Liquid evaporation on fibers

The study of evaporation phenomena is a very old subject whose pioneering work dates back to the end of the 19th century by Maxwell and Sreznevsky. Nowadays, evaporation and drying are clearly active research topics but most of the fundamental research has been done in the diffusive limit: the transport of vapor is limited by the diffusion in the gas phase. In the last decade, increasing attention has been devoted to natural convection, which is triggered by density gradients between the vapor and gas phases. Similarly, fundamental studies of evaporation by forced convection remain scarce and predictions are highly dependent on the geometry of the systems as the coupling between the air flow and the liquid is central to the description of the evaporation kinetics.

In this work, we seek to understand how evaporation and drying occurs in fibrous materials as found in various Saint Gobain applications, such as insulating wools, sandpapers, and wall coverings.

First, we establish theoretically and numerically the evaporation dynamics of a drop on a fiber in a diffusive boundary by highlighting the role of the liquid morphology which depends on the fiber radius and the wetting properties. This allows us to establish laws equivalent to the sessile drops well known in the literature, for this geometry still insufficiently explored.

In a second step, we compare our experimental data of evaporation dynamics in diffusive boundary and forced convection with these models to demonstrate that the fiber contributes to significant thermal effects on the droplet lifetime. We propose a simple model indicating the correct parameter to estimate the fiber-liquid and air-liquid thermal contributions.

Finally, we study the deposition of solute on fibers. On the one hand, we show experimentally and theoretically that the dynamics of solute transport is different from the well known case of the sessile drop. We attribute these effects to the particular morphology induced by the fiber. These effects are then studied for other materials of industrial interest.

This work is conducted in collaboration with C. Poulard (LPS) for the numerical simulations and J. Dervaux (MSC, Paris) for the theoretical developments on evaporation.