Evaporation of an isolated liquid plug moving inside a capillary tube

The paper reports an experimental study to understand the evaporation mechanism of a partially wetting isolated liquid plug (methanol) of length L moving inside a long, dry, horizontal circular glass capillary tube (ID = 1.5 mm). The plug (with specified range of non-dimensional L/D ratios) is pushed from rest by controlled injection of air from one side, till a quasi-steady terminal plug velocity is achieved in the adiabatic section (non-heated length) of the capillary tube. Under such conditions, the drainage of thin-film occurring at the receding interface and its subsequent dewetting is well predicted by existing literature. The plug is then allowed to move through the heated section maintained at constant wall temperature (lesser than the saturation temperature of methanol). The drained film now starts evaporating rapidly, drastically affecting the bulk transport behavior. High resolution videography, coupled with laser confocal microscopy provides vital bulk as well as local information, including time-varying plug length, film thickness and local dewetting behavior near the contact line. Experimental results obtained for different wall temperatures and different initial L/D ratios of liquid plug suggests that the Taylor's law for predicting drainage characteristics under adiabatic flow conditions is valid, even for cases where there is a continuous evaporation of thin-film. The study thus provides a framework for modeling evaporative flux based on simple hydrodynamic theory of film drainage.