Contact angle measurement for LiBr aqueous solutions on different surface materials used in absorption systems

Wetting surface is a very important issue for the design of absorption applications and heat exchangers. The contact angle is deemed essential in wettability studies; However, LiBr aqueous solution contact angle studies are limited. This work analyses the contact angle of LiBr aqueous solution in the range of 0–55% mass fraction on different material surfaces: copper, aluminum, stainless-steel and polytetrafluoroethylene (PTFE) under atmospheric conditions. A sessile drop technique was used for measuring the contact angles, and a linear relation between solution surface tension and contact angle is observed in the tested materials. The study of three metals show hydrophilic performance ( θ < 90 °), whereas the PTFE shows hydrophobic performance. Additionally, the effect of the selection of the material, and consequent effect on the contact angle, on the minimum wetting rate and film thicknesses is presented under the working conditions of the absorption technologies. From the wettability point of view, the results show that using stainless-steel and aluminum leads to a slightly better performance than a copper made heat


Introduction
Absorption technologies have recently become more and more attractive, e.g. absorption chillers, absorption heat transformers and liquid desiccant systems ( Rivera et al., 2015 ). One of the most critical component in these technologies is the absorber, where the refrigerant is directly absorbed by a hygroscopic solution ( Garimella, 1999 ). The falling film configuration is usually used for water-LiBr heat exchangers, due to the large vapor-absorbent interface that optimizes both heat and mass transfer. However, en-suring wettability in falling film components is one of the critical aspects, since the heat exchanger energy efficiency is strongly affected by this parameter. A large effort in order to understand and optimize the performance of the falling film components has been done in the last years, e.g. analysis of the surface structure (micro and macro) of the tubes ( Kim et al.,20 03a,20 03b;Park et al.,20 03,20 04;Kang and Kim,20 06 ) and heat exchanger configuration, mainly determined by the tube diameter and spacing ( Yoon et al., 2008 ). The heat and mass transfer improvements in these studies was directly attributed to the improvement of the tube's wettability. Even the use of hydrophilic coatings has been analyzed by different authors: Yoon et al. 2002 observed that the wetted area of the bundle increased between 110% and 30% when a plasma treated tube was used; an improvement of the wetted

Nomenclature
Symbols T -temperature / °C g -acceleration of gravity / m · s −2 Greek Symbols -dimensionless film thickness δ -liquid film thickness / mm -dimensionless mass flow-rate per unit length γ -mass flow rate per unit length / kg · s −1 ·m −1 μ -dynamic viscosity / mPa · s −1 ρ -density / kg · m −3 σ -surface tension / mN · m −1 θ -contact angle / °ξ -solution mass concentration / %  Qi et al. (2015) when a hydrophilic surface was used. Also the wettability of generators for absorption heat transformers has been visually analyzed ( Lazcano-Véliz et al., 2014 ). Due to the importance of the wettability analysis, the contact angle is being considered in the absorption technologies studies in the last years. The contact angle measurement is usually considered in surface characterization and wetting studies, being a simple and versatile methodology ( Kowk-Yee and Zhao, 2015 ). Therefore, investigating the contact angle between working fluid and heat exchanger materials is considered crucial, as the contact angle is indispensable for understanding the wetted area. The measurement of the contact angle is being considered in the works of different authors, although it is still limited in the absorption technologies. Qi et al. (2015) studied the contact angle of the commonly used LiCl and LiBr solutions on stainless-steel plates, proposing empirical correlations for the prediction of the contact angle depending on fluid surface tension and solid surface roughness. The author ensures contact angle values, and therefore wettability, is critical for the performance of the falling film components and for the accurate performance prediction of the theoretical models.
The new theoretical researches are including the static contact angle and its influence in the LiBr solution flow spreading over horizontal tube bundles ( Ji et al., 2017;Fernandez-Arroiabe et al., 2018 ) or other configurations, such as over vertical finned plates ( Mortazavi et al., 2015 ). Giannetti et al. (2017) included the effect of partial wetting effect in the mass transfer analytic solution and they observed that the Sherwood number is decreased up to 90% for very low Reynolds numbers when partial wetting is considered. In order to include the effect of the film partial wetting at low Reynolds numbers correlations which, in turn, are dependent on the contact angle are being used.
The aim of this paper is to present the effect of the LiBr concentration, and therefore liquid surface tension, into the contact angle in commonly used absorption technologies materials, copper and stainless-steel, as well as alternative materials pro- posed by other authors due to corrosion problems, such as aluminum ( Romero et al., 2011 ) and polytetrafluoroethylene (PTFE) ( Zhang, 2010 ). However, wettability issues were not considered in these studies. The obtained results will help in the future development of more accurate heat and mass transfer performance prediction models.

