Talks Soundbites

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Marjolein Dijkstra (Utrecht)
Colloidal self-assembly
The behaviour of colloidal spheres moving around in a solvent resembles that of ordinary atoms, but the size variation of the colloids allows for an unprecedented degree of manipulation and control.
In this talk, I will show how one can exploit this picture of colloids as extremely tunable
"superatoms or building blocks" to obtain new phases and new structures which may lead to "advanced materials", e.g., e-ink displays, photonic bandgap structures, and electrorheological fluids. I will show that mixtures of two sizes of hard spheres can organize spontaneously in laves phases. In addition, mixtures of oppositely charged colloids display a remarkably rich variety of crystal structures as the crystal structure is not dictated by the charge neutrality condition as in the case of ionic crystals. The resulting phase diagram of oppositely charged colloids displays colloidal analogs of simple-salt structures, and of the doped fullerene structures, but also novel structures with large-small stoichiometries 1:6 and 1:8 that do not have an atomic or molecular analog. Finally, I will show that many new phases can be manipulated by external fields, e.g., an electric field stabilizes the hexagonal-close-packed, body-centered-orthorhombic, and body-centered-tetragonal phase, while triangular, square, buckling, and prism phases are found in confined hard spheres.

Yun Yan (Wageningen)
Novel micelles composed of ions, ligands, and block copolymers
Metal-bisligand coordination complexes are self-assembled supramolecules1.  The water solvable metal-bisligand coordination complexes with negative charges on coordination centre can be regarded as negatively charged polyelectrolytes.  This polyanions are found to form hierarchical self-assemblies with polyneutral-polycations block copolymer2.  In this research, we report the formation of coacervate micelles in poly(2-vinyl-N-methyl-pyridium iodide)-polyethylene oxide (P2MVP41-PEO204) /Zn-L2EO4 coordination complexes.  The latter is formed by mixing 1,11-bis(2,6-dicarbonate potassium-4-yloxy)-3,6,9-trioxaundecane, L2EO4, with zinc nitrate. By variation of the charge mixing ratio, both small spherical micelles with radius of 25 nm and large worm-like micelles with radius of 150 nm are formed in the mixed systems.

Leon van Dijk (Eindhoven)
Self-assembly and optical properties of supramolecular aggregates
Optical spectra of self-assembling -conjugated oligomers in solution reveal the self-assembly process as a function of temperature. For oligomers with a stereocenter, there is a transition from a phase with mostly non-helical bonds to one with mostly helical bonds, due to cooperativity of the bonding [1]. We describe the statistical properties of this system with a one-dimensional Ising model.  The spectrum of exciton ground-state energies is obtained as an average over the statistical ensemble, using a tight-binding model to describe the delocalization of the exciton. A comparison is made between the predicted and measured spectra.
[1] P. Jonkheijm, P. van der Schoot, A.P.H.J. Schenning, E.W. Meijer, Science 313, 80 (2006)

Wouter Ellenbroek (Leiden)
Steric hindrance close to the jamming transition
Many disordered materials (like granular media, foams, colloidal suspensions) show a transition between solid-like and fluid-like behavior, known as the jamming transition. In the (numerical) linear response of jammed granular model systems to shear and compression, we identify a relation between distance to jamming and the amount by which particles "appear to be in each other's way". This steric hindrance picture of jamming is also observed in experiments on vibrated dense granular liquids and sheared 2D foams, thus contributing to the idea of a unified description of jamming in these materials.

Kinga Lörincz (VU Amsterdam)
An edge on power-laws of granular avalanches
Many punctuated phenomena in nature are claimed (e.g. by the theory of Self-Organized Criticality (SOC)) to be power-law distributed. However, the archetype of SOC, the sand pile, does not seem to display power-law behavior in controlled experiments. In our experiments on a three dimensional pile of long grained rice, we find that if the foot of the pile rests on a horizontal surface we observe power law behavior. However, if the avalanches can drop off from the edge of the pile, in addition quasi-periodic system-spanning avalanches are present. This explains the previously reported non power-law behavior in sand pile experiments.

Chantal Valeriani (Amolf)
Homogeneous bubble nucleation in a Lennard-Jones fluid:
Pathway analysis and rate calculation.
Molecular simulations with efficient sampling techniques have proven to be a valuable tool to explore the microscopic basis of nucleation, both for crystals and for liquid droplets.A few years ago, Shen and Debenedetti [1] used biased Monte Carlo simulations to study homogeneous bubble nucleation in a super-heated Lennard-Jones fluid. Performing a geometric analysis of the early bubble cavities in the simulation, they concluded that the critical nucleus had a web-like, system-spanning structure, rather than the compact, spherical shape predicted by classical nucleation theory.In our study, we use a normal approach, based on the so-called Forward Flux Sampling technique, to probe the transition from a metastable super-heated Lennard-Jones fluid to a stable vapor phase. Our simulations indicate that the early state of bubble formation is the formation of a compact nano-bubble. However, many mysteries remain.
[1] Shen, V.K. and P.G. Debenedetti, J.Chem.Phys., 1999. 111(8): p.3581-3589.

