Bouwen met een microscopische blokkendoos : van simpele modellen tot geavanceerde materialen

In de onderzoeksgroep zachte gecondenseerde materie van de Universiteit Utrecht turen we dag in, dag uit naar microscopisch kleine plastic bolletjes, zogeheten ‘colloïden’. Doel: een beter begrip van processen zoals kristalliseren en smelten in vloeistoffen of vaste stoffen van nog veel kleinere, atomaire deeltjes, maar ook de creatie van nieuwe ‘zelforganiserende’ materialen, waaronder fotonische kristallen die licht kunnen schakelen. Voor beide doeleinden is een goede controle over het karakter van de colloïdale bouwstenen onontbeerlijk. Onlangs hebben we de eerste stappen gezet op de weg naar complexere systemen met een rijker gedrag en een groter potentieel voor toepassingen

Manipulating colloids with charges and electric fields

This thesis presents the results of experimental investigations on a variety of colloidal suspensions. Colloidal particles are at least a hundred times larger than atoms or molecules, but suspended in a liquid they display the same phase behavior, including fluid and crystalline phases. Due to their relatively large size, colloids are much easier to investigate and manipulate, though. This makes them excellent condensed matter model systems. With this in mind, we studied micrometer-sized perspex (‘PMMA’) spheres, labeled with a fluorescent dye for high-resolution confocal microscopy imaging, and suspended in a low-polar mixture of the organic solvents cyclohexyl bromide and cis-decalin. This system offered us the flexibility to change the interactions between the particles from ‘hard-sphere-like’ to long-ranged repulsive (between like-charged particles), long-ranged attractive (between oppositely charged particles) and dipolar (in an electric field). We investigated the phase behavior of our suspensions as a function of the particle concentration, the ionic strength of the solvent and the particles’ charges. In this way, we obtained new insight in the freezing and melting behavior of like-charged and oppositely charged colloids. Interestingly, we found that the latter can readily form large crystals, thus defying the common belief that plus-minus interactions inevitably lead to aggregation. Moreover, we demonstrated that these systems can serve as a reliable model system for classical ionic matter (‘salts’), and that opposite-charge interactions can greatly facilitate the self-assembly of new structures with special properties for applications. On a slightly different note, we also studied electrostatic effects in mixtures of the cyclohexyl bromide solvent and water, both with and without colloidal particles present. This provided new insight in the stabilization mechanisms of oil-water emulsions and gave us control over the self-assembly of various useful colloidal structures. Besides modifying the particle charge, we employed the sensitivity of colloids to ‘external fields’ to manipulate the structure and dynamics of our suspensions. In particular, we used an electric field, in which the particles acquired a dipole moment. The induced dipole-dipole interactions gave rise to uniquely different crystalline and non-crystalline structures, due to their anisotropic nature. We explored the phase behavior as a function of the particle concentration, the electric field strength and the field geometry, and showed how one can rapidly switch from one structure to another. The latter is particularly interesting for applications. Finally, we also studied much weaker, inhomogeneous electric fields. In this case, the dipole moment of the particles was too small to change the phase behavior, but large enough to induce dielectrophoretic motion, driving the particles to the areas with the lowest field strength. We demonstrated how this can be used to manipulate the local particle concentration inside a sealed sample, on a time scale of minutes-weeks. The combination with real-time confocal microscopy allowed us to follow all particle rearrangements during the densification. Such controlled compression is of interest to colloidal model studies and the fabrication of high-quality crystals for applications. After all, for all suspensions the particle concentration is one of the most important factors determining the behavior

Epitaxial nucleation and growth of n-alkane crystals on graphite (0001)

Contains fulltext : 60394.pdf (publisher's version ) (Closed access)To study the heteroepitaxial growth of apolar organic compounds on apolar inorganic substrates, the n-alkanes dotriacontane (C32H66) and tritriacontane (C33H68), dissolved in n-heptane, were deposited onto the (0001) face of highly oriented pyrolytic graphite (HOPG). It was found that both n-alkanes display epitaxial crystal growth. The hexagonal symmetry of the substrate surface is reflected by the three preferred orientations of the platelike crystals. The orientation of the alkane crystals was determined using polarization microscopy and atomic force microscopy. A Stranski-Krastanov mechanism for three-dimensional epitaxial growth is proposed, in which the first layer of n-alkane molecules on the graphite surface is assumed to be one of the well-known monolayer structures. Three-dimensional nucleation of the n-alkane crystals occurs on this layer, in such a way that the hydrocarbon chains of the crystal are parallel to those in the monolayer. From the point group symmetry of the n-alkane contact plane and the substrate surface, it was deduced that the epitaxial nuclei Of C32H66 and C33H68 can be deposited in six and three orientations, respectively, which is in agreement with the experimental results

