Janne-Mieke Meijer

Alexander von Humboldt Research Fellow
University of Konstanz

Department of Physics
Soft Matter Physics
Colloidal dispersions consist of particles with a size between 1 and 1000 nm dispersed in a liquid. Because colloids exhibit thermal motion (Brownian motion) the properties of colloidal dispersions are similar to molecular systems. For example both exhibit diffusion, gas-liquid condensation and crystallization. Due to their size, the length and time scales of the structural dynamics are easily observable on single particle levels using optical microscopy and on a global level using scattering techniques. However, unlike atomic systems, the interaction potential of the colloidal particles can be tuned easily from attractive to repulsive by addition of polymers or surface chemistry. In addition, with recent advances in synthesis many different anisotropic shapes have become available. 

Exploring the geometrical effects of anisotropic colloidal particles on the dense structures that form addresses the fundamental problem of the relation between the building block shape and the resulting phase diagram and dynamics. Moreover, the directed assembly of colloidal particles is a promising route for the fabrication of functional nanomaterials. My research focusses on the effect of shape and interaction on the colloidal phase behaviour and particularly on the crystal and defect structures. The local structures are investigated using optical (confocal) and electron microscopy techniques, which are complemented with light and x-ray scattering techniques to determine the bulk crystal structures.
  1. Spheres
    By studying crystallization of different types of colloidal spheres using microscopy and small angle x-ray scattering insight into the colloidal crystal structures in 2D and 3D is obtained.
  2. Cubes/Superballs
    The assemblies of colloidal cubes/superballs are if interest for new functional materials, such as photonic crystals. Using optical and electron microscopy combined with small angle x-ray scattering the 2D and 3D structures are investigated in detail.
  3. Bowls
    Soft repulsive particles with a bowl like shape are studied with confocal laser scanning microscopy to determine the phase behavior, structures and dynamics, as well as the directed assembly.
Phase behaviour of Bowl-Shaped Colloids 
Soft repulsive charged bowl-shaped colloids can be obtained by nanoengineering spherical composite microgels, consisting of a polystyrene core surrounded by a crosslinked poly(N-isopropylmethacrylamide) shell. We study the self-assembly of these bowl-shaped colloids interacting via long-range repulsions and compare our finding to those of spherical particles. We investigate the effects of shape on the phase behavior and dynamics as well as their self-assembly directed by an external electric field.

Phase behaviour of Superballs
Hollow silica cubes posses rounded corners and can be described by a superball shape, which described all shaped between a sphere and a cube. We experimentally investigate the phase behaviour of these particles using a combination of confocal microscopy and small angle x-ray scattering with microradian resolutuib.

We found that by that tuning the shape, size and interactions we can probe the phase behaviour in a region where an enrichment of the phase behaviour is predicted.  we find an even richer phase behavior than predicted as three distinct crystal phases are uncovered.

J.M. Meijer, A. Pal, S. Ouhajji, H.N.W. Lekkerkerker, A.P. Philipse and A.V. Petukhov, (2017) Nature Communications, 8, 14352

Driven assembly of Superballs
The 2D and 3D self-assembly of colloidal superballs is induced by employing several convective assembly techniques, such as horizontal deposition and flow controlled vertical deposition. Their assembly, structure formation and resulting deposits are studied with optical and scanning electron microscopy. We want to assess whether the Λ0- and Λ1-lattices that were predicted by simulations and theory indeed occur in the silica cubes packings formed in this way. In addition, we analyse the local order and defect structures that form.

Meijer, J.M., Hagemans, F., Rossi, L., Byelov, D., Castillo, S.I.R., Snigireva, I., Philipse, A.P. & Petukhov, A.V. (2012) Langmuir, 28, 7631-7638
Crystals of Magnetic Colloidal Hematite Cubes and the Influence of an External Magnetic Field
We studied the organization of the colloidal cubes of hematite with a permanent magnetic dipole into three dimensional (3D) structures. The effect of their superball shape, surface charge and alignment in a magnetic field on the resulting self-assembled structures is investigated. Cube structures were characterized by small angle X-ray scattering (SAXS) with microradian resolution that provides structural information via the structure factor peaks. An external magnetic field was applied to induce particle alignment and led to the formation of a well-ordered crystal of silica coated hematite cubes that was investigated in detail. In addition the fluid-solid interface of the forming sediment was imaged in real space using Hard Transmisson X-ray Microscopy (TXM).

J.M. Meijer, Byelov, D., Rossi, L., Snigirev, A., Snigireva, I., Philipse, A.P. & Petukhov, A.V. (2013) Soft Matter, 9(45), 10729-10738
Colloidal crystals of spheres
Colloidal crystals and their internal structure have been the focus of many investigations because of their possible application as functional nanomaterials, for instance as photonic crystals. The presence of defects in the crystal structure influences the functional properties of the colloidal crystal and therefore it is important to fully characterize both the crystal and defect structures in detail. Furthermore, investigations of the optical colloidal crystal properties, e.g. response of the photonic structure to strong optical fields, are important for technical feasibility of the crystals as photonic devices.

Using recent developed X-ray techniques, such as hard X-ray microscopy (HXRM), high resolution small angle x-ray scatterin (SAXS) and coherent X-ray diffraction imaging (CXDI) we study the internal structure of specifically prepared colloidal crystals, such as, bulk crystals, films or small grains.

A. G. Shabalin, J.M. Meijer, R. Dronyak, O. M. Yefanov, A. Singer, R. P. Kurta, U. Lorenz, O. Y. Gorobtsov, D. Dzhigaev, S. Kalbfleisch, J. Gulden, A. V. Zozulya, M. Sprung, A. V. Petukhov, and I. A. Vartanyants. (2016) Phys. Rev. Lett. 117, 138002 

J.M. Meijer, A.G. Shabalin, Dronyak, R., Yefanov, O.M., Singer, A., Kurta, R.P., Lorenz, U., Gorobstov, O., Dzhigaev, D., Gulden, J., Byelov, D.V., Zozulya, A.V., Sprung, M., Vartanyants, I.A. and Petukhov, A.V. (2014) J. Appl. Cryst., 47, 1199–1204

J.M. Meijer, V.W.A. de Villeneuve and A.V. Petukhov (2007) Langmuir, 23, 3554-3560.
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