
Publications
[AL43] A. Lindner
Flow of complex suspensions
Phys. Fluids 26 101307 (2014)
.
Gels are a functional template for microparticle fabrication
and microbiology experiments. The control and knowledge of their
mechanical properties is critical in a number of applications, but
no simple in situ method exists to determine these properties. We
propose a novel microfluidic based method that directly measures
the mechanical properties of the gel upon its fabrication. We
measure the deformation of a gel beam under a controlled flow
forcing, which gives us a direct access to the Young's modulus of
the material itself. We then use this method to determine the
mechanical properties of poly(ethylene glycol) diacrylate (PEGDA)
under various experimental conditions. The mechanical properties
of the gel can be highly tuned, yielding two order of magnitude in
the Young's modulus. The method can be easily implemented to allow
for an in situ direct measurement and control of Young's moduli
under various experimental conditions.
[AL42] C. Duprat, H. Berthet, J. S. Wexler,
O. du Roure and A. Lindner
Microfluidic in situ mechanical testing of photopolymerized gels
Lab Chip, 2014, DOI: 10.1039/C4LC01034E.
.
Gels are a functional template for microparticle fabrication
and microbiology experiments. The control and knowledge of their
mechanical properties is critical in a number of applications, but
no simple in situ method exists to determine these properties. We
propose a novel microfluidic based method that directly measures
the mechanical properties of the gel upon its fabrication. We
measure the deformation of a gel beam under a controlled flow
forcing, which gives us a direct access to the Young's modulus of
the material itself. We then use this method to determine the
mechanical properties of poly(ethylene glycol) diacrylate (PEGDA)
under various experimental conditions. The mechanical properties
of the gel can be highly tuned, yielding two order of magnitude in
the Young's modulus. The method can be easily implemented to allow
for an in situ direct measurement and control of Young's moduli
under various experimental conditions.
[AL41] Chelsea S. Davis, Florian Lemoine,
Thierry Darnige, David Martina, Costantino Creton, and Anke
Lindner
Debonding Mechanisms of Soft Materials at Short Contact Times
Langmuir 2014, 30, 10626?10636, dx.doi.org/10.1021/la5023592
.
A carefully controlled, custombuilt adhesion testing device was
developed which allows a precise, short dwell time on the order of
milliseconds to be applied during a contact adhesion experiment.
The dwell time dependence of the adhesive strength of crosslinked
polydimethyl siloxane (PDMS) in contact with glass and
uncrosslinked styrene butadiene rubber (SBR) in contact with glass
and with itself was tested with a spherical probe in a confined
JohnsonKendallRoberts (JKR) geometry. Analysis of the contact
images revealed several unique separation mechanisms which are
dependent on dwell time and interfacial properties. PDMSGlass
interfaces show essentially no dependence of adhesion on the dwell
time while the adhesive strength and separation mechanisms of SBR
interfaces are shown to vary drastically for dwell times ranging
from 40 ms to 10,000 ms. This influence of dwell time is
particularly pronounced for polymerpolymer (SBRSBR) interfaces.
Observations of cavitation due to trapped air pockets in the
center of the contact at very short contact times illustrate a
transition between a defectcontrolled debonding and an
interfacecontrolled debonding which has not been previously
reported.
[AL40] Jeremie Gachelin, Annie Rousselet, Anke Lindner, Eric Clement
Collective motion in an active suspension of Escherichia coli bacteria
New J. Phys. 16 (2014) 02
.
We investigate experimentally the emergence of collective motion
in the bulk of an active suspension of E. coli bacteria. When
increasing the concentration from a dilute to a semidilute
regime, we observe a continuous crossover from a dynamical
cluster regime to a regime of `bioturbulence' convection
patterns. We measure a length scale characterizing the collective
motion as a function of the bacteria concentration. For bacteria
fullly supplied with oxygen, the increase of the correlation
length is almost linear with concentration and at the largest
concentrations tested, the correlation length could be as large as
24 bacterial body sizes (or 78 when including the agella bundle).
In contrast, under conditions of oxygen shortage the correlation
length saturates at a value around 7 body length.
[AL39] François Tanguy, Matteo Nicol, Anke
Lindner, and Costantino Creton
Quantitative analysis of the debonding structure of soft adhesives
Eur. Phys. J. E (2014) 37: 3
.
We experimentally investigate the growth dynamics of cavities
nucleating during the first stages of debonding of three different
model adhesives. The material properties of these adhesives range
from a more liquidlike material to a soft viscoelastic solid and
are carefully characterized by small strain oscillatory shear
rheology as well as large strain uniaxial extension. The debonding
experiments are performed on a probe tack setup. Using high
contrast images of the debonding process and precise image
analysis tools we quantify the total projected area of the
cavities, the average cavity shape and growth rate and link these
observations to the material properties. These measurements are
then used to access corrected effective stress and strain curves
that can be directly compared to the results from the uniaxial
extension.
[AL38] J. Zilz, R.J. Poole, M. Alves, C.
Wagner, C. Schäfer and A. Lindner
Serpentine channels: micro – rheometers for fluid relaxation times
Lab Chip, 2014, 14 (2), 351 – 358
.
We propose a novel device capable of measuring relaxation times
of viscoelastic fluids as small as 1 ms. In contrast to most
rheometers, which by their very nature are concerned with
producing viscometric or nearlyviscometric flows, here we make
use of an elastic instability that occurs in the flow of
viscoelastic fluids with curved streamlines. To calibrate the
rheometer we combine simple scaling arguments with relaxation
times obtained from first normalstress difference data measured
in a classical shear rheometer. As an additional check we also
compare these relaxation times to those obtained from Zimm theory
and good agreement is observed. Once calibrated, we show how the
serpentine rheometer can be used to access smaller polymer
concentrations and lower solvent viscosities where classical
measurements become difficult or impossible to use due to inertial
and/or resolution limitations. In the absence of calibration, the
serpentine channel can still be a very useful comparative or index
device.
