182 research outputs found
Free radially expanding liquid sheet in air: time- and space-resolved measurement of the thickness field
The collision of a liquid drop against a small target results in the
formation of a thin liquid sheet that extends radially until it reaches a
maximum diameter. The subsequent retraction is due to the air-liquid surface
tension. We have used a time- and space-resolved technique to measure the
thickness field of this class of liquid sheet, based on the grey level
measurement of the image of a dyed liquid sheet recorded using a fast camera.
This method enables a precise measurement of the thickness in the range
, with a temporal resolution equal to that of the
camera. We have measured the evolution with time since impact, , and radial
position, , of the thickness, , for various drop volumes and impact
velocities. Two asymptotic regimes for the expansion of the sheet are
evidenced. The scalings of the thickness with and measured in the two
regimes are those that were predicted in \citet{Rozhkov2004} fort the
short-time regime and \citet{Villermaux2011} for the long time regime, but
never experimentally measured before. Interestingly, our experimental data also
evidence the existence of a maximum of the film thickness at
a radial position corresponding to the crossover of these
two asymptotic regimes. The maximum moves with a constant velocity of the order
of the drop impact velocity, as expected theoretically. Thanks to our
visualization technique, we also evidence an azimuthal thickness modulation of
the liquid sheets.Comment: accepted for publication in Journal of Fluid Mechanic
Inwardly curved polymer brushes : Concave is not like Convex
Inwardly curved polymer brushes are present in cylindrical and spherical
micelles or in membranes tubes and vesicles decorated with anchored polymers,
and influence their stability. We consider such polymer brushes in good solvent
and show that previous works, based on a self-similar concentric structure of
the brush, are physically inconsistent. We use scaling laws to derive very
simply the leading term of the free energy in the high curvature limit, where
the osmotic pressure is the relevant physical ingredient. We also derive the
complete conformation at all curvatures using a self-consistent field approach.
The free energy is computed therefrom using a local scaling description.Comment: Subm. to Eur. Phys. J. E., rev. version, 12 pages plus 9 figures,
PACS : 36.20.Ey / 82.35.Gh / 82.70.-y. Figure 1 modified. In introduction,
discussion added on concentration gradients near the edge of the brush.
[email protected] [email protected] [email protected] [email protected]
Linear viscoelasticity of entangled wormlike micelles bridged by telechelic polymers : an experimental model for a double transient network
We survey the linear viscoelasticity of a new type of transient network:
bridged wormlike micelles, whose structure has been characterized recently
[Ramos and Ligoure, (2007)]. This composite material is obtained by adding
telechelic copolymers (water-soluble chains with hydrophobic stickers at each
extremity) to a solution of entangled wormlike micelles (WM). For comparison,
naked WM and WM decorated by amphiphilic copolymers are also investigated.
While these latter systems exhibit almost a same single ideal Maxwell behavior,
solutions of bridged WM can be described as two Maxwell fluids components
blends, characterized by two markedly different characteristic times, t_fast
and t_slow, and two elastic moduli, G_fast and G_slow, with G_fast >> G_slow.
We show that the slow mode is related to the viscoelasticity of the transient
network of entangled WM, and the fast mode to the network of telechelic active
chains (i.e. chains that do not form loops but bridge two micelles). The
dependence of the viscoelasticity with the surfactant concentration, phi, and
the sticker-to-surfactant molar ratio, beta, is discussed. In particular, we
show that G_fast is proportional to the number of active chains in the
material, phi beta. Simple theoretical expectations allow then to evaluate the
bridges/loops ratio for the telechelic polymers
Brittle fracture of polymer transient networks
We study the fracture of reversible double transient networks, constituted of
water suspensions of entangled surfactant wormlike micelles reversibly linked
by various amounts of telechelic polymers. We provide a state diagram that
delineates the regime of fracture without necking of the filament from the
regime where no fracture or break-up has been observed. We show that filaments
fracture when stretched at a rate larger than the inverse of the slowest
relaxation time of the networks. We quantitatively demonstrate that dissipation
processes are not relevant in our experimental conditions and that, depending
on the density of nodes in the networks, fracture occurs in the linear
viscoelastic regime or in a non-linear regime. In addition, analysis of the
crack opening profiles indicates deviations from a parabolic shape close to the
crack tip for weakly connected networks. We demonstrate a direct correlation
between the amplitude of the deviation from the parabolic shape and the amount
of non linear viscoelasticity
Instabilities in freely expanding sheets of associating viscoelastic fluids
We use the impact of drops on a small solid target as a tool to investigate
the behavior of viscoelastic fluids under extreme deformation rates. We study
two classes of transient networks: semidilute solutions of supramolecular
polymers and suspensions of spherical oil droplets reversibly linked by
polymers. The two types of samples display very similar linear viscoelastic
properties, which can be described with a Maxwell fluid model, but contrasting
nonlinear properties due to different network structure. Upon impact, weakly
viscoelastic samples exhibit a behavior qualitatively similar to that of
Newtonian fluids: A smooth and regular sheet forms, expands, and then retracts.
By contrast, for highly viscoelastic fluids, the thickness of the sheet is
found to be very irregular, leading to instabilities and eventually formation
of holes. We find that material rheological properties rule the onset of
instabilities. We first provide a simple image analysis of the expanding sheets
to determine the onset of instabilities. We then demonstrate that a Deborah
number related to the shortest relaxation time associated to the sample
structure following a high shear is the relevant parameter that controls the
heterogeneities in the thickness of the sheet, eventually leading to the
formation of holes. When the sheet tears-up, data suggest by contrast that the
opening dynamics depends also on the expansion rate of the sheet.Comment: accepted for publication in Soft Matte
A double rigidity transition rules the fate of drying colloidal drops
The evaporation of drops of colloidal suspensions plays an important role in
numerous contexts, such as the production of powdered dairies, the synthesis of
functional supraparticles, and virus and bacteria survival in aerosols or drops
on surfaces. The presence of colloidal particles in the evaporating drop
eventually leads to the formation of a dense shell that may undergo a shape
instability. Previous works propose that, for drops evaporating very fast, the
instability occurs when the particles form a rigid porous solid, constituted of
permanently aggregated particles at random close packing. To date, however, no
measurements could directly test this scenario and assess whether it also
applies to drops drying at lower evaporation rates, severely limiting our
understanding of this phenomenon and the possibility of harnessing it in
applications. Here, we combine macroscopic imaging and space- and time-resolved
measurements of the microscopic dynamics of colloidal nanoparticles in drying
drops, measuring the evolution of the thickness of the shell and the spatial
distribution and mobility of the nanoparticles. We find that, above a threshold
evaporation rate, the drop undergoes successively two distinct shape
instabilities. While the second instability is due to the permanent aggregation
of nanoparticles, as hypothesized in previous works on fast-evaporating drops,
we show that the first one results from a reversible glass transition of the
shell, unreported so far. We rationalize our findings and discuss their
implications in the framework of a unified state diagram for the drying of
colloidal drops
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