Is Unidirectional Drying in a Round Capillary Always Diffusive?

The unidirectional drying of water in cylindrical capillaries has been described since the discovery of Stefan's solution as a vapor diffusion-controlled process with a square root of time kinetics. Here we show that this well-known process actually depends on the way the capillary is closed . Experiments are performed on the evaporation of water in capillaries closed at one end with a solid material or connected to a fluid reservoir. While we recover Stefan's solution in the first case, we show that in the second situation the water plug evaporates at a constant rate with the water-air meniscus remaining pinned at the exit where evaporation proceeds. The presence of the liquid reservoir closing the capi l l a r y combined with a capillary pumping effect induces a flow of t h e water plug toward the evaporation front leading to a constant-rate drying , substantially faster than the prediction of Stefan's equation. Our results show that a transition from a constant-rate evaporation regime at short times to a diffusion-driven evaporation regime at long times can be observed by increasing the viscosity of the fluid in the reservoir blocking the other end of the capillary. Such transition can also be observed by connecting the capi l l a r y end to a solidifying fluid like epox y glue.

Automated summary

The unidirectional drying process of water in cylindrical capillaries, which was previously thought to be vapor diffusion-controlled with a square root of time kinetics, is actually influenced by the way the capillary is closed. Experimental results show that when the capillary is closed with a solid material, Stefan's solution is recovered; however, when it is connected to a fluid reservoir, the water plug evaporates at a constant rate with the water-air meniscus remaining pinned at the exit. The presence of a liquid reservoir and capillary pumping effect increases the flow of the water plug towards the evaporation front, leading to a drying process that is substantially faster than predicted by Stefan's equation. Increasing the viscosity of the fluid in the reservoir or connecting the capillary end to a solidifying fluid like epoxy glue can induce a transition from constant-rate evaporation to diffusion-driven evaporation.