CuAu structure in the restricted primitive model and oppositely charged colloids

We study the phase behavior of oppositely charged equal-size hard spheres both theoretically and experimentally, using Monte Carlo simulations and confocal microscopy. In the simulations, two systems are considered: the restricted primitive model (RPM) and a system of screened Coulomb particles. We construct the phase diagrams of both systems by computer simulations and predict a novel solid phase that has the CuAu structure. In addition, the CuAu structure is observed experimentally in a system of oppositely charged colloids. The qualitative agreement between the RPM, the screened Coulomb system, and the experiments shows that colloids form a suitable model system to study phase behavior in ionic systems

Gel formation in suspensions of oppositely charged colloids: mechanism and relation the equilibrium phase diagram

We study gel formation in a mixture of equally-sized oppositely charged colloids both experimentally and by means of computer simulations. Both the experiments and the simulations show that the mechanism by which a gel is formed from a dilute, homogeneous suspension is an interrupted gas-liquid phase separation. Furthermore, we use Brownian dynamics simulations to study the relation between gel formation and the equilibrium phase diagram. We find that, regardless of the interaction range, an interrupted liquid-gas phase separation is observed as the system is quenched into a state point where the gas-liquid separation is metastable. The structure of the gel formed in our experiments compares well with that of a simulated gel, indicating that gravity has only a minor influence on the local structure of this type of gel. This is supported by the experimental evidence that gels squeezed or stretched by gravity have similar structures, as well as by the fact that gels do not collapse as readily as in the case of colloid-polymer mixtures. Finally, we check whether or not crystallites are formed in the gel branches; we find crystalline domains for the longer ranged interactions and for moderate quenches to the metastable gas-liquid spinodal regime

Structure, stability, and formation pathways of colloidal gels in systems with short-range attraction and long-range repulsion

We study colloidal gels formed upon centrifugation of dilute suspensions of spherical colloids (radius 446 nm) that interact through a long-range electrostatic repulsion (Debye length ≈ 850 nm) and a short-range depletion attraction (∼12.5 nm), by means of confocal scanning laser microscopy (CSLM). In these systems, at low colloid densities, colloidal clusters are stable. Upon increasing the density by centrifugation, at different stages of cluster formation, we show that colloidal gels are formed that significantly differ in structure. While significant single-particle displacements do not occur on the hour time scale, the different gels slowly evolve within several weeks to a similar structure that is at least stable for over a year. Furthermore, while reference systems without long-range repulsion collapse into dense glassy states, the repulsive colloidal gels are able to support external stress in the form of a centrifugal field of at least 9g

Nucleation of colloidal crystals on configurable seed structures

Nucleation is an important stage in the growth of crystals. During this stage, the structure and orientation of a crystal are determined. However, short time- and length-scales make nucleation poorly understood. Micrometer-sized colloidal particles form an ideal model system to study nucleation due to more experimentally accessible time- and length-scales and the possibility to manipulate them individually. Here we report experiments and simulations on nucleation in the bulk of a hard-sphere fluid, initiated by seed structures configured using optical tweezers. We find that the defect topology of the critical nucleus determines the crystal morphology. From the growth of the crystals beyond the critical nucleus size, new insights into the role of defects in crystal growth were gained that are incompatible with the assumption of equilibrium growth. These results explain the complex crystal morphologies observed in experiments on hard spheres

Out-of-equilibrium processes in suspensions of oppositely charged colloids: Liquid-to-crystal nucleation and gel formation

We study the kinetics of the liquid-to-crystal transformation and of gel formation in colloidal suspensions of oppositely charged particles. We analyse, by means of both computer simulations and experiments, the evolution of a fluid quenched to a state point of the phase diagram where the most stable state is either a homogeneous crystalline solid or a solid phase in contact with a dilute gas. On the one hand, at high temperatures and high packing fractions, close to an ordered-solid/disordered-solid coexistence line, we find that the fluid-to-crystal pathway does not follow the minimum free energy route. On the other hand, a quench to a state point far from the ordered-crystal/disordered-crystal coexistence border is followed by a fluid-to-solid transition through the minimum free energy pathway. At low temperatures and packing fractions we observe that the system undergoes a gas-liquid spinodal decomposition that, at some point, arrests giving rise to a gel-like structure. Both our simulations and experiments suggest that increasing the interaction range favors crystallization over vitrification in gel-like structures