A Practical Procedure for Measuring Contact Angles in Wettability Studies by the Sessile Drop Method
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MRS Advances © 2019 Materials Research Society DOI: 10.1557/adv.2019.373
A Practical Procedure for Measuring Contact Angles in Wettability Studies by the Sessile Drop Method J. López-Cuevas*, M.I. Pech-Canul, J.L. Rodríguez-Galicia, and J.C. Rendón-Angeles Cinvestav Unidad Saltillo, Calle Industria Metalúrgica No. 1062, Parque Industrial Saltillo - Ramos Arizpe, 25900, Ramos Arizpe, Coahuila, México.
*Author to whom all correspondence should be addressed. E-mail: [email protected]
ABSTRACT
An old procedure used to carry out a graphical derivation of curves, which is based on the optical properties of plane mirrors, has been adapted for the measurement of the contact angle (θ) formed between a liquid drop and a flat solid substrate in wettability experiments carried out by the so-called “sessile drop” method. The method was tested for mercury on soda-lime glass at room temperature in air as well as for Cusil (Ag-28wt.%Cu) and IncusilABA (Ag-27wt.%Cu-12wt.%In-2wt.%Ti) brazing alloys on pressureless-sintered silicon carbide (PLS-SiC) at 850 °C, under a vacuum of 10-4/10-5 Torr. The proposed method is fast, simple and accurate enough from high (~140°) to relatively low ( 10°) contact angles. Although the proposed method has been tested for metal-ceramic systems, it is of general application, so that it would be useful for any liquid-solid system. The method is applicable for any temperature, pressure and atmospheric experimental conditions employed, as well as for any chemical composition of liquid and solid. It is also useful for both low and high contact angles, as well as for reactive and non-reactive systems, as long as a photograph of a liquid drop resting on a flat solid surface is available for the studied system.
INTRODUCTION In the so-called “sessile drop” method, a small drop, frequently of a liquid metal or alloy weighting only a few mg, is placed resting freely on a flat and horizontal solid surface, frequently a ceramic substrate, under various conditions of temperature, pressure, atmospheric environment, and chemical composition of liquid and solid [1]. Then, the change of the contact angle (θ) is monitored as a function of time. This angle is measured between the tangent to the surface of the liquid and the surface of the solid
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substrate, at the so-called “three-phase contact point (TPCP)”, where both materials make contact simultaneously with the furnace atmosphere. Under non-reactive conditions, the drop shape is determined by the balance of forces established between the surface tension of the liquid, which tends to pull it up to form a sphere, in order to minimize its surface free energy, and gravity, which tends to flatten and spread the liquid [2]. This balance of forces is described by the equation of Laplace (Eq. 1) [3], which is usual
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