Is Unidirectional Drying in a Round Capillary Always Diffusive?

Area of Science:

• Physics
• Physical Chemistry
• Fluid Dynamics

Background:

• Unidirectional drying in capillaries typically follows Stefan's solution, exhibiting square root of time kinetics due to vapor diffusion.
• This established model assumes a specific capillary closure, which may not universally apply.

Purpose of the Study:

• To investigate the influence of capillary end conditions on water evaporation kinetics.
• To determine if alternative drying mechanisms exist beyond diffusion control.

Main Methods:

• Experimental evaporation of water in cylindrical capillaries.
• Capillaries were sealed with either a solid material or a fluid reservoir.
• Viscosity and solidification of the reservoir fluid were varied.

Main Results:

• Stefan's solution (square root of time) was observed when capillaries were sealed with a solid.
• A constant-rate drying regime was observed when capillaries were connected to a fluid reservoir.
• This constant rate is driven by capillary pumping, inducing water plug flow towards the evaporation front.

Conclusions:

• The method of capillary closure fundamentally alters water evaporation kinetics.
• Constant-rate drying, faster than diffusion-limited evaporation, occurs when a fluid reservoir drives water flow.
• A transition from constant-rate to diffusion-limited drying can be induced by increasing reservoir fluid viscosity or solidification.