In our group we study colloidal suspensions, where particles on the order of a micrometer (the colloids) are dispersed in a solvent. Colloidal systems are highly interesting and relevant, as they find numerous applications in several industrial branches such as coatings, food, cosmetics but also in more technical applications as photonic crystals and data storage devices. In addition, colloids are widely accepted as a versatile model system for atoms and molecules as their phase behaviour is analogous to that of atomic and molecular systems; they display rich phase behaviour involving colloidal ‘crystal’, ‘liquid’ and ‘gas’ phases. The typical colloidal length and time scales, i.e. micrometers and seconds, make it possible to directly observe colloidal particles in real-space and real-time using video- and/or confocal microscopy. Advanced colloid chemistry techniques are available to tune the chemical and physical properties of the particles or even to develop completely new and unique colloidal model systems. In addition, colloidal systems are easily deformed and manipulated using external fields such as optical laser tweezers.
- Colloidal model systems
- Optical tweezers
- Grain boundaries and frustrated crystallisation
- Crystallization in confinement
- Solid-fluid interfaces
- Hydrodynamics at ultralow interfacial tension
- Demixing in confinement
- Direct experimental verification of the free volume theory for colloid-polymer mixtures
- Developing a colloidal surfactant system
- And many more…
Synthesis and Characterization of New Colloidal Model Systems
Well defined colloidal model systems are of upmost importance in the study of colloidal dispersions. Using colloid chemistry the chemical and physical properties of the colloidal model system can be precisely adjusted to the physical experiments in mind. Therefore, the synthesis and characterization of colloidal model systems plays a central role in our research.A wide variety of chemical techniques is available to synthesize many different colloidal particles with very specific properties. For example, very monodisperse silica or latex spheres can be made, bit also rods, magnetic and fluorescent colloids can be prepared. Also the specific interactions between colloids can be controlled using surface chemistry. To characterize the particles several techniques such as light scattering, optical (confocal) microscopy and electron microscopy will be used.
- Alfons van Blaaderen, “Colloids get complex”, Nature (News and Views) 439, 545 (2006)
- Roel P.A. Dullens, “Colloidal hard spheres: Cooking and Looking”, Soft Matter, 2, 805 (2006)
An optical tweezer is a strongly focussed laser beam that can trap small objects, such as colloids, using the forces that are exerted by the light. The scattering forces push the particles down and the gradient forces pull the particle towards the center of the beam. Combining optical tweezers and colloidal systems facilitates the ability to control, manipulate and deform colloidal systems on the microscopic, i.e. single-particle level (see MOVIE on the left!).
- D. G. Grier, Nature 424, 810 (2003)
- D. L. J. Vossen et al, Rev. Sci. Instrum. 78, 2960 (2004)
- M. J. Lang, S. M. Block, Am. J. Phys. 71, 201 (2003)
Grain Boundaries and Frustrated Crystallisation
The strength of materials is closely related to the grain size of the material. However, grain boundary stability is still far from understood. Using geometrical frustration, crystals which are rich in grain boundaries can be prepared. By studying the structural and dynamical behaviour of both colloidal single crystals and crystal imperfections insight will be gained into the relation between frustration and the stability of grain boundaries.Further reading:
- Roel P.A. Dullens, Maurice C.D. Mourad, Dirk G.A.L. Aarts, Jacob P. Hoogenboom and W.K. Kegel, Shape-induced frustration of hexagonal order in polyhedral colloids, Phys. Rev. Lett., 96, 028304 (2006)
- Volkert W.A. de Villeneuve, Roel P.A. Dullens, Dirk G.A.L. Aarts, Esther Groeneveld, Johannes H. Scherff, Willem K. Kegel and Henk N.W. Lekkerkerker, Colloidal hard sphere crystal growth frustrated by large spherical impurities, Science, 309, 1231 (2005)
Crystallization in Confinement
Suspensions of hard-sphere colloids display an entropy-driven fluid-crystal transition. This remarkable phenomenon widely serves as a simple model of crystallization in atomic systems. The ordered colloids scatter light in a well-defined manner leading to sharp Bragg reflections as can be seen in the image on the right (clicking on it will bring you to a very informative website on small-angle X-ray scattering by Andrei Petukhov). Confinement changes both the crystallization kinetics and the crystalline structures, which has important consequences for the material properties such as strength, elasticity etc. This can again be studied by combining microfluidics and colloids. Moreover, such small systems are particularly suited to explore with computer simulations (see the simulation snapshots on the right).
