202 research outputs found
Hydrodynamic fingering instability of driven wetting films: hindrance by diffusion
Recent experimental and theoretical efforts have revealed the existence of a fingering instability at the moving front of thin liquid films forced to spread under gravitational, rotational or surface shear stresses, as for example by using the Marangoni effect. The authors describe how the presence of a precursor film in front of the spreading macroscopic film, whether it is by prewetting the substrate or by surface diffusion or multilayer absorption, can prevent the development of the instability
Precursor films in wetting phenomena
The spontaneous spreading of non-volatile liquid droplets on solid substrates
poses a classic problem in the context of wetting phenomena. It is well known
that the spreading of a macroscopic droplet is in many cases accompanied by a
thin film of macroscopic lateral extent, the so-called precursor film, which
emanates from the three-phase contact line region and spreads ahead of the
latter with a much higher speed. Such films have been usually associated with
liquid-on-solid systems, but in the last decade similar films have been
reported to occur in solid-on-solid systems. While the situations in which the
thickness of such films is of mesoscopic size are rather well understood, an
intriguing and yet to be fully understood aspect is the spreading of
microscopic, i.e., molecularly thin films. Here we review the available
experimental observations of such films in various liquid-on-solid and
solid-on-solid systems, as well as the corresponding theoretical models and
studies aimed at understanding their formation and spreading dynamics. Recent
developments and perspectives for future research are discussed.Comment: 51 pages, 10 figures; small typos correcte
Rescaling the dynamics of evaporating drops
The dynamics of evaporation of wetting droplets has been investigated
experimentally in an extended range of drop sizes, in order to provide trends
relevant for a theoretical analysis. A model is proposed, which generalises
Tanner's law, allowing us to smooth out the singularities both in dissipation
and in evaporative flux at the moving contact line. A qualitative agreement is
obtained, which represents a first step towards the solution of a very old,
complex problem
Late stage kinetics for various wicking and spreading problems
The kinetics of spreading of a liquid drop in a wedge or V-shaped groove, in
a network of such grooves, and on a hydrophilic strip, is re-examined. The
length of a droplet of volume Omega spreading in a wedge after a time t is
predicted to scale as Omega^(1/5) * t^(2/5), and the height profile is
predicted to be a parabola in the distance along the wedge. If the droplet is
spreading radially in a sparse network of V-shaped grooves on a surface, the
radius is predicted to scale as Omega^(1/6) * t^(1/3), provided the liquid is
completely contained within the grooves. A number of other results are also
obtained.Comment: 5 pages, 2 figures, RevTeX
Diffusive Spreading of Chainlike Molecules on Surfaces
We study the diffusion and submonolayer spreading of chainlike molecules on
surfaces. Using the fluctuating bond model we extract the collective and tracer
diffusion coefficients D_c and D_t with a variety of methods. We show that
D_c(theta) has unusual behavior as a function of the coverage theta. It first
increases but after a maximum goes to zero as theta go to one. We show that the
increase is due to entropic repulsion that leads to steep density profiles for
spreading droplets seen in experiments. We also develop an analytic model for
D_c(theta) which agrees well with the simulations.Comment: 3 pages, RevTeX, 4 postscript figures, to appear in Phys. Rev.
Letters (1996
Instability patterns in ultrathin nematic films: comparison between theory and experiment
Motivated by recent experimental observations [U. Delabre et al, Langmuir 24,
3998, 2008] we reconsider an instability of ultrathin nematic films, spread on
liquid substrates. Within a continuum elastic theory of liquid crystals, in the
harmonic approximation, we find an analytical expressions for the critical
thickness as well as for the critical wavenumber, characterizing the onset of
instability towards the stripe phase. Comparing theoretical predictions with
experimental observations, we establish the utility of surface-like term such
as an azimuthal anchoring.Comment: 6 pages, 3 figures, LaTeX macros EPL draft, accepted for publication
in EP
Reverse undercompressive shock structures in driven thin film flow
We show experimental evidence of a new structure involving an
undercompressive and reverse undercompressive shock for draining films driven
by a surface tension gradient against gravity. The reverse undercompressive
shock is unstable to transverse perturbations while the leading
undercompressive shock is stable. Depending on the pinch-off film thickness, as
controlled by the meniscus, either a trailing rarefaction wave or a compressive
shock separates from the reverse undercompressive shock
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