[AL37] H. Berthet, M. Fermigier and A.
Lindner
Single fiber transport in a confined channel: microfluidic
experiments and numerical study.
Phys. Fluids 25, 103601 (2013)
.
We present an experimental and numerical study on the transport
of a single fiber confined in a microfluidic HeleShaw geometry.
The fiber has a square crosssection and a typical aspect ratio of
ten. We address the question of the fiber velocity as it is freely
transported by the flow, and study in particular its dependence on
the fiber orientation and confinement in the channel, defined as
the ratio of the fiber height with the channel height. Both
experiments and simulations are set so that the fiber suspended in
the middle of the channel height does not interact with the
lateral flow boundaries. At low confinements, the fiber velocity
is independent of the fiber orientation with the flow direction
and tends to the maximal velocity of the fluid when the
confinement tends to zero. The fiber slows down as the confinement
increases.We find that as the confinement reaches approximately
0.5, the orientation affects the fiber velocity: a fiber
perpendicular to the flow direction moves faster than a parallel
one. Consequently, a confined fiber transported in a microchannel
at an angle different from 0° or 90° with the flow direction will
drift towards a lateral wall, in the opposite direction found in
sedimenting fibers. We also characterize the perturbation caused
by the presence of the fiber on the flow field, and find that it
drops very quickly as the fiber confinement decreases.
[A36] R.J. Poole, A. Lindner, M.A. Alves
Viscoelastic secondary flows in serpentine channels
Journal of NonNewtonian Fluid Mechanics 201 (2013) 10–16
.
We report the results of a detailed numerical investigation of
inertialess viscoelastic fluid flow through threedimensional
serpentine (or wavy) channels of varying radius of curvature and
aspect ratio using the OldroydB model. The results reveal the
existence of a secondary flow which is absent for the equivalent
Newtonian fluid flow. The secondary flow arises due to the
curvature of the geometry and the streamwise first normal–stress
differences generated in the flowing fluid and can be thought of
as the viscoelastic equivalent of Dean vortices. The effects of
radius of curvature, aspect ratio and solventtototal viscosity
ratio on the strength of the secondary flow are investigated. The
secondary flow strength is shown to be a function of a modified
Deborah number over a wide parameter range.
[A35] Julia Nase, Osvanny Ramos, Costantino
Creton, and Anke Lindner
Debonding energy of PDMS: A new analysis of a classic adhesion
scenario
Eur. Phys. J. E (2013) 36: 103, DOI 10.1140/epje/i2013131033
.
We investigated the debonding energy between confined layers of
a soft elastic solid (PDMS) and a circular steel indenter in a
flat punch geometry. PDMS is extensively used in applications, but
also a widespread model system for fundamental research. Varying
systematically the pulling speed and the viscoelastic properties,
notably the modulus, we determined scaling laws for the debonding
energy. We showed that the debonding energy is independent of the
sample thickness. Applying a new approach and separating the crack
initiation and the propagation part of the force curves, we
analyzed the thickness dependence more precisely and we
demonstrated that the energy to propagate the crack at given
average speed does not only depend on the modulus, but also on the
sample thickness.
[A34] Satyam Bhuyan, François Tanguy, David
Martina, Costantino Creton, Anke Lindner and Matteo Ciccotti
Propagating cracks at the interface between viscoelastic Pressure
Sensitive Adhesives and model surfaces
Soft Matter, (2013), 9, 6515, DOI: 10.1039/c3sm27919g
.
The motion of the triple line during the debonding of a soft
viscoelastic adhesive from a rigid polymer surface has been
investigated quantitatively. In order to make the debonding
geometry simpler than the probe tack test, a new technique, called
the sticky wedge test, has been developed where the probe is
constituted by a horizontal cylinder instead of a flat punch. The
crack propagates in the elongated contact area between the
cylinder and the flat surface, thereby, allowing us to measure
optically in situ the cracktip velocity and the receding contact
angle of the debonding adhesive. Two model pressure sensitive
adhesives (PSA) based on poly(nbutyl acrylatecoacrylic acid)
with different molecular weights and branching levels and four
polymer substrates (rubbery or glassy at room temperature) were
used. Due to the soft and incompressible nature of the adhesive,
the strain energy release rate for this test geometry has been
estimated by the equations for the pure shear test geometry. The
results show three main new insights: first, despite significant
approximations, this novel approach holds promise to characterize
more quantitatively the interfacial crack propagation between soft
viscoelastic solids and hard surfaces and the relationship between
the applied energy release rate G and the cracktip velocity v has
been reliably established for eight combinations of the model
viscoelastic adhesives and surfaces. Second, at equivalent values
of the thermodynamic work of adhesion, the adhesion energy of both
the adhesives against the rubbery surface was much lower than that
against the glassy polymers. Third, surprisingly, the measured
receding contact angle of debonding was close to 90_ for all
adhesive/ surface combinations used.
[A33] J. Gachelin, G. Miño, H. Berthet, A.
Lindner, A. Rousselet and E. Clément
NonNewtonian viscosity of Ecoli suspensions
PRL 110, 268103 (2013), 10.1103/PhysRevLett.110.268103
.
The viscosity of an active suspension of E. coli bacteria is
determined experimentally as a function of the shear rate using a
Yshaped microfluidic channel. From the relative suspension
viscosity, we identify rheological thickening and thinning regimes
as well as situations at low shear rate where the viscosity of the
bacteria suspension can be lower than the viscosity of the
suspending fluid. In addition, bacteria concentration and velocity
profiles in the bulk are directly measured in the microchannel.
