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Scientific Highlights 2011

Two step yielding in attractive colloids: transition from gels to attractive glasses

N. Koumakis and G. Petekidis
SoftComp partner: FORTH, Greece
Soft Matter 7 (2011) 2456

Steady and oscillatory rheology was utilized to study the mechanical response of colloidal glasses and gels with particular emphasis in their yielding behaviour. We used a suspension of hard sphere colloidal particles with short-range depletion attractions induced by the addition of non-adsorbing linear polymer. While high volume fractions hard sphere glasses exhibit a single yield point due to cage breaking, attraction dominated glasses show a two-step yielding reflecting bond and cage breaking respectively. Here we investigated the yielding behaviour of frustrated colloid-polymer systems with equal attraction strength and range, varying the particle volume fraction, φ, spanning the region from an attractive glass (φ=0.6) to a low volume fraction (φ=0.1) attractive gel. Yielding throughout this range, probed both by oscillatory and steady shear, is found to remain a two step process until very low φ’s. The first yield strain related with in-cage or inter-cluster bond braking remains constant for φ>0.3 while the second yield strain, attributed to braking of cages or clusters into smaller constituents, increases as volume fraction is decreased due to enhancement of structural inhomogeneities in the gel. Steady shear tests indicated distinct shear rate regimes: At steady state, low and intermediate shear rates create denser or smaller flowing clusters, whereas high rates may lead to complete break-up into independent particles. When the range of attraction was increased, both yield strains increased and scaling with the range of attraction and accompanied structural changes. Finally, ageing leads to an overall strengthening of both the gel and the attractive glass accompanied by an enhancement of the second stress overshoot in steady shear, while the attractive glass also becomes more brittle.

Left: Step rate test in a repulsive glass of 4 1⁄4 0.60, an attractive glass at 4 1⁄4 0.60 and a gel at 4 1⁄4 0.44 for a rate of 0.5 s-1. Arrows indicate single and double yielding for repulsive and attractive glass/gel respectively. Right: Structure representation of a lower 4 gel at rest and under shear. Increasing from left to right is the applied strain in both steady (g) and oscillatory (g0) experiments. Increasing from bottom to top is the shear rate (g_ ) or frequency of oscillation. The scheme describes how different regimes of strain rate and applied strain lead to different structural properties under shear.

Blood Rheology

D.A.Fedosov, W.Pan, B.Caswell, G.Gompper and G.E.Karniadakis
SoftComp partner: Forschungszentrum Juelich (Group Prof. Gompper), Germany
Proc. Natl. Acad. Sci. USA 108 (2011) 11772

We study blood rheology using a particle-based mesoscopic simulation technique. The blood model accurately predicts the dependence of blood viscosity on shear rate and hematocrit. Specifically, RBC aggregation interactions lead to the formation of reversible rouleaux structures and to a tremendous increase of blood viscosity at low shear rates. The simulations provide the first quantitative estimate of the magnitude of adhesive forces between RBCs and to relate the non-Newtonian blood behavior with the suspension's microstructure, deformation and dynamics of single RBCs. The model can easily be generalized to study a much wider class of complex fluids including capsule and vesicle suspensions.

A Simulation snapshot of reversible rouleaux formation with hematocrit H=10% and shear rate 0.4 s-1. An attractive force of only 3 to 7 pico-Newton between red blood cells is estimated from the comparison the experimental data.

Friction between Individual Star Polymers

S.P.Singh, R.G.Winkler and G.Gompper
SoftComp partner: Forschungszentrum Juelich (Group Prof. Gompper), Germany
Physical Review Letters 107 (2011) 158301

We focus here on the (time-dependent) friction between individual star polymers in solution. Our studies provide insight into the universal non-equilibrium effective friction forces and structural changes. In particular, we show that on departure polymer repulsion turns into attraction at larger drag velocities. This behavior can be traced back to the retardation of polymer relaxation and symmetry breaking of the polymer conformations relative to the mid-plane between the polymer centers.

Simulation snapshot of two stars dragged past each other with constant velocity vd and separated by a constant vertical distance Rd.

