. Guthmann, R. Gombrowicz, V. Repain, and E. Rolley, Phys. Rev.
Lett. {\bf 80}, 2865 (1998)].Comment: 16 pages, 6 encapsulated figure
The Complex Ginzburg-Landau Equation in the Presence of Walls and Corners
We investigate the influence of walls and corners (with Dirichlet and Neumann
boundary conditions) in the evolution of twodimensional autooscillating fields
described by the complex Ginzburg-Landau equation. Analytical solutions are
found, and arguments provided, to show that Dirichlet walls introduce strong
selection mechanisms for the wave pattern. Corners between walls provide
additional synchronization mechanisms and associated selection criteria. The
numerical results fit well with the theoretical predictions in the parameter
range studied.Comment: 10 pages, 9 figures; for related work visit
http://www.nbi.dk/~martine
On the zeta-function regularization of a two-dimensional series of epsten-Hurwitz type
For a few years now, the study of quantum field theories in partially compactified space-time manifolds has acquired increasing importance in several domains of quantum physics. Let me just mention the issues of dimensional reduction and spontaneous compactification, and the multiple questions associated with the study of quantum field theories in the presence of boundaries (like the Casimir effect) and on curved space-time (manifolds with curvature and nontrivial topology), a step towards quantum gravity
A reliable scheme for fabricating sub-5 nm co-planar junctions for single-molecule electrons.
We demonstrate a high yield production scheme to fabricate sub-5 nm co-planar metalâinsulatorâmetal junctions. This involves determining the relationship between the actual gap between the metallic junctions for a given designed gap, and the use of weak developers with ultrasonic agitation to process the exposed resist. This results in an improved process to achieve narrow inter-electrode gaps. The gaps were imaged using an AFM equipped with a carbon nanotube tip to achieve a high degree of accuracy in measurement. The smallest gap unambiguously measured was ~ 2 nm. Gaps with †5 nm spacing were produced with a very high yield of about 75% for a designed inter-electrode distance of 0 nm. The leakage resistance of the gaps was found to be of the order of 1012 Ω. The entire junction structure was designed to be co-planar to better than 1 nm over 1 ÎŒ m2