Smoothing of capillary waves
In microfluidics, the field that deals with the use of liquids for example in sensors at a microscale, it is important to understand very well the behaviour of extremely thin liquid layers and their interfaces. Such an interface always contains very small waves that spontaneously occur, the so-called capillary waves. Researchers of the Foundation for Fundamental Research on Matter (FOM), Utrecht University and the Ecole Normale Supérieure in Paris have discovered that setting a shear flow smoothes the waves. One would assume that the flow would intensify the waves, like a strong wind over a water surface leads to more and larger surface waves. The researchers made their discovery by using a model system in which a fluid and gas are existing together, just like water and water vapour, but which has the advantage that the interface between these two phases is so rough that it is visible under a microscope. Their results have been published in the Physical Review Letters of 21 July 2006.
The interface of two media that are in a different phase - for example a fluid with gas on top - is constantly moving by arbitrary movements of the molecules as a result of the temperature. As a consequence, microscopically small fluctuations originate spontaneously, the so-called capillary waves. The smaller the interfacial tension, the higher the waves and the rougher the interface becomes. Unlike wind - a stronger wind causes more and higher waves - an increasing shear flow suppresses the capillary waves at a microscale. So, the interface is smoothed through stronger shear.
Researchers of the FOM Foundation, Utrecht University and the Ecole Normale Supérieure in Paris discovered this phenomenon for the first time with the aid of a specially built setting in a model system. This system consists of a mixture of colloids - dispersed particles - and polymers that provide the colloids to attract a little. This attraction sees to it that a similar mixture subdivides into a phase consisting of many colloids and few polymers (colloidal liquid) and a different phase consisting of many polymers and few colloids (colloidal gas). As the phase with many colloids is heavier, it falls to the bottom of the cell and a horizontal interface develops with the colloidal gas (see figure and also /live/nieuws/archief_persberichten/2004/artikel.pag?objectnumber=26543).
As colloids are much larger and slower than molecules, such a colloidal system is very sensitive to the effect of an external field, which the researchers* have made use of. They created a cell in which they were able to construct and maintain a shear flow very accurately parallel to the interface. Besides, they were able to keep the interface in the cell constantly in its place, despite the flow. They examined this interface with a so-called confocal microscope, with which cross sections of the interface were made. This is shown in the figure.
The observations had a surprise in store: the shear flow does not make the interface rougher but smoother, as is visible in the figure. In itself, one could suppose that more and higher waves will develop at the flow, but that appeared not to be the case with these microscopic little waves. If there is not a flow, free fluctuating of capillary waves becomes evident in a low interface tension. In case of flow the slower part of the waves is hampered to fluctuate freely. By calculating the researchers discovered that the interface tension became effectively larger. Consequently, the then existing waves become lower and the interface itself becomes smoother.
This is very interesting knowledge in order to formulate new theories and to describe the hydrodynamics of interfaces at a microscale. Capillary waves and shear flow play also an important role in the coalescence of droplets and in microfluidics. So, this newly discovered phenomenon should play an important role.
*Didi Derks, Dirk Aarts, Daniel Bonn, Henk Lekkerkerker and Arnout Imhof
For more information contact Didi Derks or Arnout Imhof, Soft Condensed Matter, Universiteit Utrecht, phone +31 (0)30 253 24 23.