Dynamics of entangled polymers in the presence of nanoparticles

T.E. Ouldridge, A.A. Louis and J.P.K. Doye
SoftComp partner: Univ. Oxford, United Kingdom
J. Chem. Phys. 134 (2011) 085101

DNA nanotechnology is one of the "hottest" topics in nanotechology, because the selectivity of Watson-Crick base pairing allows self-assembling nanoscale structures and devices to be designed with unprecedented control and addressability. However, the contribution of molecular simulation to this field has so far been minimal, both because atomistic simulations cannot address the relevant time and length scales, and because there has not been a suitable coarse-grained model available. This is set to change with the DNA model introduced in this paper. It is simple enough to allow DNA self-assembly to be efficiently simulated, yet also provides an accurate description of the structural, mechanical and thermodynamic properties of both single-stranded and duplex DNA. The potential applications of the model are considerable, as it will enable the self-assembly processes associated with DNA nanotechnology to be visualized for the first time, and potentially contribute to the rational design of new nanostructures and devices.

A 12-base DNA duplex, and an illustration of the relative flexibility of (a) duplex DNA, and (b) stacked and (c) unstacked single-stranded DNA in our model.

Acceleration of Complex Fluids Near a Wall and its Meaning for Enhanced Oil Recovery

H.Frielinghaus, M.Kerscher, O.Holderer, M.Monkenbusch and D.Richter
SoftComp partner: Forschungszentrum Juelich (Group Prof. Richter), Germany
Physical Review Letter, submitted; Physical Review, E83 (2011) 030401

In the enhanced oil recovery complex fluids are pumped to the oil field for various reasons. The fracturing fluid deposits the pressure energy inside the sandstone close to the bore hole due to the high viscosity. The generated cracks serve higher recovery rates after the application. Often microemulsions form when the aqueous surfactant system comes in contact with the oil.

The dynamics of surface near microemulsions have been characterized by grazing incidence neutron spin echo spectroscopy (GINSES). From this method we obtained detailed depth information of the dynamics for the first time. We have found a three times faster relaxation adjacent to the surface compared to the bulk. The wall-reflected hydrodynamic field explains the faster dynamics. The static structure of this system has been characterized in a previous study. While in the bulk a bicontinuous structure is formed, the surface near structure is lamellar.

Relaxation times obtained from a single stretched exponential fit as a function of the scattering depth. The solid line arises from intensity ratios of static measurements.

Continuous droplet interface crossing encapsulation (cDICE) for high throughput monodisperse vesicle design

M.Abkarian, E.Loiseau and G.Massiera
*SoftComp partner: CNRS Montpellier, France
Soft Matter, 7 (2011) 4610

A vesicle is a capsule made of a closed amphiphile bilayer separating an inner fluid from an outer one. Vesicles are thus of particular interest in application such as microencapsulation, controlled release, drug or O2 carriers and on a more fundamental point of view because they serve as elementary models of living cells. By continuously dripping droplets off a capillary and forcing their passage through an interface using a centrifugal force on a single set-up, content- and size-tunable monodisperse vesicles suspensions can be produced in minutes. Materials as diverse as actin filaments, high molecular weight polymers, high ionic strength solutions, micron-sized particles or even red blood cells are examples of aqueous solutions that can be encapsulated. Overcoming some of the current techniques limitations, such a simple technique called continuous droplet interface crossing encapsulation (cDICE) has a great potential in various fields from encapsulation to the design of biomimetic cells and artificial tissues.

Schematic side view of the setup. Examples of the suspensions encapsulated in the vesicles. From left to right: 1 micron polystyrene colloids at 4% v/v, actin filament bundles, vesicle foam. The scale bar is 10 μm in all the panels.

Structural Signature of a Brittle-to-Ductile Transition in Self-Assembled Networks

L.Ramos, A.Laperrousaz, P.Dieudonné and C.Ligoure
SoftComp partner: CNRS-Montpellier, France
Physical Review Letters 107 (2011) 148302

How do complex fluids rupture? Are the concepts of ductile and brittle fractures relevant for complex fluids? How could ductile and brittle fractures be characterized and what drives the transition from a brittle-like behavior to a ductile-like behavior?

To answer these questions, we have studied the nonlinear rheology of a novel class of transient networks, made of surfactant micelles of tunable morphology reversibly linked by block copolymers. We have coupled couple rheology and time-resolved structural measurements, using synchrotron radiation, to characterize the highly nonlinear viscoelastic regime, and have proposed the fluctuations of the degree of alignment of the micelles under shear as a probe to identify a fracture process. We have shown a clear signature of a brittle-to-ductile transition in transient gels, as the morphology of the micelles varies, and have quantified, by analogy with solid materials, the amount of ductility in a complex fluid.

Cartoon of the transient network of tunable and morphology (top), structural parameter allowing the evidence of a brittle-to-ductile transition when the network morphology changes (bottom left) and link between the fracture of solid and of viscoelastic fluid (bottom right).