Materials
Material samples were prepared by the abrasive polishing process recommended by the Standard Guide for Preparation of Metallographic Specimens ( ASTM, 2011 ). The specimens were cut and installed into a rotating wheel covered with abrasive. Different grinding papers were used from 240 to 1200, finally using 6 μm and 1 μm polishing cloths and diamond paste to obtain smooth and homogeneous surfaces. Cleaning (by ethanol) between stages was required to prevent carryover of abrasives and contamination of subsequent preparation surfaces grinding discs or cloths for fine grinding. Five specimens were prepared for each material, and the selected materials were as follows: 1. Copper (Cu-ETP 99.9); 2. Aluminum (6063-T5); 3. Stainless-Steel (AISI 316 L); 4. PTFE.
The surface roughness has a strong influence on the values of contact angles and, thus, on the wettability.The R a , R z and R q values were below 0.01 μm, 0.11 μm and 0.02 μm in the case of the three metal analyzed and below 0.14 μm, 1.78 μm and 0.22 μm for the PTFE.

Wet drop tests
Five different LiBr solution concentrations, including water (0% Solution), were tested. Solutions were prepared using commercial LiBr solution provided by Leverton-Clarke (CAS Number: 7550-35-8). Density and temperature of the solutions were measured by an Anton Paar DMA 35 density-meter, while concentrations, presented in Table 1 , were calculated by the correlation proposed by Patek and Klomfar (2006) .
The contact angle was measured by the static sessile drop method, using an OCA 15plus goniometer (Neurtek Instruments), with an image resolution of 752 × 582 pixels, 50 fps video recorder and ± 0.1 °accuracy. The drop volume was 3.0 μl, and measurements were performed in a controlled temperature and humidity ambient (23 °C and 45%). The equipment allows monitoring of the spreading dynamic contact angles. Once the drop was deposited, the contact angle was reduced by the effect of the spreading. The drop was allowed to stabilize and let it reaching its final static state. This procedure was monitored, and then the drop profile was analyzed. In Fig. 1 the contact angle percentage variation compared to the previous instant contact angle measurement is observed, on one hand for the water and the LiBr 55% and, on the other hand, for the four material analyzed. Due to the higher viscosity,  the static state is reached later in the LiBr aqueous solution. As a result of this analysis, the measurement of the contact angle after 60 s from drop deposition was considered, since drop profile changes below 0.1% were observed for the four materials analyzed. This procedure considers dynamic decays to the steady state and the Young contact angle (smooth and chemically heterogeneous surfaces are considered), but by minimization of the evaporation or absorption influence. The measurements of each solution-solid system were repeated over 10 times in different areas of the samples.

