Study of Evaporation in Capillary Tubes by Infrared Thermography and Ombroscopy Techniques

We study the evaporation of a volatile liquid in a capillary tube of square internal cross section (side length = 1mm) by combining an infrared thermography technique with ombroscopy visualizations. This makes it possible to obtain the temperature profile along the outer surface of the capillary together with the evolution of the bulk meniscus position within the tube. Evaporation in such a tube is significantly faster than in tubes of circular section owing to the liquid films developing along the tube internal corners under the effect of capillary forces as the bulk meniscus recedes inside the tube. When the tube is held horizontal the temperature minimum stays at the tube entrance and the evaporation rate reaches a stationary value. This is the signature of the liquid films, which transport the liquid up to the tube entrance, where the mass transfer is important. When the tube is vertical and open at the top, the temperature minimum stays at the entrance (top) for a while. In contrast with the horizontal case, however, the position of the temperature minimum changes when the bulk meniscus has sufficiently receded inside the tube. The rate of evaporation then decreases significantly. The different behaviour between the vertical case and the horizontal one is explained by the thinning of the corner films in the vertical tube entrance region under the conjugated effects of gravity and viscous forces. When the tube is held horizontal, the capillary effects are dominant and the film thickness remains essentially constant in the tube entrance region.