General


The term “colloidal matter” characterizes a class of materials consisting of large assemblies of colloidal particles. As the individual particles are substantially larger than the atomic scale but still much smaller than a macroscopic size, these materials have unusual thermodynamic, rheological and optical properties bridging the gap between the molecular and macroscopic world. This makes colloidal matter interesting, both from the fundamental and applied point of view. In addition, the size of the building blocks makes these materials amenable to relatively simple chemical modifications, allows for quantitative 3D analysis with confocal microscopy and makes it possible to manipulate the structures with external fields. Computer simulations on the same systems that are studied experimentally in real-space provide a powerful combination to increase our understanding of these (soft) condensed matter model systems. Our fundamental work focuses on using colloids as a way to extend our knowledge of condensed matter problems like, freezing/melting, the glass transition. Our interest in more applied use of soft condensed matter focuses on photonics.


Colloidal Model Systems

Using chemical synthesis techniques a variety of colloidal core-shell particles can be prepared. For fluorescence confocal microscopy the core consists of a fluorescently labeled material, while the shell consists of a non-fluorescent material like pure silica. In this case the core enables the confocal detection while the larger shell makes it possible to detect even touching spheres and to chemically modify the surface. Most of the syntheses and characterization are done at the Van ‘t Hoff Laboratory of the Debye Institute (Utrecht University).

Schematic picture of fluorescent core-silica shell
particle used for confocal microscopy

SEM picture of fluorescent core-silica shell particle used for confocal microscopy

TEM picture of silica particles with a gold core
(gold radius 15 nm, total 150 nm)

SEM picture of a high refractive index ZnS-cores (radius 1.6 mm)

Different core-shell morphologies give the particles different specific properties suited for various applications: optical tweezers, photonic crystals, ER fluids

Fluorescent-silica (200 nm radius)-core PMMA-
shell colloid (final radius 600 nm). These
spheres can be index and density matched.
Ellipsoidal silica spheres (original size 300 nm and 1000 nm) made by ion beam irradiation (together with the group of Albert Polman, AMOLF)


Experimental Techniques

Confocal Microscopy





In confocal microscopy a pinhole is used in the focal plane both at illumination and at detection. In this way out of focus emitted light is effectively rejected by the detection pinhole and an increased resolution is obtained (top left). By scanning through the focal plane an image of a slice inside the sample can be taken (top right). From several slices taken at different heights a 3D-image of the sample can be reconstructed (bottom left) and particle coordinates can be obtained (right). A 3D reconstruction of a colloidal liquid-crystal interface is shown (bottom right).


Computer Simulations (Marjolein Dijkstra)

Projects


Condensation of Charged Spheres and Swelling of Clay Platelets (01PR1983: Antti-Pekka Hynninen)

The explanation of the experimentally observed phenomena of (i) attraction between like-charged colloidal spheres and (ii) the sol-gel transition in clay suspensions are two of the most profound problems in colloid science. We propose to study both phenomena by simulation using the classical analogue of the Car-Parinello method. In this recently introduced method the mesoscopic (colloidal) particles are treated explicitly by Molecular Dynamics simulation, whereas the microions are treated on the level of their density profiles that follow from minimising the free energy functional. We plan to extend this novel method to systems coupled to a salt-reservoir, and to combine it with standard techniques for determining phase equilibria, such as the Gibbs Ensemble method and thermodynamic integration. This combination of extensions allows us to determine the phase diagram and the structure of suspensions of colloidal spheres and clay particles, not only in the well-understood regime of high salt concentrations, but also in the low-salt regime, where the unexplained phenomena (i) and (ii) were observed.