LiBr contact angle in different materials
The surface tension increased with concentration and therefore the contact angle in the four studied materials. Whereas the three metals analyzed showed hydrophilic performance ( θ < 90 °), PTFE showed hydrophobic performance. For the copper, aluminum and stainless-steel, contact angles were between 70.0 °and 90.8 °, 62.1 °a nd 85.6 °and, 63.8 °and 89.7 °for water and 55% solution, respectively. For the PTFE surface, contact angles measured were between 106.2 °and 115.8 °, which the mentioned extreme cases are directly shown in Fig. 2 . According to wettability theory, the contact angle is strongly influenced by liquid surface tension ( Kowk-Yee and Zhao, 2015 ). The four materials showed a high linear relation between contact angle and solution surface tension, as shown in Fig. 3 , which was calculated by the empirical correlation proposed by Medrano (2001) . Fig. 3 shows how the contact angle increased by 1.07, 1.38, 1.45 and 0.53 °/mN · m −1 for copper, aluminum, stainless-steel and PTFE, respectively.
In Fig. 4 , experimental data from Bernett and Zisman (1968) and the experimental contact angles measured on copper and aluminum are compared. Zisman and co-workers proposed the first contact angle interpretation theories studying solid surface excess energy, and they observed that the contact angle, represented by cos θ , varies smoothly with the liquid surface tension. The data from Bernett and Zisman (1968) was obtained by liquids with surface tensions between 44.00 and 50.80 mN · m −1 , but the fluids in this work have surface tensions between 72.28 and 90.83 mN · m −1 . The surface tension of the fluids analyzed in these two works are not in the same range, and therefore, they are not directly comparable. However, the linear relation between the cos θ and the surface tension reported by Bernett and Zisman (1968) was also observed in the measurements of this work. Thus, the linear relation described by the authors was considered for fitting in the range of the surface tension values of both works. The critical surface tensions obtained by the adjustment proposed in this work, and presented in Fig. 4 , were 43.5 mN · m −1 (slope of −0.0223 m · mN −1 ) for copper and 46.8 mN · m −1 (slope of −0.0240 m · mN −1 ) for aluminum, they are in good agreement with values calculated by Bernett and Zisman (1968) .  Experimental results presented by Bueno (2005) were used in the case of the stainless-steel. The surface tension of the liquids used by the author were between 48 mN · m −1 (Ethylene Glycol) and 72.8 mN · m −1 (Water). However, direct comparison in water shows a deviation of −8.7%. The linear relation between the contact angle cos θ and surface tension is observed by combination of the results in both works, and the linear function was adjusted in the range of the surface tension (see Fig. 5 ), A critical surface tension of 39.5 mN · m −1 (slope of −0.01782 m · mN −1 ) was calculated.
Finally, results obtained in PTFE were compared. In this case, the range of the liquid surface tension studied in this work moves away from the linear relation described by Zisman method. According to Kowk-Yee and Zhao (2015) , this is "attributable to Hbonding and polar interactions between the liquid and the solid surface". Fig. 6 shows that the results obtained by this work and data presented by Li and Neumann (1992) and Kwok et al. (1995) . Analyzed surface tensions were not in the same range and were not directly comparable. Except water, the results in this work present a contact angle deviation of −4.8% comparing with the results from Li and Neumann (1992) . For this reason, data was adjusted to the equation of states described by Kwok and Neumann (1999) : Modified Berthelots rule ( Eq. (1) ) and Alternative Formulation ( Eq. (2) ): Modified Berthelots rule:  Alternative Formulation: least-square analysis ( Kwok et al., 1995 ).

Effect of the material selection in the minimum wetting rate
Results obtained in this work have been used to study the effect of the use of different materials in the minimum wetting rate (MWR) and therefore in the design of the heat exchangers of the absorption technologies. MWR is defined as the lowest flow rate needed to ensure that the surface remains covered by a continuous thin liquid film, which directly depends on the wettability performance and on the contact angle. Different analytic expressions has been proposed in the bibliography ( Hartley and Murgatroyd, 1964;Hobler, 1964;Mikielewicz and Moszynskl, 1976;Doniec, 1991;El-Genk and Saber, 2001 ). A comparison of different authors expressions and experimental works are presented in Fig. 7 .
In this work, expressions proposed by El-Genk and Saber (2001) for predicting the minimum liquid film thickness (MLFT) and the MWR were used. These MWR-Contact angle and MLFT-Contact angle relations, which are based on the minimum total energy show the best agreement with the experimental data from Ponter et al. (1967) and Munakata et al. (1975) , as shown in Fig. 7 .
The author calculates MLFT, min , as follows: And then MWR, min , may be calculated by the following empirical correlation: Where dimensionless flow rate and dimensionless film thickness are defined as follows: In this MWR and MLFT study, performance parameters, i.e., absorber and generator inlet and outlet temperature and solution concentration, proposed by Herold et al. (2016) for both absorption chillers and absorption heat transformers were considered. First, for both proposed cycles, average solution temperature and concentration were considered, and then density, viscosity and surface tension were calculated by correlations proposed by Patek and Klomfar (2006) and Medrano (2001) . Second, contact angles for each component depending on possible materials were predicted (by use of the results presented in Fig. 3 ). Finally, these contact angles were used to calculate the minimum film thickness according to Eq. (3) and the minimum solution flow rate by Eq. (4) to ensure fully and continuously wet surface, which the results are presented in Table 2 .
According to the results obtained in this study, angles obtained on aluminum and stainless-steel surfaces are lower than the angles obtained on copper surfaces. These differences become greater when higher temperature applications are required, reducing contact angles between 1.98 and 6.04% in the case of the aluminum and between 4.06% and 9.24% in the case of stainless-steel. Results in Table 2 show that due to the improved wettability of aluminum and stainless-steel, lower mass flow rates per unit length, and therefore lower film thicknesses, would be needed in these materials to obtain a uniform film similar to the film on copper. The specific mass flow rates for the copper would lied between 0.1328 and 0.2293 kg · s −1 ·m −1 for the absorbers in the Heat Transformer and the Chiller respectively, which are the studied components with highest and lowest working temperatures. These flow rates would be reduced to 0.1203 and 0.2220 kg · s −1 ·m −1 if the heat exchanger would be made of aluminum, and to 0.1139 and 0.2144 kg · s −1 ·m −1 if stainless-steel is considered. The lower flow  Ponter et al. (1967) and glycerol-water mixtures ( -points, from Munakata et al. (1975) .