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Markus Bier (Utrecht University)
Relaxation dynamics of fluids of colloidal platelets
We model fluids of colloidal platelets by means of dynamic density functional theory in order to study relaxation to equilibrium. After sketching the formalism preliminary results will be mentioned briefly.

Maarten Biesheuvel (MPI Potsdam, Germany)
Random curvature motion of self-propelled biomimetic colloids
We analyze the 2D motion of 5 micron-sized colloids propelled by surface-directed actin-gel polymerization, which is an in-vitro model system for motor-protein-free motion in biosystems. We focus on the curvature of the particle trajectories and present a microscopic, stochastic, "multi-spring" model to explain the curvature distributions.

Peter Bolhuis (UvA Amsterdam)
Simulating rare events in soft matter and complex fluids
Our research group develops and employs computer simulation techniques to investigate activated processes in complex fluids and soft matter, for instance the self-assembly of surfactants, protein.

Daniel Bonn (UvA Amsterdam)
Unjamming of jammed systems

Martien Cohen Stuart (Wageningen University)
Hierarchical self-assembly: three-component, reversible nanoparticles
When metal ions are mixed with water soluble bisligands, a reversible, water soluble coordination-polyelectrolyte is formed. In turn, this polyanion can be combined with a cationic-neutral diblock copolymer to form fully equilibrated micelles with some unusual properties.

Antonia Denkova (TU Delft)
Influence of ethanol and salt on the morphological changes of Poly(ethylene oxide)-Poly(propylene oxide)-Poly(ethylene oxide) block copolymer assemblies
Amphiphilic triblock copolymers are used as templating agents to synthesise large-pore mesoporous silica materials. Depending on the surfactant choice and reaction conditions (temperature, pH, additives, stirring rate, etc?) mesoporous silicas with different pore sizes and structures can be prepared [1,2]. The formation mechanism of these materials is insufficiently clear in spite of the numerous syntheses carried out by various research groups. A more fundamental understanding of the assembling mechanism of these systems is needed in order to better control the synthesis, and, ultimately, to rationally design new mesoporous materials.
The interactions between the surfactant and the various additives (silica, acids, salts, solvents) during the synthesis are believed to play an important role in the formation of these highly structured materials [3]. Here, we present a study on the interactions of Poly(ethylene oxide)-Poly(propylene oxide)-Poly(ethylene oxide) block copolymer, the template for the ordered mesoporous material SBA-15, at different surfactant and ethanol concentrations in the presence and in the absence of inorganic salt. We have also examined the effect of salt alone on the morphology of the block copolymer micelles. Rheology and light scattering techniques were used to examine the influence of these additives on the micelle morphology and flow properties. The results show that both ethanol and salts strongly affect the micelle morphology in different ways. In the presence of salt a large number of micelles and some micelle aggregates are formed. The shape and distribution of these aggregates cannot be determined conclusively at this stage. However, the effect of varying solvent quality by continuous addition of ethanol leads to the formation of rod-like micelles. Interestingly, the length of rod-like micelles increases with increasing ethanol concentration until an optimal ethanol concentration (8 - 10 vol%) is reached, above which the length of the micelles decreases again. Furthermore, such non-monotonic variations of micelle size with ethanol concentration define a region in the phase diagram where a gel is observed.
[1] Zhao, D.; Huo, Q.; Feng, J.; Chmelka, B. F.; Stucky, G. D. J. Am. Chem. Soc. 1998, 120, 6024.
[2] Flodstr”m, K.; Alfredsson, V.; K„llrot, N. J. Am. Chem. Soc. 2003,125, 402.
[3] Edler, K.J. Aust. J. Chem. 2005, 58, 627."

Eelco Eggen (Utrecht University)
A generalization of Parsons-Lee theory for hard-rod mixtures
Parsons-Lee theory is a miraculously successful theory for describing fluids of rodlike particles. However, due to its phenomenological nature, it can be interpreted in several ways. It is interesting to see how these different interpretations can lead to different approaches, concerning its generalization to mixtures.