[A32] Dadhichi Paretkar, Marleen Kamperman,
David Martina, Jiahua Zhao, Costantino Creton, Anke Lindner, Anand
Jagota, Robert McMeeking and Eduard Arzt
Preload responsive adhesion: effects of aspect ratio, tip shape,
and alignment
J. R. Soc. Interface 2013 10, 20130171, (2013)
.
We tested the adhesive response of polymer surfaces structured
with arrays of cylindrical fibrils having diameters of 10–20 mm
and aspect ratios 1–2.4. Fibrils had two different tip shapes of
endflaps and round edges. A preloadinduced mechanical buckling
instability of the fibrils was used to switch between the states
of adhesion and nonadhesion. Nonadhesion in fibrils with round
edges was reached at preloads that caused fibril buckling, whereas
fibrils with endflaps showed adhesion loss only at very high
preloads. The round edge acted as a circumferential flaw
prohibiting smooth tip contact recovery leading to an adhesion
loss. In situ observations showed that, after reversal of
buckling, the endflaps unfold and reform contact under
prevailing compressive stress, retaining adhesion in spite of
buckling. At very high preloads, however, endflaps are unable to
reform contact resulting in adhesion loss. Additionally, the
endflaps showed varying contact adaptability as a function of the
fibril–probe alignment, which further affects the preload for
adhesion loss. The combined influence of preload, tip shape and
alignment on adhesion can be used to switch adhesion in
bioinspired fibrillar arrays.
[A31] M. van Deen, T. Bertrand, N. Vu, D.
Quéré, E. Clément and A. Lindner
Particles accelerate the detachment of viscous liquids
Rheologica Acta (2013), DOI 10.1007/s0039701306919
.
During detachment of a viscous fluid extruded from a nozzle a
filament linking the droplet to the latter is formed. Under the
effect of surface tension the filament thins until pinch off and
final detachment of the droplet. In this paper we study the effect
of the presence of individual particles trapped in the filament on
the detachment dynamics using granular suspensions of small volume
fractions (? < 6%). We show that even a single particle
strongly modifies the detachment dynamics. The particle perturbs
the thinning of the thread and a large droplet of fluid around the
particle is formed. This perturbation leads to an acceleration of
the detachment of the droplet compared to the detachment observed
for a pure fluid. We quantify this acceleration for single
particles of different sizes and link it to similar observations
for suspensions of small volume fractions. Our study also gives
more insight into particulate effects on detachment of more dense
suspensions and allows to explain the accelerated detachment close
to final pinch off observed previously (Bonnoit et al 2012).
[A30] J. Wexler, P. Trinh, H. Berthet, N.
Quennouz, O. du Roure, H. Huppert, A. Lindner and H. Stone
Bending of elastic fibres in viscous flows: the influence of
confinement
J. Fluid Mech. (2013), vol. 720, pp. 517544,
doi:10.1017/jfm.2013.49
.
We present a mathematical model and corresponding series of
micrfluidic experiments examining the flow of a viscous fluid past
an elastic fibre in a threedimensional channel. The fibre's axis
lies perpendicular to the direction of flow and its base is
clamped to one wall of the channel; the side walls of the channel
are close to the fibre, confining the flow. Experiments show that
there is a linear relationship between deflection and flow rate
for highly confined fibres at low flow rates, which inspires an
asymptotic treatment of the problem in this regime. The
threedimensional problem is reduced to a twodimensional model,
consisting of HeleShaw flow past a barrier, with boundary
conditions at the barrier that allow for the effects of
flexibility and threedimensional leakage. The analysis yields
insight into the competing effects of flexion and leakage, and an
analytical solution is derived for the leadingorder pressure
field corresponding to a slit that partially blocks a
twodimensional channel. The predictions of our model show
favourable agreement with experimental results, allowing
measurement of the fibre's elasticity and the flow rate in the
channel.
[A29] E. Altshuler, G. Miño, C.
PérezPenichet, L. del R?o, A. Lindner, A. Rousselet, and E.
Clément
Flowcontrolled densification and anomalous dispersion of E. coli
through a constriction
SoftMatter, 2012, DOI:10.1039/C2SM26460A
.
Dispersion and migration of bacteria under flow in tortuous and
confined structures such as porous or fractured materials is
related to a large spectrum of practical interest, but is still
poorly understood. Here, we address the question of transport and
dispersion of an E. coli suspension flowing through a
microfluidic channel with a funnellike constriction in its
center. We show a counterintuitive symmetry breaking of the
bacterial concentration, which increases significantly past the
funnel. This concentration enhancement persists over large
distances from the funnel and disappears at large flow rate
values. We map our results onto a one dimensional
convection–diffusion equation predicting quantitatively the
experimental results, without free parameters, when a conservative
nonlocal source term is introduced. This last term, measured
experimentally, represents a long range memory effect due to the
unbalance of wall adsorption and desorption processes past the
constriction. Our model experiment points out the generic
importance of considering such constriction effects in the
description of transport properties of active matter in porous
media. It also opens the possibility to control the concentration
of bacterial suspensions in microfluidic channels by simply
tuning the flow intensity or direction.
[A28] C. Davis, D. Martina, C. Creton, A.
Lindner and A. Crosby
Enhanced Adhesion of Elastic Materials to SmallScale Wrinkles
Langmuir (2012) DOI: 10.1021/la302314z
.