For further reading:
- R.P.A. Dullens, D.G.A.L. Aarts and W.K. Kegel, Dynamic broadening of the crystal-fluid interface of colloidal hard spheres, accepted for publication in Phys. Rev. Lett. (2006)
- R.P.A. Dullens, D.G.A.L. Aarts, W.K. Kegel, and H. N. W. Lekkerkerker, Mol. Phys., The Widom insertion method and ordering in small hard sphere systems, 103, 3195 (2005)
- V.W.A. de Villeneuve, R.P.A. Dullens, D.G.A.L. Aarts, E. Groeneveld, J.H. Scherff, W.K. Kegel and H.N.W. Lekkerkerker,Colloidal hard sphere crystal growth frustrated by large spherical impurities, Science, 309, 1231-1233 (2005)
- A.V. Petukhov, D.G.A.L. Aarts, I.P. Dolbnya, E.H.A. de Hoog, K. Kassapidou, G.J. Vroege, W. Bras, and H.N.W. Lekkerkerker, High-Resolution Small-Angle X-Ray Diffraction Study of Long-Range Order in Hard-Sphere Colloidal Crystals, Phys. Rev. Lett., 88, 208301 (2002)
Suspensions of hard-sphere colloids display an entropy-driven fluid-crystal transition. This remarkable phenomenon widely serves as a simple model of crystallization in atomic systems. Detailed knowledge of the structure and dynamics of the solid-fluid interface is of central importance for processes such as crystal nucleation and growth. Using specially developed ‘core-shell’ colloidal hard spheres and confocal microscopy the equilibrium and non-equilibrium properties of the interface can be studied at the single particle, model atomic level.Further reading:
- Roel P.A. Dullens, Dirk G.A.L. Aarts and Willem K. Kegel, Colloidal crystal-fluid interfaces, accepted for publication Philosophical Magazine Letters (2007)
- Roel P.A. Dullens, Dirk G.A.L. Aarts and Willem K. Kegel, Dynamic broadening of the crystal-fluid interface of colloidal hard spheres, Phys. Rev. Lett., 97, 228301 (200)
Hydrodynamics at Ultralow Interfacial Tension
In many hydrodynamic instabilities the interfacial tension plays a driving role. For example, in the first stages of droplet coalescence it leads to velocities of order 10 m/s in molecular fluids, but only of order µm/s in our model system. This allows studying the hydrodynamics in great detail. There is a wide number of instabilities that may be explored in microfluidics; droplet breakup and coalescence (click the movies on the right), the Saffman-Taylor (viscous fingering) instability, the Kelvin-Helmholtz instability, etc. A better understanding of these instabilities may e.g. lead to a better understanding of spraying and other droplet formation processes, underlining besides the fundamental, also the practical relevance of such studies.
Furthermore, by studying these instabilities we are now beginning to understand at what length scales classical hydrodynamics starts to break down and statistical mechanics takes over.
For further reading:
- D. Derks, D.G.A.L. Aarts, D. Bonn, H.N.W. Lekkerkerker, and A. Imhof, Suppression of thermally excited capillary waves by shear flow, Phys. Rev. Lett. 97 038301 (2006)
- D.G.A.L. Aarts, H.N.W. Lekkerkerker, H. Guo, G. Wegdam and D. Bonn, Hydrodynamics of droplet coalescence, Phys. Rev. Lett. 95, 164503 (2005)
- D.G.A.L. Aarts, M. Schmidt, and H.N.W. Lekkerkerker, Direct visual observation of thermal capillary waves, Science, 304, 847 (2004)
Demixing in Confinement
A fluid-fluid phase separation proceeds in several stages – in molecular fluids these are difficult to follow due to the large (interfacial) driving forces. In colloidal systems the separation is again much slower (see the movies!). By confining a phase separating system one directly affects the thermodynamic instability that is at the base of the demixing. One may affect the spectrum of unstable density fluctuations, or even prohibit a critical nucleus from forming. In such instants surface and wetting effects will become dominant. Given the current trend of miniaturization these problems of fundamental nature are now encountered in industry as well. Through the appropriate choice of colloids and microfluidic cells this can be explored in detail.
For further reading:
- D.G.A.L. Aarts, R.P.A. Dullens, and H.N.W. Lekkerkerker, Interfacial dynamics in demixing systems with ultralow interfacial tension, New J. Phys. 7, 40 (2005)
- D.G.A.L. Aarts and H. N. W. Lekkerkerker, Confocal scanning laser microscopy on fluid-fluid demixing colloid-polymer mixtures, J. Phys.: Condens. Matter, 16, S4231 (2004)
- D.G.A.L. Aarts, J.H. van der Wiel, and H.N.W. Lekkerkerker, Interfacial dynamics and the static profile near a single wall in a model colloid-polymer mixture, J. Phys.: Condens. Matter, 15, S245-S250 (2003)