Colloidal hard spheres: cooking and looking

In this article we highlight our recent work on the development of crosslinked core-shell polymethyl-methacrylate colloids and their application as colloidal model hard spheres for quantitative confocal microscopy. Moreover, we demonstrate that synthesizing colloids (cooking) does not only lead to the final core-shell particles, but also to "intermediate-product- particles", which are interesting in their own right and offer additional possibilities for various physical experiments (looking). In particular, we focus on the application of crosslinked latex particles as microgel particles, non-spherically shaped particles as model particles for shape-induced geometrical frustration and the final core-shell particles for the direct measurement of thermodynamic properties using quantitative real-space confocal microscopy. © The Royal Society of Chemistry 2006

Kijken naar colloïdale knikkers

Harde bollen zijn de ‘fruitvliegjes’ van de statistische mechanica omdat ze vaak als basis gebruikt worden om meer ingewikkelde systemen te bestuderen [1]. Colloïden bieden unieke mogelijkheden om deze ‘fruitvliegjes’ experimenteel te bestuderen. Zo is het mogelijk om langs chemische weg de eigenschappen van colloïden naar wens aan te passen. Ook de typische colloïdale tijd- en lengteschalen zijn gemakkelijk toegankelijk met bijvoorbeeld optische microscopie. Dit artikel laat zien hoe deze kenmerkende combinatie van chemie, tijd- en lengteschalen het mogelijk maakt om direct de vrije energie te meten en het kristal-vloeistofgrensvlak van colloïdale harde bollen letterlijk in beeld te brengen

Second-type disorder in colloidal crystals

The presence of second-type disorder in self-organised colloidal crystals is demonstrated on the basis of confocal microscopy. The study is performed for crystals consisting of colloidal hard spheres and hard polyhedral colloids. Using confocal microscopy single-crystalline domains were imaged and the particle coordinates were retrieved using particle tracking routines. To distinguish different types of disorder present in the crystals the corresponding diffraction patterns were computed from the real-space coordinates. We show that second-type disorder is present in the crystals of both the spheres and the polyhedrals. The amount of second-type disorder is significantly larger in the crystal of the polyhedrals. This shows that colloidal crystals form an ideal model system to study various types of disorder since the analysis is possible in both real and reciprocal space. Simulating diffraction patterns from real-space coordinates therefore provides a useful route to a better understanding and interpretation of diffraction patterns

Topological lifetimes of polydisperse colloidal hard spheres at a wall

Confocal scanning laser microscopy was used to study the behavior of dense suspensions of model colloidal hard spheres at a single wall. Due to the slight polydispersity, our system shows a reentrant melting transition at high densities involving a hexatic structure [Phys. Rev. Lett 92, 195702 (2004)]]. The reentrant melting transition is accompanied by an increase in the mean-squared displacement. The correlation between structure and dynamics was quantitatively analyzed on a single-particle level. In particular, the topological lifetime, being the average time that a particle spends having the same coordination number, is determined for all coordination numbers and as a function of volume fraction. The defective (non-sixfold-coordinated) particles exhibit shorter lifetimes than sixfold-coordinated particles, indicating that the mobility of the system is larger at or close to defective particles. The lifetime itself is a strong function of volume fraction. In particular, the global behavior of the mean-squared displacement is proportional to the hopping frequency (the inverse of the lifetime), showing that particles changing their coordination number contribute most to the local mobility

Reentrant surface melting of colloidal hard spheres

Concentrated suspensions of model colloidal hard spheres at a wall were studied in real space by means of time-resolved fluorescence confocal scanning microscopy. Both structure and dynamics of these systems differ dramatically from their bulk analogs (i.e., far away from a wall). In particular, systems that are a glass in the bulk show significant hexagonal order at a wall. Upon increasing the volume fraction of the colloids, a reentrant melting transition involving a hexatic structure is observed. The last observation points to two-dimensional behavior of matter at walls

Dynamic broadening of the crystal-fluid Interface of colloidal hard spheres

We investigate the structure and dynamics of the crystal-fluid interface of colloidal hard spheres in real space by confocal microscopy. Tuning the buoyancy of the particles allows us to study the interface close to and away from equilibrium. We find that the interface broadens from 8-9 particle diameters close to equilibrium to 15 particle diameters away from equilibrium. Furthermore, the interfacial velocity, i.e., the velocity by which the interface moves upwards, increases significantly. The increasing gravitational drive leads to supersaturation of the fluid above the crystal surface. This dramatically affects crystal nucleation and growth, resulting in the observed dynamic broadening of the crystal-fluid interface

Interfacial dynamics in demixing systems with ultralow interfacial tension

We report measurements on fluid-fluid phase separation in a colloid-polymer mixture, which can be followed in great detail due to the ultralow interfacial tension. The use of the real-space technique, laser-scanning confocal microscopy, leads to clear, well-defined images making quantitative comparisons to theory possible and being highly instructive. Simple scaling arguments are given why, in experiment, three steps of the phase separation can be observed: an interfacial-tension-driven coarsening, gravity-driven flow and finally the interface formation. All these processes are observed in a single experiment. The first stage can be quantitatively described by viscous hydrodynamics. Coarsening occurs through pinch-off events. The second stage begins at a typical size of ∼2π times the capillary length reminiscent of the Rayleigh-Taylor instability. The liquid phase breaks up and becomes discontinuous. There is strong directional flow in the system, but the Reynold's number remains much smaller than unity. Finally, the macroscopic interface is formed, growing upwards, with a velocity comparable to the coarsening velocity in the initial stage. Again, viscous hydrodynamics apply with a characteristic velocity of the interfacial tension over the viscosity. © IOP Publishing Ltd and Deutsche Physikalische Gesellschaft

Direct measurement of thermodynamic properties of colloidal hard spheres

Recently, we have shown how to measure thermodynamic properties of colloidal hard sphere suspensions by microscopy [Dullens et al. (2006) PNAS 103, 529]. Here, we give full experimental details on how to acquire three dimensional snapshots of a colloidal hard sphere suspension over a wide range of densities by means of confocal laser scanning microscopy. Furthermore, we elaborate on the analysis of the data sets, in which we measure the available volume to insert an additional sphere and the surface area of that volume. These geometrical properties are related to key thermodynamic quantities by statistical geometry. This enables us to measure in a direct and non-interfering way the pressure, the chemical potential and the free energy density of a hard sphere suspension, which are in good agreement with theoretical predictions