Hayley Every (TUDelft)
Homeotropically aligned thin-films of sulfonated polyaramide and polyallyamine complexes through electrostatic interaction
The anionically charged sulfonated poly(p-phenylene terephthalamide) (S-PPTA) self-assembles in water to form needle-like aggregates consisting of between 5 and 7 polymer molecules. These aggregates spontaneously align to form a nematic liquid crystalline phase at concentrations as low as 2wt%. The ordered structure is retained upon film casting, with x-ray scattering revealing a perpendicular (homeotropic) alignment with respect to the polymer film and an order parameter of 0.77. The addition of the cationically charged polyallylamine hydrochloride (PAA-HCl) to the S-PPTA solutions forms a complex through electrostatic interaction between the sulfonate and amine groups. Films cast from these solutions were also homeotropically aligned, although upon increasing the amount of polyallylamine, the degree of alignment decreased slightly (0.74 for a 20:1 S-PPTA:PAA-HCl complex). As most electrostatic complexes exhibit alignment in the plane of the film, this appears to be the first evidence of a perpendicularly aligned structure in such polymer complexes.

Hua Guo (UvA Amsterdam)
Colloidal phase behavoir in a binary liquid mixture
Negatively charged polystyrene particles suspended in the quasi binary 3-methylpyridine/water/heavy water mixture undergo a phase transition when the solution approaches the coexistence line of the binary liquid mixture .This transition is reversible, and can be induced by using temperature as a control parameter. We characterize the formed phases by measuring the structure factor with small angle x-ray scattering (SAXS) as: face centered cubic crystal, glass, and liquid. Images in real space give further insight into the evolution of the phase behavior.

Gijs Katgert (Lion)
Bubble-wall friction in surfactant solutions
We investigate the influence of interfacial rheology on the velocity of a gas bubble creeping against an inclined plate. We find powerlaw scaling of the bubble-wall friction with the velocity

Asmae Khaldoun (UvA Amsterdam)
Quicksand and quickclay
Quicksand is the generic name for unstable soils reputed to trap anyone who treads on it. Popular wisdom has it that one should not move when trapped in quicksand, as motion makes one sink in even deeper and that once trapped, it is difficult to escape. Here we provide an explanation for these observations by studying the most commonly encountered form of natural quicksand. We show that a spectacular liquefaction of the material occurs when a stress is applied to the material: the liquefaction is the reason why one sinks away, and it is more pronounced for larger stresses. By constructing laboratory quicksand, we demonstrate that the liquefaction is due to the structure: quicksand is a loose granular packing of sand particles stabilized by a clay matrix that forms a particulate gel. The stress liquefies the clay matrix, and the granular assembly collapses, expulsing water. This results in a densely packed system that practically impossible to dilate: it is for this reason that once trapped it is difficult to get out of quicksand. A sinking test demonstrates that, due to buoyancy, it impossible to drown in the quicksand.

Arkadiusz Kotlewski (TUDelft)
Rheological properties and structural aspects of discotic liquid crystal based organogels
Great number of low molecular weight organic molecules are able to form gels in organic solvents. Numerous publications explaining mechanisms of gelation for variety of systems, describing diversity of new morphologies, molecular packing and trying to correlate it with optical, thermodynamical and mechanical properties. We report here results on structure and rheological properties of a hydrogen bond free triphenylene derivative that is able to make two kinds of thermoreversible organogels in polar solvents.

Daniela Kraft (Utrecht University)
Anisotropic colloids
Self-assembly of cone-like colloids can provide interesting insight into biological systems such as micelles or the assembly of virus capsids. Such colloids can be produced by swelling cross-linked polymer spheres with different monomers. Subsequent heating and polymerization leads to the formation of protrusions. My PhD project focuses on the variation of the geometric and chemical properties of these colloids in order to influence the self-assembly into new structures.

Andriy Kyrylyuk (TU Eindhoven)
Percolation thresholds in the carbon black and carbon nanotubes systems
Conductivity percolation of carbon-based objects, e.g. carbon black or carbon nanotubes, in polymer matrices is an important route to achieve low-cost conductive polymer composites with as low as possible a loading. To gain insight in how to lower the percolation threshold of such mixtures, we study the connectedness percolation of sticky carbon-based objects in an effective medium. We discuss the differences and similarities in percolation behavior of spherical (carbon black) and rod-like (carbon nanotubes) particles. We find that the percolation threshold of carbon nanotubes is very sensitive to the range of the cluster criterion and to any attractive interaction between them. We show that the attractive interactions lower the percolation threshold and find different percolation regimes, depending on the strength of the interaction potential. We establish a relation between connectedness criterion and electrical percolation by considering the tunneling of charge carriers (electrons) between the carbon nanotubes. We also find the dependence of the characteristic tunneling distance on the material parameters, in particular, on the dielectric constant of the medium.

