Extremely low surface tension in suspensions of colloidal platelets
Researchers of Utrecht University (financially supported by the Netherlands Organisation for Scientific Research (NWO, Chemical Sciences) and the Foundation for Fundamental Research on Matter (FOM) have shown, together with fellow researchers in Eindhoven, Düsseldorf, Mainz and Bristol, that the surface tension in a suspension of colloidal platelets is extremely low. They have been investigating interfaces between a disorderly state and a fluid crystalline state. The surface tension, however, is 25 million times lower than that of the interface between water and air. This extremely low surface tension offers new possibilities to investigating the merging and breaking of drops, the fluid meniscus at a glass wall and the waves on the interface. The researchers have published the results in Physical Review Letters on 25 August 2006.
The whirligig beetle (Het Schrijverke)
Anyone who knows Guido Gezelles poem 'Het Schrijverke' or has ever been gazing at the waterlevel of a pond, does know that some insects are able to stand and walk on water. The fact is that the insect pushes down the interface between the air and the water underneath him by using gravity (see figure 1). Then, the interface is transforming somewhat and is pushed back like a tightened rubber. The ease with which this happens, is characterised by the so-called surface tension. The lower the surface tension, the simpler the interface is likely to be transformed. So, the size of the surface tension determines, among others, the shape of a drop, the speed at which the drops will merge, the height of the waves on the interface, or the shape of a fluid meniscus at a glass wall.
Extremely low surface tension
Colloids are tiny particles, the size of about one thousand times larger than the separate atoms. Physicists use colloidal suspensions, in which these particles are floating in a solvent, as a model system for studying the phase changes in matter such as melting and freezing. It has been known for a long time, also thanks to the investigation by researchers at Utrecht University, that disc-shaped colloidal particles in a suitable solvent are able to break spontaneously into two states (see figure 2). The one state is called fluid crystalline, in this state all platelets are pointing in the same direction. The other state is a disorderly one. In this so-called isotropic state the colloids are gyrating criss-cross.
For the first time researchers David van der Beek and Henk Lekkerkerker of Utrecht University have been succeeded in determining the surface tension of the interface between the two states. They were experimenting with so-called gibbsite platelets, a mineral of aluminum hydroxide of 240 nanometer in diameter and 18 nanometer thick, dispersed in toluene. Surprisingly, they found an extremely low value for the surface tension. This surface tension is 25 million times lower than that of the interface between water and air, on which Gezelles ‘schrijverke’ (whirligig beetle) is able to walk.
Measurement and theory
Despite the low value of the surface tension, it appears to correspond very well to theoretical analyses that have been executed by René van Roij (Utrecht University/FOM), Paul van der Schoot (Eindhoven), Henrik Reich (Düsseldorf), and Matthias Schmidt (Düsseldorf/Bristol), and also to the computer simulations by Marjolein Dijkstra (Utrecht University), Richard Vink (Düsseldorf) and Tanja Schilling (Mainz). Although insects will not be able to stand or walk on this extremely weak interface, the weakness does offer unique, new possibilities to further investigation: the interface tension that is measured and computed is an important first step in studying important phenomena like the breaking and merging of drops or of capillary waves on the surface of so-called 'complex fluids'.
For more information, please contact Dr. René van Roij, Utrecht University, phone: (030) 253 75 79.