Physical Research

Liquids on Fibers: Slipping or Flowing?

Scientists reveal dynamics of droplet formation on fibers

15.7.2015 | An international team, including Karin Jacobs and Sabrina Haefner from Saarland University, together with Oliver Bäumchen from the Max Planck Institute for Dynamics and Self-Organization in Göttingen, and colleagues from Canada and France, has been able to show with novel experiments and mathematical models how a liquid film moves on a fiber, depending on the fiber coating.
Thin fibers play an important role in daily life, from the use of glass fibers in ultra-fast data transmission to textile fibers. In order to enable special properties of these fibers, they are often coated with a thin liquid layer that should be stable and homogeneous. On the other hand, the production of drinking water requires different features: if water is harvested from fog, for instance, it should flow smoothly along fibers as a liquid film or as liquid droplets. Now, scientists have been able to reveal whether liquid films flow slowly along the fiber or if they can slip faster on the fiber.

Many examples for liquids on fibers are known in nature. Just think about dew droplets on spider webs that you can observe during a walk in the morning. Indeed, humidity is collected on the fiber as droplets, as the liquid surface can be minimized this way. This phenomenon, which can also be observed for a stream of water flowing out of a faucet, is named the Rayleigh-Plateau instability. "All systems drive towards their energetic minimum, and that is the droplet shape in this case," says Sabrina Haefner, a physicist in the research group of Karin Jacobs. This instability can be very useful in very dry and remote regions of the world. For example, in Chile's Atacama desert, the getting drinking water is essential for the locals, and they harvest water from humidity by means of fiber nets.

Professor Karin Jacobs
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Professor Karin Jacobs
In industrial applications, however, it is often necessary to realize stable and homogeneous liquid films on fibers. So how can droplet formation be avoided? "The surface energy of the liquid, its viscosity, the thickness of the liquid film, as well as the diameter of the fiber, play an important role," explains Karin Jacobs. The international team of researchers has now found that the properties of the fiber itself also have a strong impact. "The contact between the liquid and the fiber is indeed very important," Oliver Bäumchen from the Max Planck Institute for Dynamics and Self-Organization explains. "If the liquid slips on the fiber surface, the droplet formation is much faster than in the case of just flowing along the fiber."

The team of physicists tested this for liquid films supported by uncoated and Teflon-coated fibers. On uncoated fibers, the liquid film moved rather slowly, and droplet formation took longer than on coated fibers, where the liquid film was able to slip. "In line with mathematical models, these experiments allow for quantifying ’slippage’ of liquid films and to precisely predict the dynamics of the droplet formation process," says Sabrina Haefner from Saarland University. The team of researchers agrees: their results are very important for the design of novel fiber coatings. Besides the German researchers, teams from McMaster University in Hamilton, Canada, and the ESPCI in Paris, France were involved in this project.
  (© Saarland University, AcademiaNet)
Gerhild Sieber

More information

Source

  • S. Haefner, M. Benzaquen, O. Bäumchen, T. Salez, R. Peters, J.D. McGraw, K. Jacobs, E. Raphaél, K. Dalnoki-Veress: "Influence of Slip on the Plateau-Rayleigh Instability on a Fibre", Nature Communications, 6, Article number: 7409, doi:10.1038/ncomms8409

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