Abstracts Talks 

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Jay Fineberg (Israel)
Slip-Stick: The Evolution of Frictional Strength
The evolution of frictional strength has great fundamental and practical importance. Applications range from earthquake dynamics to hard drive read/write cycles. Frictional strength is governed by the resistance to shear of the large ensemble of discrete contacts that forms the interface that separates two sliding bodies. An interface's overall strength is determined by both the real contact area and the contacts' shear strength. While the average motion of large, slowly sliding bodies is well-described by empirical friction laws, interface strength is a dynamic entity which is inherently related to both fast processes such as detachment/re-attachment and the slow process of contact area rejuvenation. Here we show how frictional strength evolves from extremely short to long time scales, by continuous measurements of the concurrent local evolution of the real contact area and the corresponding interface motion (slip) from the first microseconds when contact detachment occurs to large (100sec) timescales. We identify four distinct and inter-related phases of evolution. First, all of the local contact area reduction occurs within a few microseconds, upon the passage of a crack-like front. This is followed by the onset of rapid slip over a characteristic time, whose value suggests a fracture-induced reduction of contact strength before any slip occurs. This rapid slip phase culminates with a sharp transition to slip at velocities an order of magnitude slower. Immediately upon slip arrest, "aging" commences as contact area increases with a characteristic time scale provided by the system's local memory of its effective contact time prior to slip arrest. We show how the singular logarithmic behavior generally associated with aging is regularized at short times. These results provide a comprehensive picture of how frictional strength evolves from the short times and rapid slip velocities at the onset of motion to aging at the long times following slip arrest.

Katia Bertoldi (Twente)
The Use of Instabilities to Create Materials with Tunable Properties
Nature makes extensive use of structures characterized by well defined microstructures - often either regular or periodic – to achieve different properties and attributes. The brilliant colors of many birds, butterflies and fishes, as well as the hydrophobic character of desert beetles and lotus leaves, originate in their surface structures.
Mimicking the complex patterns of the natural world in manufactured physical devices is a significant challenge. While materials characterized by a microstructure on the millimeter scale have already been intensively used for a long time, structures in which the periodicity is in the sub-micron scale are relatively novel and have attracted increased attention over the last years. However, the demand for higher density, faster speed and lighter devices drives the need for developing new materials with more complex microstructure and responsive to several external stimuli.
Traditionally, instabilities have been viewed as an inconvenience with research focusing on how they might be avoided. The results of this investigation show that microscopic instabilities can be used to create a new class of materials with switchable properties.
Periodic elastomeric solids are subjected to uniaxial compression and novel transformations of the patterned structures are found upon reaching a critical value of applied load. Numerical analyses clearly show the mechanism of the pattern switch to be a form of microstructural elastic instability, giving reversible and repeatable transformation events as confirmed by experiments. This behaviour provides opportunities for transformative phononic/photonic crystals  and materials with tunable negative Poisson’s ratio.

Mirjam Leunissen (New York / Amolf)
Self-Protected Interactions in DNA-Functionalized Colloids: A Nano-Contact Glue
Surface functionalization with DNA is a powerful tool for guiding the self-assembly of nano- and micrometer-sized particles into larger-scale ordered structures. Surprisingly, the ability of single-stranded DNA to form folded secondary structures has not been explored for controlling (nano)colloidal assembly processes, despite its frequent use in DNA nanotechnology. Here, I will show how loop and hairpin formation in the DNA coatings of micrometer-sized particles gives us in situ control over the inter-particle binding strength and association kinetics. We can finely tune and even switch off the attractions between particles, rendering them inert unless they are heated or held together - like a nano-contact glue. The novel kinetic control offered by the switchable self-protected attractions is explained with a simple quantitative model that emphasizes the competition between intra- and inter-particle hybridization, while the practical utility is demonstrated by the assembly of designer clusters in concentrated suspensions.

Jasper van der Gucht (Wageningen)
Self-assembled polymer networks with precisely defined functionality
We study transient networks formed by monodisperse telechelic polypeptides with collagen-like end blocks and a random coil-like middle block. These artificial proteins are created using recombinant DNA techniques. Upon cooling, the end blocks associate reversibly into triple helices, leading to gels with well-defined, trifunctional crosslinks. The well-defined topology of the gels allows us to describe both the linear rheology and the gelation kinetics in a quantitative manner with an analytical model that requires no adjustable parameters, and accounts for the molecular structure of the gel, and the presence of loops and dangling ends. At high shear rates, the gels fracture due to activated rupture of crosslinks. The mechanism of fracture is studied using rheology and particle image velocimetry.

Antina Ghosh (UvA Amsterdam)
Density of states and soft modes : ordered and disordered colloidal systems

Ronald Otten (Eindhoven)
Percolation of polydisperse carbon nanotubes
In order to explain huge variations in the onset of electrical percolation in particular in carbon nanotube composites, we analyze a continuum connectedness model that incorporates an arbitrary polydispersity in both length and width and show that the percolation threshold is quite sensitive to both. The presence of even very small quantities of longer nanotubes lowers the threshold significantly, whereas wider ones tend to raise it. The theory allows us to model electrical percolation in non-additive mixtures of conductive and (effectively) insulating nanotubes, such as is the case in dispersions of single-walled carbon nanotubes, showing that the latter raise the percolation threshold proportionally to their relative abundance.


