18 research outputs found
Stability of liquid ridges on chemical micro- and nanostripes
We analyze the stability of sessile filaments (ridges) of nonvolatile liquids
versus pearling in the case of externally driven flow along a chemical stripe
within the framework of the thin film approximation. The ridges can be stable
with respect to pearling even if the contact line is not completely pinned. A
generalized stability criterion for moving contact lines is provided. For large
wavelengths and no drive, within perturbation theory, an analytical expression
of the growth rate of pearling instabilities is derived. A numerical analysis
shows that drive further stabilizes the ridge by reducing the growth rate of
unstable perturbations, even though there is no complete stabilization. Hence
the stability criteria established without drive ensure overall stability.Comment: 10 pages, 6 figure
Post-Tanner spreading of nematic droplets
The quasistationary spreading of a circular liquid drop on a solid substrate
typically obeys the so-called Tanner law, with the instantaneous base radius
R(t) growing with time as R ~ t^{1/10} -- an effect of the dominant role of
capillary forces for a small-sized droplet. However, for droplets of nematic
liquid crystals, a faster spreading law sets in at long times, so that R ~
t^alpha with alpha significantly larger than the Tanner exponent 1/10. In the
framework of the thin film model (or lubrication approximation), we describe
this "acceleration" as a transition to a qualitatively different spreading
regime driven by a strong substrate-liquid interaction specific to nematics
(antagonistic anchoring at the interfaces). The numerical solution of the thin
film equation agrees well with the available experimental data for nematics,
even though the non-Newtonian rheology has yet to be taken into account. Thus
we complement the theory of spreading with a post-Tanner stage, noting that the
spreading process can be expected to cross over from the usual
capillarity-dominated stage to a regime where the whole reservoir becomes a
diffusive film in the sense of Derjaguin.Comment: 15 pages, 4 figures, accepted in JPCM special issu
Contact line stability of ridges and drops
Within the framework of a semi-microscopic interface displacement model we
analyze the linear stability of sessile ridges and drops of a non-volatile
liquid on a homogeneous, partially wet substrate, for both signs and arbitrary
amplitudes of the three-phase contact line tension. Focusing on perturbations
which correspond to deformations of the three-phase contact line, we find that
drops are generally stable while ridges are subject only to the long-wavelength
Rayleigh-Plateau instability leading to a breakup into droplets, in contrast to
the predictions of capillary models which take line tension into account. We
argue that the short-wavelength instabilities predicted within the framework of
the latter macroscopic capillary theory occur outside its range of validity and
thus are spurious.Comment: 6 pages, 1 figur
Post-Tanner stages of droplet spreading: the energy balance approach revisited
The spreading of a circular liquid drop on a solid substrate can be described
by the time evolution of its base radius R(t). In complete wetting the
quasistationary regime (far away from initial and final transients) typically
obeys the so-called Tanner law, with R t^alpha_T, alpha_T=1/10. Late-time
spreading may differ significantly from the Tanner law: in some cases the drop
does not thin down to a molecular film and instead reaches an equilibrium
pancake-like shape; in other situations, as revealed by recent experiments with
spontaneously spreading nematic crystals, the growth of the base radius
accelerates after the Tanner stage. Here we demonstrate that these two
seemingly conflicting trends can be reconciled within a suitably revisited
energy balance approach, by taking into account the line tension contribution
to the driving force of spreading: a positive line tension is responsible for
the formation of pancake-like structures, whereas a negative line tension tends
to lengthen the contact line and induces an accelerated spreading (a transition
to a faster power law for R(t) than in the Tanner stage).Comment: 12 pages, 1 figur
Software-hardware speech module
The article considers the software-hardware speech module and its use in dolls and interactive toysВ статье рассмотрен программно-аппаратный речевой модуль и его использование в куклах и интерактивных игрушка
Generic behaviours in impact fragmentation
We present a simple numerical model for investigating the general properties of fragmentation. By use of molecular dynamics simulations, we study the impact fragmentation of a solid disk of interacting particles with a wall. Regardless of the particular form of the interaction potential, the fragment size distribution exhibits a power law behaviour with an exponent that increases logarithmically with the energy deposited in the system, in agreement with experiments. We expect this behaviour to be generic in fragmentation phenomena
Granular gases in mechanical engineering: on the origin of heterogeneous ultrasonic shot peening
International audienceThe behavior of an ultrasonic shot peening process is observed and analyzed by using a model of inelastic hard spheres in a gravitational field that are fluidized by a vibrating bottom wall (sonotrode) in a cylindrical chamber. A marked heterogeneous distribution of impacts appears when the collision between the shot and the side wall becomes inelastic with constant dissipation. This effect is one order of magnitude larger than the simple heterogeneity arising from boundary collision on the cylinder. Variable restitution coefficients bring the simulation closer to the general observation and allow the investigation of peening regimes with changing shot density. We compute within this model other physical quantities (impact velocities, impact angle, temperature and density profile) that are influenced by the number N of spheres