The adhesive properties of a material can be greatly affected
simply by wrinkling its surface. We show the importance of
selecting the wrinkle feature sizes (amplitude, b; and wavelength,
?) that complement the materialdefined length scale related to
the adhesion energy and modulus. A rigid circular cylindrical
punch patterned with aligned wrinkles ranging in amplitude from
0.5 to 5.0 ?m with a fixed aspect ratio of 0.1 is used to
characterize the adhesion of elastic films of smooth poly(dimethyl
siloxane) (PDMS). The crosslinker concentration used to form the
PDMS layers is varied to determine the impact of material
properties on wrinkled surface adhesion. The elastic films have an
average thickness of 240 ?m and the average probe radius is 1 mm,
leading to a confined contact scenario. The separation stress and
work of debonding are presented for each crosslinker
concentration with testing rates ranging over 3 orders of
magnitude. For stiffer films (10 wt % crosslinker, E' ? 3.00
MPa), small wrinkles (b ? 0.5 ?m) increase the separation stress
by nearly 200% relative to a smooth interface whereas large
wrinkles (b ?5.0 ?m) are shown to reduce adhesion significantly. A
substantial increase in the debonding energy is also observed for
these smallamplitude wrinkles contacting stiff materials. No
discernible impact of wrinkled surface topography on the adhesion
of softer (2 and 4 wt % crosslinker, 0.05 MPa < E' < 0.30
MPa) films is measured.
[A27] J. Zilz, R. J. Poole, M. A. Alves, D.
Bartolo, B. Levache and A. Lindner
Geometric scaling of purelyelastic flow instabilities
J. Fluid Mech. 712 (2012) 203218
.
A combined experimental, numerical and theoretical investigation
of the geometric scaling of the onset of a purely elastic flow
instability in serpentine channels is presented. Good qualitative
agreement is obtained between experiments, using dilute solutions
of flexible polymers in microfluidic devices, and
threedimensional numerical simulations using the upperconvected
Maxwell model. The results are confirmed by a simple theoretical
analysis, based on the dimensionless criterion proposed by Pakdel
& McKinley (Phys. Rev. Lett., vol. 77, 1996, pp. 2459–2462)
for onset of a purely elastic flow instability. Threedimensional
simulations show that the instability is primarily driven by the
curvature of the streamlines induced by the flow geometry and not
due to the weak secondary flow in the azimuthal direction. In
addition, the simulations also reveal that the instability is
timedependent and that the flow oscillates with a welldefined
period and amplitude close to the onset of the supercritical
instability
[A26] J. Boujlel, M. Maillard, A. Lindner,
G. Ovarlez, X. Chateau, P. Coussot
Boundary layers in pastes  Displacement of a long object through
a yield stress fluid
J. Rheol. 56(5), 10831108 (2012)
.
When it moves through a yield stress fluid, a solid object
continuously reaches and liquefies new solid regions, so that both
flow in liquid regions and deformations in solid regions occur. In
the present work, we focus on the displacement of a plate through
simple yield stress fluids (nonthixotropic). Through force vs
velocity and particle imaging velocimetry measurements with a
detailed analysis of the deformation history, we are able to
identify the solid and liquid regions and their respective role in
the flow characteristics. It is shown that the displacement of a
long object through a yield stress fluid gives rise to the
formation of a liquid boundary layer (BL) of uniform thickness at
short distance from the leading edge, while the rest of the
material remains solid. The original result is that the thickness
of this boundary layer, which is of the order of 10mm, only
slightly increases with velocity and does not tend to zero when
the velocity tends to zero, in contrast with usual flows of yield
stress fluids along solid surfaces. Moreover, it does not change
for significant variations of the rheological characteristics of
the fluid in its liquid regime. We show that these specific
characteristics of the liquid layer are mainly governed by the
progressive transition from an elastic solid to a liquid, starting
slightly ahead of the leading edge of the plate.
[A25] C. Bonnoit, T. Bertrand, E. Clement
and A. Lindner
Accelerated drop detachment in dense granular suspensions
Phys. Fluids 24, 043304 (2012)
.
We experimentally study the detachment of drops of granular
suspensions using a density matched model suspension with varying
grain volume fraction (? = 15% to 55%) and grain diameter (d =
20?m to 140?m). We show that at the beginning of the detachment
process, the suspensions behave as an effective fluid. The
detachment dynamics in this regime can be entirely described by
the shear viscosity of the suspension [R. J. Furbank and J. F.
Morris, Int. J. Multiphase Flow 33(4), 448–468 (2007)]. At later
stages of the detachment, the dynamics become independent of the
volume fraction and are found to be identical to the dynamics of
the interstitial fluid. Surprisingly, visual observation reveals
that at this stage, particles are still present in the neck. We
suspect rearrangements of particles to locally free the neck of
grains, causing the observed dynamics. Close to the final pinch
off, the detachment of the suspensions is further accelerated,
compared to the dynamics of pure interstitial fluid. This
acceleration might be due to the fact that the neck diameter gets
of the order of magnitude of the size of the grains and a
continuous thinning of the liquid thread is not possible any more.
The crossover between the different detachment regimes is a
function of the grain size and the initial volume fraction. We
characterize the overall acceleration as a function of the grain
size and volume fraction.
[A24] D. Martina, C. Creton, P. Damman, M.
Jeusette and A. Lindner
Adhesion of soft viscoelastic adhesives on periodic rough surfaces
Soft Matter, 2012, 8 (19), 5350  5357
.
We experimentally study adhesion of soft viscoelastic materials
on surfaces with periodic roughness. The materials tested are
partially crosslinked PDMS layers with a low shear elastic modulus
(below 0.1 MPa) and variable viscoelasticity. We use an elastic
surface instability to produce wrinkled PDMS surfaces. The hard
periodic surfaces are then molded in Norland resin from the master
surfaces. The periodic surface roughness is typically of the order
of 100 microns. For identical surface roughness, we can use two
surface topologies: peaks and holes. Using our model material and
the model surfaces, we study in detail the interplay between the
elastic penalties needed to conform the adhesive to the rough
surface and the enhanced viscous dissipation during debonding. In
this way we show, that adhesion on these surfaces can either be
enhanced or decreased as a function of the surface topology and
the viscoelastic properties of the material.