Colloidal Rods at Interfaces (UU: Svyatoslav Savenko)

In this project we plan to study suspensions of rodlike particles in contact with a planar hard wall in which rodlike particles are embedded parallel to the wall. In order to investigate the wetting behaviour, we recently developed a new Monte-Carlo method for simulating fluids in contact with a single wall. In this method, we simulate the suspension in contact with a single wall, while a flat density profile is imposed far from this wall using a penalty function that suppresses large density fluctuations from a self-consistently determined averaged bulk density. In this way, we can avoid simulations with two walls, which can induce capillary condensation/nematization. Using this technique, we were indeed able to follow the logarithmic growth of a thick nematic film at the wall-isotropic interface (see figure on the cover page). We propose to investigate the interfacial behaviour of hard-rod fluids in contact with corrugated walls by extending this technique to structured walls. We hope that our simulations can give some insight in the origin of the alignment effects of LC molecules on corrugated/rubbed surfaces.


Entropic Wetting in Colloidal Mixtures (01SFFSM22: Andrea Fortini)

The focus of this proposal is on two different systems where the bare interactions are either hard o r ideal: (i) a simple model for a mixture of colloids and ideal polymer and (ii) a mixture of thin a nd thick rod-like particles. Our group has a proven track record in the determination of the bulk phase behaviour of both systems with theory and simulations, which gives us an excellent position to study the wetting behaviour. For both systems, we plan to investigate how the topology of the phase diagram influences the location of wetting transitions.


Colloidal Epitaxy

Colloidal epitaxy provides a means to direct the growth of colloidal crystals. Under the influence of gravity colloids settle at the bottom of the container, forming close packed crystalline domains. By using a corrugated wall where the pattern of holes equals a well chosen crystal plane, a colloidal crystal can be grown epitaxially. In this way an almost perfect face centered cubic crystal of hard-sphere like silica particles was grown. At the moment we are trying to grow a hexagonal close packed crystal, which, for hard spheres, has a slightly higher free energy.

View cut out of a 3D-stack of confocal microscopy images of a crystal grown on a fcc (100)-template Computer-generated reconstruction of the coordinates obtained from this dataset.

Confocal microscopy images of two fcc (100)-template with different lattice constants
In the intermediate (unpatterned) region a reconstruction to a hexagonal structure can be seen.
When the distance between the patterned regions is decreased, the (100)-structure extends over the unpatterned region.

Manipulation of colloidal crystal growth has applications for the production of photonic crystals. Understanding the dynamics of epitaxial crystal growth is on the other hand an interesting and fundamental research question. Even the role of gravity in the epitaxial growth process, which is not yet understood, can be examined by using particles that can be density matched with their suspending liquid.


Inverse Photonic Crystals (02POM10: J.H.J. Thijssen)

Photonic crystals are 3D structures in which the refractive index varies periodically throughout space. Such structures form a periodic potential for photons, causing strong interactions with light. In a way, these interactions can be considered as Bragg-reflections. Provided the refractive index contrast is large enough and a suitable structure is chosen, photonic crystals can exhibit a photonic band gap. This means that light of a certain frequency range cannot propagate within the crystal in any direction regardless of its polarization. In other words, light of a frequency within the gap will be Bragg-reflected such that standing waves are formed in all directions for all polarizations. Thus, a photonic band gap is the optical analogue of an electronic band gap in semiconductors. However, since Maxwell's equations are scale-invariant, a structure can be designed to have a band gap at any desired wavelength. For example, if the gap is tuned at a wavelength of 1.3 or 1.5 micron, photonic crystals could be used in fibres for longe-distance telecommunication.


White-light-illumination image of Bragg colors produced by a photonic crystal of 1.4 micron diameter silica spheres.

One way of fabricating photonic crystals is by colloidal self-assembly. Colloids are particles with sizes on the order of 1 nm up to 1 micron. These colloidal particles could be ideal building blocks for photonic crystals, for they feature sizes on the order of visible and near-infrared light, and they are known to self-assemble under the right conditions. Furthermore, the single particle properties of colloids can be chemically modified. Regular structures of colloids are called colloidal crystals and if the refractive index of the colloids is different from that of the host, which is almost always the case, these crystals are photonic crystals, by definition.

