|Research proposals for
Part II students
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.
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.
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:
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: