Abstract
The measurement of the equilibrium contact angle (ECA) of a weakly evaporating sessile drop becomes very challenging when the temperatures are higher than ambient temperature. Since the ECA is a critical input parameter for numerical simulations of diabatic processes, it is relevant to know the variation of the ECA with the fluid and wall temperatures. Several research groups have studied the effect of temperature on ECA either experimentally, with direct measures, or numerically, using molecular dynamic simulations. However, there is some disagreement between the authors. In this paper two possible theoretical models are presented, describing how the ECA varies with the surface temperature. These two models (called Decreasing Trend Model and Unsymmetrical Trend Model, respectively) are compared with experimental measurements. Within the experimental errors, the equilibrium contact angle shows a decrease with increasing surface temperatures on the hydrophilic surface. Conversely the ECA appears approximately constant on hydrophobic surfaces for increasing wall temperatures. The two conclusions for practical applications for weakly evaporating conditions are that (i) the higher the ECA, the smaller is the effect of the surface temperature, (ii) a good evaluation of the decrease of the ECA with the surface temperature can be obtained by the proposed DTM approach.
Highlights
The cohesive forces between molecules of the same phase are responsible for the interphase tension of the liquid
The uncertainty on the calculation of equilibrium contact angles does not allow to conclude that the ECA varies with temperature in this case
The Unsymmetrical trend model (UTm) is in contradiction with a thermodynamic approach base on the second law principle, which shows that for a homothermous system in equilibrium, an increase of the temperature cannot induce an increase of the ECA
Summary
The cohesive forces between molecules of the same phase (liquid, solid and gas/vapor) are responsible for the interphase tension of the liquid. Surface temperature (from 23 °C up to the oil smoke point at 200 °C) are measured and compared to values predicted by the Girifalco-Good-Fowkes-Young (GGFY) equation[21] This equation, in combination with the Eötvös rule (that assumes an interface tension linear with the temperature), predicts a reduction of contact angle with increase of the surface temperature. Kandlikar et al.[23] measured the CA during rapid evaporation of liquid on a heated Hydrophobic surface and the values of the CA for high surface temperatures (100 °C < Tw < 250 °C) are remarkably constant They showed that the liquid temperature has a much larger effect on the value of contact angles for polar liquids (water) than for apolar liquid (Diiodomethane), and for water they observed an increase of the CA with temperature, and a constancy of CA for diiodomethane.
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