[A23] T. Bertrand, C. Bonnoit, E. Clement
and A. Lindner
Dynamics of drop formation in granular suspensions
Granular Matter (2012) 1–6:169–174
.
The presence of grains strongly modifies the detachment of drops
of a viscous liquid.We have shown previously that the detachment
of drops of granular suspensions takes place via different regimes
(Bonnoit et al. submitted, 2011). Here we study the influence of
the volume fraction of particles on the formation and shape of the
droplets by means of visual observations. We measure the minimal
neck diameter as well as the height of the detachment as a
function of time to quantify the evolution of the drop shape. We
also address the question of the thinning dynamics of the neck in
the different regimes. Linking the dynamics to the properties of
the effective fluid or to rearrangements of individual grains in
the thread gives insights in the origin of the different regimes.
[A22] J. Nase, D. Derks and A. Lindner
Dynamic evolution of fingering patterns in a lifted HeleShaw
cell
Physics of Fluids, (2011) 23, 123101
.
We present a study on pattern formation in a Newtonian liquid
during lifting of a circular Hele–Shaw cell. When a confined layer
of oil is subject to such a stretch flow, air penetrates into the
liquid from the sides and a fingering instability, a variant of
the classical Saffman–Taylor instability, evolves. This setting
has the particularity that the finger growth takes place in a
conserved volume of liquid and that the dimensionless surface
tension, the control parameter which governs the Saffman–Taylor
instability, is changing with time. This leads to a constantly
evolving pattern, which we investigate with regard to number of
fingers and finger amplitude. We distinguish in the pattern at
each instant growing fingers and stagnant fingers. Systematically
varying the properties of the viscous oil and the geometry of the
Hele–Shaw cell, we show that the number of growing fingers is at
each moment well described by a simple approach based on linear
stability analysis and depends only on the dimensionless surface
tension. In contrast, the finger amplitude and consequently the
total number of fingers (growing and stagnant fingers) depend also
on the cell confinement. We demonstrate that the finger amplitude
has a distinct influence on the debonding force. Higher finger
amplitude and number of fingers lead to lower forces.
[A21] C. Bonnoit, J. Lanuza, A. Lindner and
E. Clement
A diverging mesoscopic length scale controls the rheology of dense
suspensions
Phys. Rev . Lett. (2010) 105, 108302
.
From the flow properties of dense granular suspensions on an
inclined plane, we identify a mesoscopic length scale strongly
increasing with volume fraction. When the flowing layer height is
larger than this length scale, a diverging Newtonian viscosity is
determined. However, when the flowing layer height drops below
this scale, we evidence a nonlocal effective viscosity, decreasing
as a power law of the flow height. We establish a scaling relation
between this mesoscopic length scale and the suspension viscosity.
These results support recent theoretical and numerical results
implying collective and clustered granular motion when the jamming
point is approached from below.
[A20] E.Wandersman , N. Quennouz , M.
Fermigier, A. Lindner, and O. du Roure
Buckled in translation
Soft Matter, 2010, DOI: 10.1039/C0SM00132E
.
We report experiments on the deformation andtransport of an
elastic fiber in a viscous cellular flow, namely alattice of
counterrotative vortices. We show that the fiber can bucklewhen
approaching a stagnation point. By tuning either the flow or
fiberproperties, we measure the onset of this buckling
instability. Thebuckling threshold is determined by the relative
intensity of viscousand elastic forces, the elastoviscous number
Sp. Moreover we show thatflexible fibers escape faster from a
vortex (formed by closedstreamlines) compared to rigid fibers. As
a consequence, thedeformation of the fiber changes its transport
properties in thecellular flow.
[A19] J. Nase, O.Ramos, C. Creton, L.
Sonneberg, T. Yamaguchi and A. Lindner
A Measurement of the receeding contact angle at the interface
between aviscoelastic material and a rigid surface
Soft Matter, 6 (2010) 26852691
.
We present the first quantitative measurement ofthe receding
contact angle between a soft viscoelastic material and a solid
surface during thedebonding of the material. We developed a
newmethod of observation, based on the technique pioneered by
Yamaguchi et al.. In a probetack geometry, we obtained a 3Dlike
view ofthe debonding pattern as seen through the adhesive/probe
interface, providing thus an excellent view of the details of the
contact geometrynear the interface. Combining visual and
forcedisplacement information in a probe tack test, weidentified
three different categories of mechanismsand contact geometries as
a function of the material parameters. For very weakly
crosslinked viscoelastic materials around the gel point, air
fingersform in the bulk of the layer. For higher degrees
ofcrosslinking, the air fingers form and propagate atthe
interface, with a large deformation of the bulkof the layer. A
well defined and reproducible receding contact angle ? < 90?
depending on theviscoelastic properties of the layer was observed
atthe leading edge of the moving finger. Finally forwell
crosslinked samples the failure occurs by interfacial crack
propagation with little deformationof the layer; the contact angle
at the leading edgethen was close to 90° as expected from elastic
fracture mechanics.
