353 research outputs found
Why surface nanobubbles live for hours
We present a theoretical model for the experimentally found but
counter-intuitive exceptionally long lifetime of surface nanobubbles. We can
explain why, under normal experimental conditions, surface nanobubbles are
stable for many hours or even up to days rather than the expected microseconds.
The limited gas diffusion through the water in the far field, the cooperative
effect of nanobubble clusters, and the pinned contact line of the nanobubbles
lead to the slow dissolution rate.Comment: 5 pages, 3 figure
Capillarity of soft amorphous solids: a microscopic model for surface stress
The elastic deformation of a soft solid induced by capillary forces crucially
relies on the excess stress inside the solid-liquid interface. While for a
liquid-liquid interface this "surface stress" is strictly identical to the
"surface free energy", the thermodynamic Shuttleworth equation implies that
this is no longer the case when one of the phases is elastic. Here we develop a
microscopic model that incorporates enthalpic interactions and entropic
elasticity, based on which we explicitly compute the surface stress and surface
free energy. It is found that the compressibility of the interfacial region,
through the Poisson ratio near the interface, determines the difference between
surface stress and surface energy. We highlight the consequence of this finding
by comparing with recent experiments and simulations on partially wetted soft
substrates
Initial spreading of low-viscosity drops on partially wetting surfaces
Liquid drops start spreading directly after brought into contact with a
partial wetting substrate. Although this phenomenon involves a three-phase
contact line, the spreading motion is very fast. We study the initial spreading
dynamics of low-viscosity drops, using two complementary methods: Molecular
Dynamics simulations and high-speed imaging. We access previously unexplored
length- and time-scales, and provide a detailed picture on how the initial
contact between the liquid drop and the solid is established. Both methods
unambiguously point towards a spreading regime that is independent of
wettability, with the contact radius growing as the square root of time
Emergent hyperuniformity in periodically-driven emulsions
We report the emergence of large-scale hyperuniformity in microfluidic
emulsions. Upon periodic driving confined emulsions undergo a first-order
transition from a reversible to an irreversible dynamics. We evidence that this
dynamical transition is accompanied by structural changes at all scales
yielding macroscopic yet finite hyperuniform structures. Numerical simulations
are performed to single out the very ingredients responsible for the
suppression of density fluctuations. We show that as opposed to equilibrium
systems the long-range nature of the hydrodynamic interactions are not required
for the formation of hyperuniform patterns, thereby suggesting a robust
relation between reversibility and hyperuniformity which should hold in a broad
class of periodically driven materials.Comment: 5p, 3f, submitte
Drops on soft solids: Free energy and double transition of contact angles
The equilibrium shape of liquid drops on elastic substrates is determined by
minimising elastic and capillary free energies, focusing on thick
incompressible substrates. The problem is governed by three length scales: the
size of the drop , the molecular size , and the ratio of surface tension
to elastic modulus . We show that the contact angles undergo two
transitions upon changing the substrates from rigid to soft. The microscopic
wetting angles deviate from Young's law when , while the
apparent macroscopic angle only changes in the very soft limit . The elastic deformations are worked out in the simplifying case where the
solid surface energy is assumed constant. The total free energy turns out lower
on softer substrates, consistent with recent experiments
Elasto-capillarity at the nanoscale: on the coupling between elasticity and surface energy in soft solids
The capillary forces exerted by liquid drops and bubbles on a soft solid are
directly measured using molecular dynamics simulations. The force on the solid
by the liquid near the contact line is not oriented along the liquid vapor
interface nor perpendicular to the solid surface, as usually assumed, but
points towards the liquid.
It is shown that the elastic deformations induced by this force can only be
explained if, contrary to an incompressible liquid, the surface stress is
different from the surface energy. Using thermodynamic variations we show that
the the surface stress and the surface energy can both be determined accurately
by measuring the deformation of a slender body plunged in a liquid. The results
obtained from molecular dynamics fully confirm those recently obtained
experimentally [Marchand et al. Phys. Rev. Lett. 108, 094301 (2012)] for an
elastomeric wire
Origin of line tension for a Lennard-Jones nanodroplet
The existence and origin of line tension has remained controversial in
literature. To address this issue we compute the shape of Lennard-Jones
nanodrops using molecular dynamics and compare them to density functional
theory in the approximation of the sharp kink interface. We show that the
deviation from Young's law is very small and would correspond to a typical line
tension length scale (defined as line tension divided by surface tension)
similar to the molecular size and decreasing with Young's angle. We propose an
alternative interpretation based on the geometry of the interface at the
molecular scale
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