In our group, two routes towards photonic crystals are explored. One of them uses monodisperse colloidal silica spheres to form colloidal crystals that serve as a template. The spheres are usually labelled with a fluorescent dye, which allows individual imaging of touching particles using confocal microscopy. Other characterisation techniques that are used include laser diffraction, x-ray diffraction (see figure), Vis-NIR spectroscopy and electron microscopy.


X-ray diffraction image of an inverted bct crystal of 1.4 micron diameter silica spheres.

With a little bit of effort, it is possible to grow face-centred cubic (fcc) colloidal crystals by sedimentation of monodisperse colloidal spheres. It has been calculated that an fcc crystal of air-spheres in silicon has a band gap with a relative width of 5%. However, this gap is not very robust and its width is not spectecular. For example, a diamond crystal of air spheres in silicon exhibits a much larger and much more robust gap at a much lower index contrast However, up till now, nobody has succeeded in fabricating a photonic crystal with diamond symmetry and a sufficient index contrast.

In our group, we try to grow colloidal crystals with symmetries other than the fcc one as well. For example, sedimentation of highly charged spheres with a long-range interaction yields a body-centred cubic (bcc) crystal. Sedimentation in an electric field perpendicular to gravity yields crystals with body-centred tetragonal (bct) or face-centred orthorhombic (fco) symmetry, depending on the charge of the colloids. For example, the figure shows a confocal microscope images of the bct hexagonal (110) plane; the lower figure is a side-view ((100) plane), clearly showing bct AB-stacking. In this way, using external fields to influence the crystallization of colloidal particles, it might be possible in the future to obtain even more different structures.


Confocal xy-scan ((110)-plane) of a bct crystal of 1.4 micron diameter silica spheres.
Confocal xz-scan ((100)-plane) of the same crystal.

Usually, silica colloids are used for fabricating colloidal crystals. The refractive index of silica equals 1.45 (compare 1.33 for water and 1.5 for glass). If air is the host medium, the index contrast of such a silica colloidal crystal is 1.45, which is too low to open up a band gap. Thus, in a final step, the colloidal crystal is used as a template for infiltration using chemical vapor deposition (CVD). The colloidal crystal is infiltrated with a high-index material, such as silicon (see figure). Silicon has the advantage that it has a large refractive index (3.5) and absorption is small for wavelengths larger than 1.1 micron. Furthermore, silicon photonic crystals should be compatible with existing technologies from the semiconductor industry. In a last step, the original silica template is removed by wet etching, enlarging the index contrast (from 2.41 to 3.5).


Scanning Electron Microscope image of an fcc air-sphere crystal in a backbone of silicon (image by D.C. 't Hart).



Electro-Rheological Fluids

When a dispersion of uncharged colloidal spheres is placed in a (uniform) electric field the dielectric constant difference between particles and solvent creates dipolar interaction potentials between the spheres. If the fields are so high that the dipolar interactions between the spheres are several times kT non-equilibrium string-like structures are formed spanning the container and the dispersion starts behaving like a solid. This ability to change viscosity over several orders of magnitude in milliseconds is useful for applications like shock absorbers or a variable transmission. The proposed lowest energy structure for monodisperse spheres at high fields is a body centered tetragonal crystal (BCT). At relatively low fields (~0.5 V/mm), where interaction forces between the spheres were only several times kT, we observed such BCT crystals (figure on the right). Furthermore, intriguing metastable sheet-like structures, not yet predicted by theory, were seen as precursor to the BCT crystals (left three Figures). At higher concentrations, where the equilibrium phase without field is an FCC colloidal crystal, an interesting martensitic FCC-BCT transition was found.

Particles 1 micron diameter, fluorescent core 400 nm.Confocal micrographs, field strength: ~0.5 V/mm.

ER fluids time sequence showing sheets (perpendicular to the field and along the field). Finally BCT crystals form (last image inset is enlarged, field perpendicular).
   
FCC-BCT crystal transition

Optical Tweezers


Metallo-Dielectric Colloids


Binary Colloids


Monodisperse Anisotronic Colloids


Real-Space Analysis of Sedimentation


Coating Monodisperse Emulsions


Equipment


modified: 25-04-2018, 11.29