Research proposals for Part II students

Colloidal suspensions, in which particles of ~ 1 µm (the colloids) are dispersed in a solvent, display rich phase behaviour similar to that of molecules or atoms. Because they are so big, they are also slow, and they can thus be studied by optical microscopy. Moreover, again due to their size they can be readily perturbed by external or internal forces. Some examples of research areas for potential Part II projects are listed below. To discuss any of these in more detail contact Dirk Aarts. For other interesting part II proposals, surf to Roel Dullens' part II page.

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)
Drop coalescence Drop breakup

Demixing in confinement

Gravity driven flow Gravity driven flow

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)
  • and check: this page

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). This work is part of an ongoing collaboration with Roel Dullens and Volkert de Villeneuve .

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)