[A18] C. Bonnoit, T.Darnige, E. Clement,
and A. Lindner
Inclined plane rheometry of a dense granular suspension
J. Rheol. 54 (2010) 6579
We present a new method to measure the viscosityof a dense model
suspension using an inclinedplane rheometer. The suspension is
made of monodisperse, spherical,nonBrownian polystyrenebeads
immersed in a density matched silicon oil. We show that with
thissimple setup, theviscosity can be directly measured up to
volume fractions of ??=61% andthat particle migrationcan be
neglected. The results are in excellent agreement with
localviscosity measurementsobtained by magnetic resonance imaging
techniques by Ovarlez et al. [J.Rheol. 50(3), 259–292,2006]. In
the high density regime, we show that the viscosity is withinthe
tested range ofparameters, independent of the shear rate and the
confinement pressure.Finally, we discussdeviations from the
viscous behavior of the suspensions
[A17] A. Lindner andC. Wagner
Viscoelastic surface instabilities
C.R. Physique (2009) 712–727
We review three different types of viscoelasticsurface
instabilities: The Rayleigh–Plateau, the Saffman–Taylor and
theFaradayinstabilities. These instabilities are classical
examples ofhydrodynamic surface instabilities. The addition of a
small amount ofpolymer to pure water can alter its flow behavior
drastically and thetype of instability may change not only
quantitatively but alsoqualitatively. We will show that some of
the observed new phenomena canbe explained by the use of simple
rheological modelsthat contain most of the underlying physical
mechanisms leading to theinstability. A quantitative description
however is often onlypossible close to the onset of the
instability or for weak deviationsfrom Newtonian behavior. A
complete theoretical description isstill lacking when the system
is driven far from equilibrium or forfluids with strong
nonNewtonian behavior.
[A16] Ø. Johnsen, C.Chevalier, A. Lindner,
R. Toussaint, E. Clément, K.J. Måløy, E. Flekkøyand J. Schmittbuhl
Decompaction and fluidization of a saturated and confined
granularmedium by injection of a viscous liquid or a gas
Phys. Rev. E 78, 051302 (2008)
We compare quantitatively two experimentalsituations concerning
injection of a miscible fluid into an initiallyjammed granular
medium saturated with the same fluid, confined in aHele–Shaw cell.
Both experiments are identical, apart from theinterstitial and
injected fluid, which is in one case air injected intoa dry
granular packing, and in the other case silicone oil injectedinto
a dense suspension. In spite of the strong differences
regardingthe nature of the two fluids, strikingly similar
dynamical andgeometrical features are identified as function of
the controlparameters: cell thickness and applied fluid injection
pressure. Inboth cases an initial hydrodynamically driven
decompaction process iscontrolling the unjamming and is preparing
the final displacementprocess characterized by finger–like
patterns empty of grains. Thepattern shapes are comparable. In
addition, the mobilities of thecoupled fluid/grain flow, rescaled
by the interstitial fluid viscosityand grain diameter squared, are
of the same range and behavecomparably. The mobility proves to
depend on the initial solid fractionof the medium. Subtle
differences are observed in geometrical aspectslike the finger
width with respect to the control parameters.
[A15] J. Nase, A.Lindner and C. Creton
Pattern formation during deformation of a confined
viscoelasticlayer:From a viscous liquid to a soft elastic solid
Phys. Rev. Lett. 101, (2008), 074503
We study pattern formation during tensiledeformation of confined
viscoelastic layers. The use of a model
system[poly(dimethylsiloxane) with different degrees of
crosslinking] allowsus to go continuously from a viscous liquid
to an elastic solid. Weobserve two distinct regimes of fingering
instabilities: a regimecalled ‘‘elastic’’ with interfacial crack
propagation, where thefingering wavelength scales only with the
film thickness, and a bulkregime called ‘‘viscoelastic,’’ where
the fingering instability shows aSaffmanTaylorlike behavior. We
find good quantitative agreement withtheory in both cases and
present a reduced parameter describing thetransition between the
two regimes and allowing us to predict theobserved patterns over
the whole range of viscoelastic properties.
[A14] C. Chevalier,A. Lindner et E. Clément
Morphodynamics during air injection into a confined granular
suspension
J. NonNewtonian Fluid Mech. 158 (2009) 63–72
We study the injection of air into a granularsuspension in a
confined geometry. The grains and the suspending fluidare density
matched allowing to vary the grain fraction ? over thewhole range
accessible: from ? = 0 up to close to the maximum packingfraction
? ~ 60%. In this situation, a large variety of patterns
isobserved, that we link directly to the rheology of the
suspension inthe confined geometry. For low grain fractions ? <
50%, we showthat taking into account an effective viscosity for
the suspensionflow, resulting from particle migration, allows to
link quantitativelythe observed structures to the classical
Saffman–Taylor selectionmechanism. For higher grain fractions ?
> 55% the granularmatrix is jammed and we deal with a weak
porous medium. In this case welink the observed patterns to the
reorganization occurring in theporous medium as a consequence of
the injection process.
[A13] C. Chevalier,A. Lindner et E. Clément
Destabilization of a SaffmanTaylor Fingerlike Pattern in a
GranularSuspension
Phys. Rev. Lett, 99 (2007) 174501
We study the SaffmanTaylor instability in agranular suspension
formed by micrometric beads immersed in a viscousliquid. When
using an effective viscosity for the flow of thesuspension in the
HeleShaw cell to define the control parameter of thesystem, the
results for the finger width of stable fingers are found tobe
close to the classical results of SaffmanTaylor. One
observes,however, an early destabilization of the fingers that can
be attributedto the discrete nature of the individual grains.
Classically, thethreshold of destabilization is linked to the
noise in the cell and isthus difficult to quantify. We show that
the grains represent a‘‘controlled noise’’ and produce an initial
perturbation of theinterface with an amplitude proportional to the
grain size. The finiteamplitude instability mechanism proposed by
Bensimon et al. allows usto link this perturbation to the value of
the threshold observed.
[A12] A. Lindner, B.Lestriez, S. Mariot, T.
