Abstract
As part of a better understanding of drying liquids within porous materials, measurements from 293 to 343 K of deionized water surface tension in air as a function of relative humidity are exposed. Experimental work was carried out using the pendant drop method coupled with image analysis within an adapted instrumented climatic chamber. Results show that surface tension linearly decreases when relative humidity increases. Although the effect of humidity is less compared to that of the temperature and even less compared to a surfactant impact, it must not be neglected and values have to be mentioned when dealing with water evaporation. Modifying surface tension also affects the pendant drop shape. The drying kinetics of the pendant drop volume and its outer shell are connected to this change of shape. Steam in the air can be assimilated to a wetting agent, hence a surfactant, and can be used in an environmental-friendly way to ease the drying stage. Indeed, the challenge is to limit the risk of cracking and damaging pieces during this crucial step in material processing.
Highlights
Surface tension is due to cohesive forces between liquid molecules
Using pendant drop method within a humid environment, values of deionized water surface tension as a function of relative humidity between 20% and 100% have been calculated for temperatures from 293 to 343 K
The pendant drop shape modifications are presented and phenomenological relations are proposed which allow an evaluation of surface tension in the 15% - 100% humidity and 278 - 343 K temperature ranges
Summary
Surface tension is due to cohesive forces between liquid molecules. This phenomenon results from molecules at liquid-air interfaces which are missing some of their attractive interactions. The spreading dynamics of a drop of blood can be modified [10], as well as the surface tension of lung surfactant films [11] From this point of view, where the effects of humidity are non-negligible, Erbil did underline and deplore the lack of relative humidity information in numerous recent papers when water drop evaporation was tackled as it can alter the evaporation rate [12]. PérezDíaz et al recently provided some interesting behaviors about how partial pressure of water vapor acts on surface tension at the liquid water-air interface at 278, 283, 288 and 293 K [13] These data and those of the preceding examples were obtained at body temperature or at lower temperatures and could not be used in a drying study where, for instance, the temperature of 333 K is commonly used
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