Clogging the mesh with drops

Researchers from the Group of Research and Applications in Statistical Physics (GRASP) - CESAM Research Unit / Faculty of Sciences - in collaboration with Utah State University and Brigham Young University, have just defined the exact angle at which folded fibres can retain a maximum of liquid. The applications of this discovery are numerous, including the development of microfluidic technologies and the optimization of water collection in arid regions. This research has just been the subject of a scientific article published in the journal Soft Matter (1).

O

n your next walk through the woods, take a look at the dew droplets attached to the plants. Look, in particular, at those trapped between two fir thorns or those suspended at the end of a cypress branch: they are imposing and seem to defy the laws of physics.

Inspired by these large droplets that form at the base of a leaf or other thin filament, researchers from the University of Liège (Prof. Nicolas Vandewalle and Dr Florianne Weyer), Utah State University (Prof. Truscott et Dr Pan) and Brigham Young University (Dr. Pitt) looked at the physics of the drops attached to these unique inverted V-shaped structures. By studying how these drops cling to a bent fiber at a certain angle, the researchers were able to determine the optimal geometry allowing to suspend as much liquid as possible.

"We tested our theory by building a rigid circular frame on which we fixed a nylon fiber," explains Floriane Weyer, PhD student at GRASP – CESAM Research Unit. Then we attached a narrower fiber at the center and pulled the original horizontal fiber upward, forming an upside down V. By varying the fibre attachment locations, we could change the angle formed between the two halves of the bent fiber. Liquids were applied at the corner formed by the fiber, using a micro-pipette. The droplet volume was gradually increased until the droplet detached from the fiber. Researchers quickly found that the largest drops were obtained when the angle was small. After numerous experimental tests, they were able to determine that a curved fiber forming an angle of 36-degrees trapped the most water, three times more than a horizontal fiber could retain. A model that can predict this angle has even been developed. Their discovery is published in the journal Soft Matter(1), a prestigious journal at the intersection of physics, chemistry and biology.

"The applications of this discovery are numerous," explains Nicolas Vandewalle, Director of GRASP - UR CESAM.  « One thinks of microfluidic applications, such as the monitoring of biochemical reactions (2) at the intersection of two optical fibers whose angle must therefore be perfectly controlled. But, if the geometry strongly influences the quantity of water captured by fibers, one can also imagine optimizing the collection of water by fog nets in arid regions by an adequate choice of mesh (3). » These nets are used to recover the moisture contained in the morning fog. Their biomimetic optimization is also the central point of a WISD FNRS project (support for projects on sustainable development and transition that aim to better circumscribe nature, criteria, living conditions, dynamics and transition paths towards sustainable development) conducted by the University of Liège (Prof. Vandewalle) and the Université Libre de Bruxelles (Prof. Terwagne).

Scientific references

(1) Z.Pan, F.Weyer, W.G.Pitt, N.Vandewalle and T.T.Truscott, Drop on a bent fiber, Soft Matter, 2018

(2) M.Lismont, N.Vandewalle, B.Joris, and L.Dreesen, Fiber based optofluidic biosensors, Appl. Phys. Lett. 105, 133701 (2014).

(3) F.Weyer, A.Duchesne and N.Vandewalle, Switching behavior of droplets crossing nodes on a fiber network, Scientific Reports 7, 13309 (2017).

Contacts

GRASP – Unité de recherche CESAM

Pr Nicolas VANDEWALLE I nvandewalle@uliege.be I +32 4 3663703

Dr Floriane WEYER I f.weyer@uliege.be I +32 4 3663752