78 research outputs found
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
The Role of Solid Friction in the Sedimentation of Strongly Attractive Colloidal Gels
We study experimentally and theoretically the sedimentation of gels made of
strongly aggregated colloidal particles, focussing on the long time behavior,
when mechanical equilibrium is asymptotically reached. The asymptotic gel
height is found to vary linearly with the initial height, a finding in stark
contrast with a recent study on similar gels [Manley \textit{et al.} 2005
\textit{Phys. Rev. Lett.} \textbf{94} 218302]. We show that the asymptotic
compaction results from the balance between gravity pull, network elasticity,
and solid friction between the gel and the container walls. Based on these
ingredients, we propose a simple model to account for the dependence of the
height loss on the initial height and volume fraction. As a result of our
analysis, we show that the static friction coefficient between the gel and the
container walls strongly depends on volume fraction: the higher the volume
fraction, the weaker the solid friction. This nonintuitive behavior is
explained using simple scaling arguments.Comment: 13 pages, 5 figures. Submitted to JSTA
Structure and rheological properties of model microemulsion networks filled with nanoparticles
Model microemulsion networks of oil droplets stabilized by non ionic
surfactant and telechelic polymer C18-PEO(10k)-C18 have been studied for two
droplet-to-polymer size ratios. The rheological properties of the networks have
been measured as a function of network connectivity and can be described in
terms of simple percolation laws. The network structure has been characterised
by Small Angle Neutron Scattering. A Reverse Monte Carlo approach is used to
demonstrate the interplay of attraction and repulsion induced by the copolymer.
These model networks are then used as matrix for the incorporation of silica
nanoparticles (R=10nm), individual dispersion being checked by scattering. A
strong impact on the rheological properties is found for silica volume
fractions up to 9%
Fracture of jammed colloidal suspensions
Concentrated colloidal suspensions display dramatic rises in viscosity, leading to jamming and granulation, with increasing shear rate. It has been proposed that these effects result from inter particle friction, as lubrication forces are overcome. This suggests the jamming of concentrated colloidal suspensions should exhibit some shared phenomenology with macroscopic granular systems where friction leads to two different types of jammed state. Here we show that transient rheological measurements can be used to probe the processes of granulation in concentrated colloidal suspensions. Our results support the idea that frictional contacts are created between jammed particles. The jamming behaviour displays two qualitatively different regimes separated by a critical strain rate with qualitatively different types of fracture/break up behaviour. In the lower strain rate regime, it is found that vibrations can be used to control jamming and granulation, resulting in a flowable fluid
Bending elasticity of a curved amphiphilic film decorated anchored copolymers: a small angle neutron scattering study
Microemulsion droplets (oil in water stabilized by a surfactant film) are
progressively decorated with increasing amounts of poly ethylene- oxide (PEO)
chains anchored in the film by the short aliphatic chain grafted at one end of
the PEO chain . The evolution of the bending elasticity of the surfactant film
with increasing decoration is deduced from the evolution in size and
polydispersity of the droplets as reflected by small angle neutron scattering.
The optimum curvature radius decreases while the bending rigidity modulus
remains practically constant. The experimental results compare well with the
predictions of a model developed for the bending properties of a curved film
decorated by non-adsorbing polymer chains, which takes into account, the finite
curvature of the film and the free diffusion of the chains on the film.Comment: 30 June 200
Adsorption dynamics of hydrophobically modified polymers at an air-water interface
The adsorption dynamics of a series of hydrophobically modified polymers, PAAαCn, at the air-water interface is studied by measuring the dynamic surface tension. The PAAαCn are composed of a poly(acrylic acid) backbone grafted with a percentage α of C8 or C12 alkyl moieties, at pH conditions where the PAA backbone is not charged. The observed adsorption dynamics is very slow and follows a logarithmic behavior at long times indicating the building of an energy barrier which grows over time. After comparison of our experimental results to models from the literature, a new model which accounts for both the deformation of the incoming polymer coils as well as the deformation of the adsorbed pseudo-brush is described. This model enables to fit very well the experimental data. The two fitting parameters give expected values for the monomer size and for the area per adsorbed polymer chain.This article is uploaded in "arXiv.org"
https://arxiv.org/abs/1706.0710
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