Maevis, R. Brummer, B. Luehmann et C. Creton
Adhesive and rheological properties of lightly crosslinked
modelacrylic networks
Journal of Adhesion, 82 (2006) 267310
The viscoelastic and adhesive properties of aseries of model,
lightly crosslinked acrylic polymer networks have
beeninvestigated. The model networks were statistical copolymers
of2ethylhexyl acrylate and acrylic acid or terpolymers of
2ethylhexylacrylate, acrylic acid, and stearyl acrylate
synthesized in solution.All were lightly crosslinked after the
polymerization was completed toobtain typical properties of
pressuresensitive adhesives. The bulkrheological properties of
the networks were characterized by dynamicalmechanical
spectroscopy and in uniaxial extension. Their adhesiveproperties
were tested with an instrumented probe tester fitted with
acylindrical steel probe. The presence of acrylic acid in the
copolymercaused an increase in both elastic modulus and resistance
tointerfacial crack propagation characterized by the
criticalenergyrelease rate Gc and the incorporation of stearyl
acrylate causeda decrease in both modulus and Gc. In both cases,
however, themodification of Gc controlled the overall behavior.
The analysis of thenonlinear elastic properties of the adhesives
with the Mooney–Rivlinmodel provided new insights on the role
played by the ratio betweenentanglements and crosslink points in
controlling the formation andextension of the bridging fibrils
observed upon debonding.
[A11] C. Chevalier,M. Ben Amar, D. Bonn et
A. Lindner
Inertial effects on Saffman –Taylor viscous fingering
J. Fluid Mech., 552 (2006) 8397
For the Saffman–Taylor instability, the inertia ofthe fluid may
become important for high finger speeds. We investigatethe effects
of inertia on the width of the viscous fingersexperimentally. We
find that, due to inertia, the finger width canincrease with
increasing speed, contrary to what happens at smallReynolds number
Re. We find that inertial effects need to be consideredabove a
critical Weber number We.In this case it can be shown that the
finger width is governed by abalance between viscous forces and
inertia. This allows us to define amodified control parameter
1/B’, which takes the corrections due toinertia into account; on
rescaling the experimental data with 1/B’,they all collapse onto
the universal curve for the classicalSaffman–Taylor instability.
Subsequently, we try to rationalize ourobservations. Numerical
simulations, taking into account a modificationof Darcy’s law to
include inertia, are found to only qualitativelyreproduce the
experimental findings, pointing to the importance
ofthreedimensional effects.
[A10] C. Caballero,E. Kolb, A. Lindner, J.
Lanuza et E. Clément
Experimental investigation of granular dynamics close to the
jammingtransition
J. Phys.: Cond. Matter 17 (2005) S2503
We present different experiments on dense granularassemblies with
the aim to clarify the notion of ”jamming transition”for these
assemblies of nonBrownian particles. The experimentalsetups
differ in the way external perturbations are applied in orderto
unjamm the systems. The first experiment monitors the response to
alocally applied deformation of a model packing at rest. The two
otherexperiments study local and collective dynamics in a granular
assemblyweakly excited by vibration.
[A9] A. Lindner, D.Derks et M. Shelley
Stretchflow of thin layers of Newtonian liquids : Fingering
patternsand lifting forces
Phys. Fluids, 17, 072107, (2005)
We study the stretch flow of a thin layer ofNewtonian liquid
constrained between two circular plates. The evolutionof the
interface of the originally circular bubble is studied whenlifting
one of the plates at a constant velocity and the observedpattern
is related to the measured lifting force. By comparingexperimental
results to numerical simulations using a Darcy’s law modelwe can
account for the fully nonlinear evolution of the observedfingering
pattern. One observes an initial destabilization of theinterface
by growth of air fingers due to a Saffman–Taylorlikeinstability
and then a coarsening of the pattern toward a circularinterface
until complete debonding of the two plates occurs.
Numericalsimulations reveal that when relating the observed
patterns to thelifting force not only the number of fingers but
also the amplitude ofthe fingering growth has to be taken into
account. This is consistentwith the experimental observations.
[A8] N.MalekiJirsaraei, A. Lindner, S.
Rouhani et D. Bonn
Saffman – Taylor instability in yield stress fluids
J. Phys.: Condens. Matter, J. Phys.: Cond. Matter 17 (2005) S1219
Pushing a fluid with a less viscous one gives riseto the well
known Saffman–Taylor instability. This instability isimportant in
a wide variety of applications involving stronglynonNewtonian
fluids that often exhibit a yield stress.Here we investigate the
Saffmann–Taylor instability in this type offluid, in longitudinal
flows in Hele–Shaw cells. In particular, westudy Darcy’s law for
yield stress fluids. The dispersion equation forthe flow is
similar to the equations obtained for ordinary viscousfluids but
the viscous terms in the dimensionless numbers conditioningthe
instability now contain the yield stress.This also has
repercussions on the wavelength of the instability as itfollows
from a linear stability analysis. As a consequence of thepresence
of yield stress, the wavelength of maximum growth is finiteeven at
vanishing velocities. We study Darcy’s law and the
fingeringpatterns experimentally for a yield stress fluid in a
linear Hele–Shawcell. The results are in rather good agreement
with the theoreticalpredictions. In addition we observe different
regimes that lead todifferent morphologies of the fingering
patterns, in both rectangularand circular Hele–Shaw cells.
[A7] A. Lindner, T.Maevis, R. Brummer, B.
Lühmann et C. Creton
Subcritical failure of soft acrylic adhesives under tensile
stress
Langmuir 20 (2004) 9156
The mechanisms of failure over time of a series ofmodel acrylic
pressuresensitive adhesives under a moderate level ofstress has
been investigated with a probe method. Two competingmechanisms of
failure have been observed: a progressive nucleation ofcavities
under stress and the propagation of existing cavities at
theinterface between the probe and the adhesive layer. Homogeneous
creepof the adhesive was never observed as the only failure
mechanism. Insituations where the resistance to crack propagation
was good relativeto the resistance to cavitation, extensive
nucleation of cavities wasobserved until a materialdependent and
stable value of stress wasachieved. On the other hand in
situations were the resistance to crackpropagation was weak,
propagation led invariably to a complete failureof the adhesive
bond. In addition to the stress relaxation, the energydissipation
was studied allowing to distinguish the different adhesiveseven
further. This allowed determination of the optimal amount of
acomonomer (acrylic acid) that had to be added to improve the
longtermresistance of the adhesives under study. Further more we
investigatethe compliance of the confined adhesive layers and
compare the obtainedresults to predictions from theoretical
models.