Abstracts Soundbites

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Jens Harting (Eindhoven)
Simulation of colloidal suspensions
A short introduction on our current activities to simulate (colloidal) suspensions using mesoscopic simulation methods such as lattice Boltzmann, Stochastic Rotation Dynamics or Brownian Dynamics will be given.

Brian Tighe (Leiden)
Shear flow near the jamming transition
Dense packings of frictionless disks display an unjamming transition, or loss of rigidity, as packing fraction is decreased. Packings near unjamming are anomalously soft, with this property governed by the distance to the transition. Similarly rich physics results when the system is forced to flow under shear. I will present a scaling argument for the flow of soft spheres near jamming, in which a competition between two loading mechanisms yields non-trivial rheological regimes that are observed in simulation.

Andriy Kyrylyuk (Utrecht)
Packing of Granular Mixtures
Mixing particles of different size and shape is a long-standing problem in science and technology, which is related to the problem of dispersion and phase separation in particulate systems. Many interesting phenomena can be observed in colloidal and granular mixtures due to competing length-scales and synergetic effects between the mixture components. Here, we investigate the packing of binary granular mixtures. We observe that for some rod elongations in the rod-sphere mixtures the quality of dispersion is not important for the packing density. The packing density of the mixed and phase separated binary system is the same, meaning that the mixing entropy/volume is essentially zero.

Daniela Kraft (Utrecht)
High yield synthesis of anisotropic polyNIPAM-polystyrene particles

Tak Shing Chan (Twente)
Two-phase hydrodynamic model for air entrainment at moving contact line
The moving contact line problems are challenging because they involve multiple length scales. One interesting case arises when an advancing liquid of high viscosity entrains the surrounding phase, such as air. In this presentation, we introduce a hydrodynamic model that generalizes the lubrication theory in order to take into account the velocity fields of the two phases. Assuming that the curvature of the interface is small we derive a differential equation for the interface profile at stationary state. We found that there is a critical capillary number above which no steady meniscus can exist and instability will occur. For example, air  bubbles will be entrained into the liquid at the advancing contact line. However, we found no instability when neglecting the viscosity of the surrounding phase, illustrating the two-phase nature of the problem.

Kinga Lorincz (UvA Amsterdam)
Non-affinity of displacement fields in sheared granular systems
The jamming transition, i.e. the transition in a granular system from rest to flow is a fundamental problem of great importance to the understanding of a wide class of disordered materials. Using the experimental method of laser sheet imaging we can accurately visualize individual particles in a sheared three-dimensional granular packing immersed in an index matching liquid. We study fluctuations in the displacements of the particles as a function of varying confining pressures and shear stresses. We characterize these fluctuations by investigating the non-affine regions in the displacement fields.

Alexander Siemens (Leiden)
Compression in foams

Fatih Göncü (Twente)
Jamming in polydisperse packing of spheres: evolution of the coordination number
We investigate the evolution of the coordination number (and the packing structure) and pressure as function of the volume fraction for a polydisperse granular packing of spheres under isotropic and triaxial compression. Using the results obtained from molecular dynamics simulations, we confirm that the coordination number – starting from the jamming point density – follows a power law with exponent ~ 0.5 – even for rather large densities.  Furthermore, we find that the pressure scales with the coordination number and the volume fraction and is proportional to the volumetric strain (logarithmic behavior). Our goal is to extend the results to simulations with anisotropic deformation and frictional particles and see in how far the observations for frictionless particles can be generalized.

Nienke Geerts (Amolf)
Direct observation of size-fractionation during colloidal crystallization.
We present a confocal-microscopy study of the quasi-two-dimensional crystallisation of a binary mixture of spherical colloids coated with long DNA strands. Our experiments show that in the crystalline phase the two colloidal species are completely demixed.  Analysis of the lattice spacings in the two types of colloidal crystal shows that the diameters of the two species of colloids differ by 10%. We argue that the demixing in the crystalline phase is due to size segregation during crystallization. This phenomenon had been predicted in several theoretical studies. To our knowledge, the present study provides the first “real-space” experimental confirmation of this effect. Direct observation of size-fractionation during colloidal crystallization.

Adrian Cioroianu (Eindhoven)
Non-affine response in biopolymer networks
In the case of biological materials the affine/non-affine response is a function of cross-link density. Intuitively, densely cross-linked networks are predicted to deform affinely, in which case network stiffening comes from entropic stretching of individual filaments, while weakly cross-linked networks show a non-affine response, due to a dominant bending of individual filaments.
However, one important aspect when dealing with a non-affine response is still lacking a proper attention. Until now, the ways strains are accommodated and distributed through the bulk of the system i.e. the geometry of deformations, have not been well understood. We believe that a strong correlation between the geometry of deformations and non-affine behavior exists and we will focus on that in the following research.

