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Do water-proof pens really resist to water ?


Physicists have experimentally demonstrated that it is possible to remove the mark of a water-proof marker... with water !

Many advances in water-resistant materials have been made in recent years. Often qualified as water-proof, they allow holding of a deposit by avoiding solubilisation. Thus, these poorly water-soluble and hydrophobic materials are used in industry, especially in make-up products. Nevertheless, this advantage is a disadvantage when cleaning the stained surface, since these deposits are generally difficult to remove. Indeed, the use of aggressive solvents and significant mechanical actions on the surface are not always allowed by the application field. An international team bringing together researchers from the LPS and Princeton University examined the conditions required for detaching a hydrophobic film from its substrate using only water.

The common experience shows that by passing this surface under the tap the deposit adheres impassively on the substrate. The experimental study has highlighted that the hydrophobic film comes off under two conditions. First, the liquid must only partially wet the film. Energetically, this translates into a lower overall system energy when the film is peeled off and floating on the surface than when it adheres to the substrate. Thus, the system will tend to evolve naturally to the "peeled off" state. But for that a second condition is necessary. The interfacial fracture, filled with liquid, between the film and the substrate must be able to propagate, which requires the viscous dissipations to be low compared to the interfacial tension allowing the peeling. A viscous liquid would waste its energy to spread instead of using it to break the bonds between the substrate and the film. It also means that the rate of penetration of the substrate into the liquid must be as low as possible so that it has time to propagate and cause the detachment.

Peeling of a mark of a permanent waterproof marker (Sharpie) on a glass slide. The slide was dipped at 1 µm/s.

By making a mark with permanent marker on a glass slide, the researchers observed that this mark came off when the glass slide was dipped into a container of water at low speed, whereas this was not the case at higher speeds. The study of the composition of these commercial markers has shown that their inks consist mainly of a resin called terpene. Deposits made only with this resin have a detachment behaviour similar to commercial markers. However, the main difficulty was to make films with a well-controlled thickness. In order to carry out model experiments, the terpene was replaced by polystyrene, for which it is easy to produce homogeneous films without internal stress. This experimental study has quantitatively demonstrated the effects of viscosity and soaking rate. Subsequently, the researchers modelled these observations by expressing the energetic conditions necessary for the detachment of the film presented above.

Published in Physical Review Letters, these works provide perspectives for cleaning techniques of waterproof products that are less aggressive and more environmentally friendly.

Reference

Water-based peeling of thin hydrophobic films
S. Khodaparast, F. Boulogne, C. Poulard et H. A. Stone
Physical Review Letters 119, 154502 (2017)
doi:10.1103/PhysRevLett.119.154502

Contact

François Boulogne