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A colloidal model system with an interaction tunable from hard sphere to soft and dipolar

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Abstract

Monodisperse colloidal suspensions of micrometre-sized spheres are playing an increasingly important role as model systems to study, in real space, a variety of phenomena in condensed matter physics—such as glass transitions and crystal nucleation1,2,3,4. But to date, no quantitative real-space studies have been performed on crystal melting, or have investigated systems with long-range repulsive potentials. Here we demonstrate a charge- and sterically stabilized colloidal suspension—poly(methyl methacrylate) spheres in a mixture of cycloheptyl (or cyclohexyl) bromide and decalin—where both the repulsive range and the anisotropy of the interparticle interaction potential can be controlled. This combination of two independent tuning parameters gives rise to a rich phase behaviour, with several unusual colloidal (liquid) crystalline phases, which we explore in real space by confocal microscopy. The softness of the interaction is tuned in this colloidal suspension by varying the solvent salt concentration; the anisotropic (dipolar) contribution to the interaction potential can be independently controlled with an external electric field ranging from a small perturbation to the point where it completely determines the phase behaviour. We also demonstrate that the electric field can be used as a pseudo-thermodynamic temperature switch to enable real-space studies of melting transitions. We expect studies of this colloidal model system to contribute to our understanding of, for example, electro- and magneto-rheological fluids.

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Figure 1: Tunability of interactions.
Figure 2: Influence of softness on the phase behaviour.
Figure 3: Volume fraction–electric field phase diagram.
Figure 4: Melting of a b.c.t. crystal into a string fluid is not the reverse of crystallization of the string fluid.

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Acknowledgements

We thank G. Bosma for particle synthesis, C. van Kats for electrophoresis measurements, H. Wisman for technical support, K. van Walree for the suggestion of TCAB salt, and discussion of charge mechanisms, and J. Hoogenboom, S. Auer and D. Frenkel for discussion. We also thank M. Dijkstra and G. Patey for a critical reading of the manuscript. This work is part of the research program of the ‘Stichting voor Fundamenteel Onderzoek der Materie (FOM)’, which is financially supported by the ‘Nederlandse organisatie voor Wetenschappelijke Onderzoek (NWO)’.

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  1. Anand Yethiraj

    Present address: Department of Chemistry, 2036 Main Mall, Vancouver, British Columbia, Canada, V6T 1Z1

Authors and Affiliations

  1. Soft Condensed Matter, Debye Institute, Utrecht University, Padualaan 5, 3584CC, Utrecht

    Anand Yethiraj & Alfons van Blaaderen

  2. FOM Institute for Atomic and Molecular Physics, Kruislaan 407, Amsterdam, 1098 SJ, The Netherlands

    Anand Yethiraj & Alfons van Blaaderen

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  1. Anand Yethiraj
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Correspondence to Anand Yethiraj or Alfons van Blaaderen.

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Yethiraj, A., van Blaaderen, A. A colloidal model system with an interaction tunable from hard sphere to soft and dipolar. Nature 421, 513–517 (2003). https://doi.org/10.1038/nature01328

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