The effect of wall thickness and material on Marangoni driven convection in capillaries

Abstract

Abstract We present results of an experimental investigation of Marangoni convection for an evaporating meniscus in open air pinned at the mouth of a capillary tube. Four different liquids have been studied: ethanol, methanol, acetone and, for the first time, also FC-72. This experimental configuration has been studied before by the present authors and others. However, it is the first time that the effect of the capillary tube thickness and material is experimentally investigated. In particular, the study considered the same internal diameter for all the tubes (1 mm) and three borosilicate external diameters (1.4, 2, and 3 mm) as well as one polycarbonate (1.25 mm) and one sapphire (3 mm). These three tube materials were chosen for their very different thermal properties and their optical transparency. The evaporation rate is measured by tracking the second meniscus receding inside the capillary tube while the first one is pinned at the tube mouth. It was found that the evaporation rate is influenced more by the thermal conductivity of the tubes than the wall thickness. An infrared camera has also been employed to measure the interfacial temperature at the meniscus pinned at the tube mouth and also along the wall of the tube close to the tube mouth. In addition, a heat transfer analysis has been performed at the tube mouth along with a mass transfer analysis which allowed the evaluation of the temperature of the meniscus interface and the evaporation rate, respectively, and comparing these results with the measured ones. The deviation found is maximum 65% between measured and calculated temperature differences between wall and meniscus and 6.7% for the evaporation rate. The results are qualitatively assessed in the light of those obtained for evaporating sessile drops and the trends found are similar.