Katya Lyakhova (Eindhoven)
Mesoscopic simulations of the self-healing process in polymer coatings
We study a polycaprolactone network with a small proportion of a low surface energy perfluoro-octylethanol group.  The objective of present work is to understand under what conditions a low-surface-energy interface will be able to self-heal after the removal of the top surface.  We have already been able to model a formation of a crosslinked polymer network as well as the formation of a polymer film with a free surface.  We have demonstrated that within our approach it is possible to simulate the migration of dangling ends of low surface-energy species to the surface.

Triet Dang (UvA Amsterdam)
The anisotropy of the crystal-liquid interface
Since direct observation of the atomic dynamics at the crystal-melt interface is not possible, the crystal growth process is still poorly understood at the atomic scale. In this project, we use micron-size colloidal particles as models to visualize atomic processes of crystal growth. From data collected from the 3D confocal microscopy, we can determine all thermodynamic properties: pressure, the chemical potential and the free energy density. We use interface fluctuations to determine the interfacial tension and its dependence on the crystal orientation. Remarkably, the anisotropy of the interfacial tension that we find is very similar to that measured for real metallic crystals.

Henry E.Amuasi (Eindhoven)
Multi-scale Mechanics of Collagen: Cross-linked Wormlike Chains
We assert that the way to understand the mechanical properties of collagenous fibres is to see them as bundles of wormlike chains cross-linked together at various points along their contours.    We will briefly discuss numerical and analytic methods that enable us to test this assertion.

Silke Henkes (Leiden)
Soft modes in packings of frictional grains
Contact forces near the Coulomb threshold play an important role in the dynamics of frictional granular packings. We investigate the influence of these contacts on the vibrational density of states, and we compare the results to the frictionless case. We obtain the same critical scaling as for frictionless disks, but with respect to an isostatic line, not the isostatic point.

Florian Janoschek (Eindhoven)
Development and validation of a simplified particulate model for coarse-grained hemodynamics simulations
We couple a simple molecular dynamics algorithm for modeling suspended particles to the lattice Boltzmann method that provides us with a representation of the hydrodynamics of the surrounding fluid. Within this code, we describe the complex interactions of red blood cells by rigid particles with phenomenological model potentials. Thus we obtain a coarse-grained yet efficient particulate model for hemodynamics that performs well on current supercomputers. However, investigations regarding the choice of parameters and the validation of the model are still part of ongoing work. We will present an overview of the model and part of our preliminary results.

Eelco Eggen (Utrecht)
Orientational ordering on a lattice
We investigate the orientational ordering of heterogeneously charged colloidal spheres on a simple cubic lattice. For a certain class of charge distributions, the high-temperature regime yields an isotropic orientational distribution of these particles. In this case, we investigate the transition between a plastic and fully-ordered crystal phase, using bifurcation theory.

Niels Boon (Utrecht)
Charge behaviour of striped surfaces
We consider the electrostatic properties of inhomogeneously charged planes, immersed in electrolyte. In particular, we are interested in surfaces on which different charge species form domains of finite size. We develop a theoretical model to numerically solve the electrostatic potential within non-linear Poisson-Boltzmann (PB) theory for striped surfaces. We consider situations in which the domains bare a fixed charge density, and also more common situations in which the charging of the domains is controlled by a chemical equilibrium constant. For the latter, we find that small ionizable domains surrounded by charge-neutral surface can charge up to much higher charge densities than one would expect for large domains.

Marieke Schor (UvA Amsterdam)
Simulations of self-assembling silk-based block copolymers
Protein-based, self-assembling block co-polymers show high potential as biocompatible materials. For example, a block copolymer with a central block based on the repetitive sequence (EGAGAGAG)x found in natural silk proteins attached to hydrophilic outer blocks will form fibers when triggered (e.g. by lowering the pH). We developed a coarse grained force field to gain insight into the folding and fiber forming behaviour of such block-copolymers via MD simulations. Our model can also be applied to study characteristics of the individual fibers and interactions between them.

Haining An (Delft)
Hardening of Tri-Block Copolymer Gels in the Presence of Magnetic Fields
The rheological response of highly swollen gels containing ordered magnetic particles in the presence of external magnetic fields was investigated. Different geometries concerning combinations of the orientation of the external magnetic field, shear direction and particle strings orientation were systematically studied. Dependence of the response on volume fraction of particles and gel swelling degree was also investigated. We showed, for instance, that the storage modulus exhibit a strong dependence on the strength of the external magnetic field indicating a highly non-linear increase of the gels for all geometries.

Piotr Glazer (Delft)
Responsive polyelectrolyte gels
When a water-swollen polyelectrolyte gel is interposed between a pair of electrodes and DC current is applied, the gel undergoes electrically-induced chemomechanical contraction. Different mechanisms which might be responsible for the actuation are analyzed.

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