When two drops of water... become one
Within a project of Technology Foundation STW, partially financed by FOM, researchers from the University of Twente have shown how drops flow together on a surface. That is useful knowledge for printer manufacturer Océ, for example. They are using this result to predict how ink from an inkjet printer behaves if it is printed on complex materials. The findings were published on 1 October in the online edition of Physical Review Letters.
The researchers from the University of Twente let water drops develop on the point of a needle. These drops fell onto a surface that was successively covered with different coatings. By changing the chemical composition of the coating used, the researchers could vary the contact angle between the drops and the surface. They changed the shape of the drop by moving the needles up and down. With this method, the physicists could very accurately determine the influence of the drops' geometry on the rate and manner in which they flowed together.
Immediately after the initial contact between the two drops a bridge develops on the surface. For contact angles less than 90 degrees the height of this bridge is a function of time to the power of two thirds. However, as soon as the contact angle of the drop with the surface becomes exactly 90 degrees, which concurs with perfect half-spherical drops, the height of this bridge is a function of time to the power of a half. This means that spherical drops flow together faster than drops with an asymmetrical shape. This result agrees with previous measurements on freely moving drops , which are perfectly spherical as a consequence of the surface tension .
Vidi project
The research is part of former FOM PhD Jacco Snoeijer's Vidi project, in which he is studying the movement of fluids across surfaces. The behaviour of fluid drops is not only decisive for everyday phenomena such as condensation and cloud formation. How rapidly individual drops flow together is also important in technological applications. Printer manufacturer Océ and chip machine manufacturer ASML are therefore involved in Snoeijer's project. Océ would like to understand how ink drops flow together, especially on materials where the ink is not absorbed immediately. ASML is interested due to its immersion technology where a thin layer of water serves as an extra lens to make even smaller images still.
According to Snoeijer, his Vidi project illustrates how well universities and industry can collaborate: "Although this research has a fundamental character, it is directly relevant for Océ. They are already using our results to validate mathematical models that allow them to predict the behaviour of their inks".
Caption for a short film
Water_spheres_90degrees_Fig3.avi: at a contact angle of 90 degrees the time dependence abruptly shifts and becomes a function of time to the power of a half.
This film was made with a high-speed camera that operates at 200.000 frames per second. The film is played 10.000 times slower.
Reference: 'Influence of Droplet Geometry on the Coalescence of Low Viscosity Drops', A. Eddi, K.G. Winkels and J.H. Snoeijer, Phys. Rev. Lett.
The research is partly funded by FOM, ASML and Océ.
Further information
Link to the research group: http://stilton.tnw.utwente.nl/people/snoeijer/
Further information can be obtained from Dr. Jacco Snoeijer, +31 (0)53 489 30 85.