Table 2
Prediction of the contact angle and minimum wetting rate and liquid film thickness for copper, aluminum, stainless-steel and PTFE depending on the falling film heat exchanger operating conditions.

Single-Effect
Single rates in the aluminum and stainless-steel would result in thinner film thicknesses, reducing from coppers 0.3303-0.6055 mm thickness to 0.3228-0.6013 for aluminum and between 0.3187 and 0.5969 for stainless-steel. Flow rates would be reduced between 3.20% and 9.42% if the aluminum would be selected instead of copper, while film thickness would be reduced between 0.68% and 2.25%. If stainless-steel is considered, the minimum required flow rate would be reduced between 4.06% and 9.24%, consequently reducing film thickness between 1.41% and 3.51%. Thinner stable films would be favorable in absorption and desorption processes, since absorption mass fluxes would be greater resulting in an in-crease of the capacity and a higher efficiency of the components ( Giannetti et al., 2017 ). On the other hand, PTFE contact-angle tests showed hydrophilic performance, and this would result in worse falling-film heatexchanger performance, as shown in Table 2 . The predicted contact angles for the selected working conditions would be increased between 24.61% and 34.44% compared to those obtained in copper. Therefore, the flow rates and film thicknesses required to obtain a uniform wet PTFE surface would be increased to 0.2141-0.3259 kg · s −1 ·m −1 and 0.36 43-0.6 483 mm, i.e. between 42.11% and 61.48% and 7.07% and 10.29%, respectively; and this would result, from a wettability perspective (not considering also worse thermal conductivity), in a much lower efficient heat exchanger, than those made of copper, aluminum or stainless-steel.

Conclusions
The contact angle of the aqueous LiBr solutions with different concentrations up to 55% on copper, aluminum, stainless-steel and PTFE have been measured by sessile drop tests. As a result of the performed tests, a straight-linear relation between the contact angle and the solution surface tension, depending on the LiBr concentration, is observed. Copper, aluminum and stainless-steel show a hydrophilic performance, while PTFE showed hydrophobic performance. Contact angles on the three metals tested were between 70.0 °and 90.8 °on copper, between 62.1 °and 85.6 °on aluminum and 63.8 °and 89.7 °on stainless-steel for water and 55% solution, respectively. The critical solid surface tension of the copper, aluminum and stainless-steel was calculated by these results, together with the data available in previous research. Critical surface tensions of 44, 45 and 39.5 mN · m −1 were determined for copper, aluminum and stainless-steel, respectively. In contrast, contact angles measured in the PTFE ranged between 106.2 °and 115.8 °. Besides, the solid surface tension of the PTFE was calculated by adjusting the modified the modified Berthelots rule and the alternative formulation. Solid-vapour tensions of 17.2 and 17.3 mN · m −1 were obtained by Modified Berthelot and Alternative formulations, respectively. Results obtained in this study are in good agreement with those in previous research.
In addition, an estimation of the minimum wetting rate and the minimum film thickness to obtain a wetted uniform surface in the components composing an absorption chiller and an absorption heat transformer has been performed. By combining the wettability studies related to the contact angle and different components under typical working conditions, the required minimum wetting rates and minimum solution film thicknesses to ensure a complete wetting on different materials were calculated and quantitatively compared to copper as the usually considered material. Aluminum and stainless-steel showed very similar performances but resulted in slightly better wetting performances compared to the performance obtained by copper, which results in greater mass transfer fluxes and more efficient heat exchangers.
Wettability is considered crucial in the design of falling film heat exchangers, but, data in contact angles is still limited for LiBr aqueous solutions. Consequently, results obtained in this work may also help in the future designs when wettability aspects are considered.