Evgeny Obraztsov (AMOLF)
The road to disorder in smectic elastomers
Disorder induced by random crosslinking the backbone of a smectic polymer to form an elastomer provides a new approach to problems of randomness and disorder in condensed matter systems. We report on the effect of crosslink concentration and of varying the stiffness of the crosslink units on the transition to disorder. Detailed analysis of high-resolution x-ray measurements of the lineshape of the quasi-Bragg peak associated with the smectic layering indicates a gradual transition from algebraic decaying ordering to disorder.

Salima Rafai (UvA Amsterdam)
Evaporation of droplets

Soumyajit Roy (Utrecht University)
Spontaneous formation of inorganic peapods
An inorganic metal-oxide based salt (ammonium phosphomolybdate) in aqueous dispersions upon sonication spontaneously transform into micrometer-sized, peapod-shaped structures. The formation of these peapods is preceded by the generation of spherical aggregates. The particles have been characterized experimentally by time resolved dynamic light scattering (DLS), transmission electron microscopy (TEM), scanning TEM with high angle annular dark field detector (STEM-HAADF) for energy dispersive X-ray (STEM/EDX) elemental analyses. The phenomenon will be shown and a pathway would be proposed.

Eduardo Sanz (Utrecht University)
Kintetics of phase transitions in oppositely charged colloids
We analyse the mechanism of phase transitions in oppositely charged colloids. At high packing fractions and weak interactions we study how an overcompressed fluid transforms into a solid. At low packing fractions and strong interactions we study how a gas-liquid spinodal decomposition is arrested yielding a gel.

Kostya Shundyak (AMOLF)
Local structure of 2D random jammed packings of frictional grains
We analyse local structure of numerically generated quasi-2D static frictional packings of soft Hertzian spheres in the wide range of interparticle friction. Scaling of several quantities, related to the local contact number, with pressure, p, allows to extrapolate them to p=0 and to identify some structural invariants. Simple mean-field arguments provide explanation of the observed local structure. This allows us to estimate an upper bound  for the configurational entropy of the packing at the onset of jamming.

Eric Sultan (Leiden University)
Mesoscopic model for visco-elastic flow
We used Dissipative Particle Dynamics to model a polymer fluid. Both macroscopic and microscopic behaviors can be investigated. The statistics of polymer configurations in shear flows are compared with single polymer experiments.

Paul van der Schoot (TU Eindhoven)
Assembly of tobacco mosaic virus
Tobacco mosaic virus is a cylindrical virus that self assembles in aqueous solution containing the coat proteins and single-stranded RNA genome. Assembly is most efficient under conditions where the coat proteins form the so-called double-disk structure. We put forward a simple statistical model that explains why this is so, and why the virus does not incorporate RNAs damaged, e.g., by the action of nucleases.

Sandra Veen (Utrecht University)
Self-assembly of single molecular inorganic clusters
Self-assembly is a process seen in many different systems, ranging from surfactant micelles, lipid bilayers to virus capsids. It recently became clear that also single molecular inorganic clusters known as polyoxometallates (POMs) can organize themselves into superstructures in a globally comparable manner. They have been shown to spontaneously and reversible form large hollow spherical superstructures, much like surfactant micelles or virus capsids. Interestingly some of the POM's are spherically symmetric yet form the hollow vesicle resembling structures. For the moment we disregard this enigmatic symmetry paradox.

Yu Ling Wu (Utrecht University)
Control over colloidal crystallization by shear and electric fields
We designed a new parallel plate shear cell that can be placed on top of a confocal microscope. With this setup we examined shear melting and crystallization of colloidal dispersions. Another type of flow with which we controlled crystallization was spin coating. Colloidal crystalline films with tunable thickness were made. By using an electric field with regular field modulations we also controlled structure formation of colloidal structures in 3D.

Zorana Zeravcic (Leiden University)
Localization behavior of vibrational modes in granular packingsAn important research topic in the last few years has been the anomalous scaling of the properties of granular packings near the jamming point. We report on the first results for the localization behavior of vibrational modes of frictionless granular media as the jamming point is approached. All our data collapse within statistical errors onto a singel curve, when properly rescaled with the system size and pressure. The localization length shows no signiture of the nearness of the jamming point. Moreover our results from level statistics show that modes whose localization length is larger than the linear system size L interact, i.e. there is strong level repulsion due to the overlap of the modes, and they follow Wigner statistics. Also our results for modes with localization length smaller than L show that they basically do not interact and follow Poisson statistics.

Jos Zwanikken (Utrecht University)
Charge stabilised, additive-free emulsions: tightrope walking with polarity
By means of a non-linear Poisson-Boltzmann theory, we studied the self-charging and crystal formation of water droplets in oil. We only expect crystals to form at a specific apolarity of the oil, in agreement with recent experimental observations.

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