[A6] D. Derks, A.Lindner C. Creton et D.
Bonn
Cohesive failure of thin layers of soft model adhesives under
tension
J. Appl. Phys. 93 (2003) 1557
The cohesive failure of soft adhesives is studiedusing a yield
stress fluid as a model adhesive, which allow to relatethe
viscoelastic properties of the fluid directly to its
adhesiveperformance. We derive a theoretical expression for the
force—distancecurve as a function of the yield stress, which
describes ourexperimental results very well. The theoretical
prediction is obtainedby assuming a circular air—adhesive
interface; surprisingly, goodagreement between theory and
experiment is also obtained when strongfingering instabilities are
observed in the experiment. This result isconfirmed by the fact
that we do not find a significant reduction inthe work of adhesion
when fingering instabilities are present. Inaddition, we discuss
the morphology of the fingering instabilities.
[A5] A. Lindner, J.Vermant et D. Bonn
Elongational viscosity of dilute polymer solutions
Physica A 319 (2003) 125
Measurement of the resistance to a stretchingmotion, the
socalled elongational viscosity h_{e},has
proven
to bedifficult for dilute polymer solutions, although it is
important for alarge number of applications. We attempt to
deduce h_{e}of
dilutesolutions
of flexible polymers in two different ways. First, h_{e}can
beinferred
from measurements of the shear viscosity h
and the firstnormal stress difference N1, invoking an appropriate
constitutiveequation. Second, it is measured in the opposing jet
elongationalviscometer. Although the results of the latter method
have been muchdebated they agree reasonably well with those from
the first method.The combination of the two methods thus allows to
obtain the correctorder of magnitude of h_{e}
[A4] A. Lindner, D.Bonn, E . Corvera Poiré,
M.Ben Amar et J. Meunier
Viscous fingering in nonNewtonian fluids
J.Fluid Mech. 469 (2002) 237
We study the viscous fingering or SaffmanTaylorinstability in
two different dilute or semidilute polymer solutions.The
different solutions exhibit only one nonNewtonian property, in
thesense that other nonNewtonian effects can be neglected. The
viscosityof solutions of stiff polymers has a strong shear rate
dependence.Relative to Newtonian fluids, narrower fingers are
found for rigidpolymers. For solutions of flexible polymers,
elastic effects such asnormal stresses are dominant, whereas the
shear viscosity is almostconstant. Wider fingers are found in this
case. We characterize thenonNewtonian flow properties of these
polymer solutions completely,allowing for separate and
quantitative investigation of the influenceof the two most common
nonNewtonian properties on the SaffmanTaylorinstability. The
effects of the nonNewtonian flow properties on theinstability can
in all cases be understood quantitatively by redefiningthe control
parameter of the instability.
[A3] A. Lindner, P.Coussot et D. Bonn
Viscous fingering in a yield stress fluid
Phys. Rev. Lett. 85 (2000) 314
We study the SaffmanTaylor or viscous fingeringinstability in
yield stress fluids. The theory for yield stress fluidsshows that
the dispersion equation of the instability is similar tothat for
Newtonian fluids; however, the capillary number governing
theinstability now contains the yield stress. Experiments using
gels andfoams reveal very branched fingers in the gel. The results
are inexcellent agreement with theory for the gel, with, in
addition, acrossover from yield stress dominated to viscous
behaviour. The resultsfor foams are very different due to the
existence of wall slip.
[A2] A. Lindner, D.Bonn et J. Meunier
Viscousfingering in complexfluids
Phys.: Condensed Matter 12 (2000), A477
Viscous fingers form when a less viscous fluidpushes a more
viscous fluid in a linear channel. The instability of theinterface
results from a competition between viscous and capillaryforces. We
show here that by using complex fluids such as polymer
orsurfactant solutions one can act on the viscosity or the
surfacetension and modify the instability drastically. Two
different polymersolutions, that exhibit either shear thinning or
normal stress effects,are used. For the first fluid the viscous
forces are altered leading tofinger narrowing, whereas for the
second fluid the viscous forces aresupplemented by normal
stresses, which leads to finger widening. Forthe surfactant
solutions the modification of the capillary forces leadsto finger
widening.
[A1] A. Lindner, D.Bonn et J. Meunier
Viscousfingering in a shear thinning fluid
Physics of Fluids, 12 (2000) 256
We study the Saffman–Taylor instability in arectangular HeleShaw
cell. The driven fluid is a dilute or semidilutepolymer solution,
with a viscosity that exhibits shear thinning. OthernonNewtonian
properties such as elastic effects are negligible underthe present
experimental conditions; the system thus allows forseparate
investigation of the influence of shear thinning on
theinstability. The experiments show that, for weak
shearthinning, theresults for the width of the fingers as a
function of the capillarynumber collapse onto the universal curve
for Newtonian fluids, providedthe shearthinning viscosity is used
to calculate the capillary number.For stronger shear thinning,
narrower fingers are found. The experimentallows also for a study
of the applicability ofDarcy’s law to shear thinning fluids. For
Newtonian fluids, this lawgives the finger velocity as a function
of the pressure gradient. Forweakly shearthinning fluids, we find
that an effective Darcy’s law, inwhich the constant viscosity is
replaced by the shearthinningviscosity, gives good agreement with
the experiments. For strongershear thinning, the predictions from
the effective Darcy’s lawdeteriorate. Satisfactory agreement with
experimental data can beobtained when using a ‘‘shearthinning’’
Darcy’s law, which can bederived using a power law model for the
shear rate dependence of theviscosity.

