174 research outputs found
Sharp Transition between Coalescence and Noncoalescence of Sessile Drops
Unexpectedly, under certain conditions, sessile drops from different but
completely miscible liquids do not always coalesce instantaneously upon
contact: the drop bodies remain separated in a temporary state of
noncoalescence, connected through a thin liquid bridge. Here we investigate the
transition between the states of instantaneous coalescence and temporary
noncoalescence. Experiments reveal that it is barely influenced by viscosities
and absolute surface tensions. The main system control parameters for the
transition are the arithmetic means of the three-phase angles,
and the surface tension differences
between both liquids. These relevant parameters can be combined into a single
system parameter, a speciffic Marangoni number . This universally
characterizes the coalescence respectively transition behavior as a function of
both, the physicochemical liquid properties and the shape of the liquid body in
the contact region. The transition occurs at a certain threshold value
and is sharp within the experimental resolution. The
experimentally observed threshold value of agrees
quantitatively with values obtained by simulations assuming authentic real
space data. The simulations indicate that the absolute value of
very weakly depends on the molecular diffusivity.Comment: 9 pages, 6 figure
Non-coalescence of sessile drops from different but miscible liquids: Hydrodynamic analysis of the twin drop contour as self stabilizing, traveling wave
Capillarity always favors drop fusion. Nevertheless sessile drops from
different but completely miscible liquids often do not fuse instantaneously
upon contact. Rather, intermediate non-coalescence is observed. Two separate
drop bodies, connected by a thin liquid neck move over the substrate. Supported
by new experimental data a thin film hydrodynamic analysis of this state is
presented. Presumably advective and diffusive volume fluxes in the neck region
establish a localized and temporarily stable surface tension gradient. This
induces a local surface (Marangoni) flow that stabilizes a traveling wave i.e.,
the observed moving twin drop configuration. The theoretical predictions are in
excellent agreement with the experimental findings.Comment: 5 pages, 5 figure
Cusp-shaped Elastic Creases and Furrows
The surfaces of growing biological tissues, swelling gels, and compressed
rubbers do not remain smooth, but frequently exhibit highly localized inward
folds. We reveal the morphology of this surface folding in a novel experimental
setup, which permits to deform the surface of a soft gel in a controlled
fashion. The interface first forms a sharp furrow, whose tip size decreases
rapidly with deformation. Above a critical deformation, the furrow bifurcates
to an inward folded crease of vanishing tip size. We show experimentally and
numerically that both creases and furrows exhibit a universal cusp-shape, whose
width scales like at a distance from the tip. We provide a
similarity theory that captures the singular profiles before and after the
self-folding bifurcation, and derive the length of the fold from large
deformation elasticity.Comment: 5 pages, 4 figure
Droplets move over viscoelastic substrates by surfing a ridge
Liquid drops on soft solids generate strong deformations below the contact
line, resulting from a balance of capillary and elastic forces. The movement of
these drops may cause strong, potentially singular dissipation in the soft
solid. Here we show that a drop on a soft substrate moves by surfing a ridge:
the initially flat solid surface is deformed into a sharp ridge whose
orientation angle depends on the contact line velocity. We measure this angle
for water on a silicone gel and develop a theory based on the substrate
rheology. We quantitatively recover the dynamic contact angle and provide a
mechanism for stick-slip motion when a drop is forced strongly: the contact
line depins and slides down the wetting ridge, forming a new one after a
transient. We anticipate that our theory will have implications in problems
such as self-organization of cell tissues or the design of capillarity-based
microrheometers.Comment: 9 pages, 5 figure
Lubrication of soft viscoelastic solids
Lubrication flows appear in many applications in engineering, biophysics, and in nature. Separation of surfaces and minimisation of friction and wear is achieved when the lubrication fluid builds up a lift force. In this paper we analyse soft lubricated contacts by treating the solid walls as viscoelastic: soft materials are typically not purely elastic, but dissipate energy under dynamical loading conditions. We present a method for viscoelastic lubrication and focus on three canonical examples, namely Kelvin-Voigt-, Standard Linear-, and Power Law-rheology. It is shown how the solid viscoelasticity affects the lubrication process when the timescale of loading becomes comparable to the rheological timescale. We derive asymptotic relations between lift force and sliding velocity, which give scaling laws that inherit a signature of the rheology. In all cases the lift is found to decrease with respect to purely elastic systems
- …