Highly Nonlinear Dynamics in a Slowly Sedimenting Colloidal Gel

G. Brambilla, S. Buzzaccaro, R. Piazza, L. Berthier, and L. Cipelletti
SoftComp partner: CNRS-Montpellier, France
Physical Review Letters 106 (2011) 118302

Colloidal gels are ubiquitous in industrial applications, e.g. in the oil, food, personal care and pharmaceutical industries. They are also useful model systems for tackling a wide variety of problems, from network formation in biological systems, to protein crystallization.

Although colloidal gels have solid-like mechanical properties, they can be easily disrupted by modest applied loads, often including their own weight. A vast literature exist on the macroscopic behavior of settling gels; however, a microscopic understanding of their behavior is still lacking, ultimately limiting our ability to fully exploit these materials.

In our work, we use a novel light scattering technique to probe not only the macroscopic deformation of a gel under gravitational load, but also the gel dynamics at the particle scale. We find that the gel behavior at all scales is controlled by a single parameter, the time-dependent compression rate. Our result establish a fascinating analogy with other glassy materials such as polymer glasses, where a similar correlation between macroscopic deformation and motion at the microscopic level has been recently reported.

a) image of the gel column (speckled bottom part) as recorded in our space-resolved light scattering experiment. The cell width is 3 mm. By analyzing a time series of such images, we obtain the temporal evolution of the concentration and sedimentation velocity profiles, and the local microscopic dynamics. b) Microscopic relaxation time as a function of local compressive strain rate. c) The final decay of the intensity correlation functions measured at different heights and times collapse on a master curve when scaling time.

Polymer-Brush Lubricated Surfaces with Colloidal Inclusions under Shear Inversion

L.Spirin, A.Galuschko, T.Kreer, K.Binder and J.Baschnagel
SoftComp partner: Univ. Mainz, Germany
Physical Review Letters 106 (2011) 168301

The study of polymer-brush surfaces and their lubrication properties is an active field with a particular relevance for the design of artificial joints. Here, we characterize the response of compressed, sheared polymer-brush bilayers with colloidal inclusions to highly nonstationary inversion processes by means of molecular dynamics simulations and scaling theory. Bilayers with a simple (dimeric) solvent reveal an overshoot for the shear stress, while simulations of dry brushes without explicit solvent molecules fail to display this effect. We demonstrate that mechanical instabilities can be controlled by the inclusion of macromolecular structures, such as colloids of varying softness. Based on a recently developed theory, we suggest a scaling approach to determine a characteristic time for conformational and collective responses.

(Left) Snapshot of a simulated bilayer with polymer brushes (blue and red) and colloidal inclusions (green) at large steady shear. (Right) Nonstationary shear stress during shear inversion as a function of time. Implicit solvent systems (A) show a smooth crossover to steady state, while the dimeric solvent (B) reveals an overshoot. Macromolecular inclusions reduce this overshoot and for hard colloids of large density
(D1) it vanishes completely. Inset: Rescaled time development of the number of binary interbrush contacts. The absolute height of the maximum is the largest for systems with dimers and is small for systems that contain inclusions.

Shock Waves in Capillary Collapse of Colloids: A Model System for Two-Dimensional Screened Newtonian Gravity

J.Bleibel, S.Dietrich, A.Dominguez and M.Oettel
SoftComp partner: Univ. Mainz, Germany
Physical Review Letters 107 (2011) 128302

Interfacially trapped, micrometer-sized colloidal particlesinteract via long-ranged capillary attraction which is analogous to two-dimensional screened Newtonian gravity
with the capillary length λ as the screening length. Using Brownian dynamics simulations, density functional theory, and analytical perturbation theory, we have studied the collapse of a finitely-sized patch of colloids. Whereas the limit λ → ꝏ corresponds to the global collapse of a self-gravitating fluid, for intermediate λ we predict theoretically and observe in simulations a ringlike density peak at the outer rim of the disclike patch, moving as an inbound shock wave. For smaller λ the dynamics crosses over to spinodal decomposition showing a coarsening of regions of enhanced density which emerge from initial fluctuations. Experimental realizations of this crossover scenario appear to be well possible for colloids trapped at water interfaces and having a radius of around 10 micrometer.

Snapshots from Browninan dynamics simulations for the collapse of a patch of particles initially confined to a disc of radius 1. Clusters (i.e., particles with at least 3 neighbors within a certain maximum distance) are depicted in red. For the upper row, the screening length is λ = 1.5 and the global collapse proceeds quite uniformly and faster than the formation of individual smaller clusters. For the lower row ( λ = 0.25), small clusters predominantly form at the outer rim and collectively move towards the center (shock wave).

Last modified: 16/06/2012