Preparation of a Non-Evaporating Contact Angle Part 1

PREPARATION OF A NONEVAPORATING CONTACT ANGLE STANDARD REFERENCE MATERIAL ABSTRACT Purpose The purpose of this project was to combine previous contact angle measurement methods with improvements in digital imaging and computing to develop a standard method and reference material for calculating contact angle and therefore surface energy. Modernization of these methods and the development of this standard provide a low cost and effective way to collect and analyze data for use in the paints, cleaning and adhesives industries, to name a few. Methods Current contact angle methods were divided into three major categories, specifically the side-on, top-down and reflected angle methods. In order to compare the validity of each of the methods, a solid, non-evaporating standard needed to be created. The three standards investigated were a ruby ball, a steel ball, and a piece of pressed aluminum. Each were mounted in such a way as to mimic a sessile drop with a contact angle of less than or greater than 90 degrees. Findings The side-on method using the digital microscope with the contact angle measured using the Manual Points Procedure was defined as the lab accepted standard as it most closely matches the current industry instruments like the goniometer. After determining the accepted angle for the proposed standards, the reflected side-on method, top-down and reflected angle methods were analyzed the standards and compared to the accepted angles for all of the standards. In the end, the steel ball was the preferred standard as it was well machined and could mimic contact angles of less than and greater than 90 degrees simply by changing its mount. The applications for this type of standard are nearly limitless. The mounts and standards are inexpensive, effective ways of determining contact angles. The use of a contact angle standard can be used to verify techniques and train technicians. By using a non-evaporating standard, a variety of liquids with an array of surfaces can be tested and analyzed. The non-evaporating contact angle standard fills an important void in the coating and adhesive industry. CHAPTER I INTRODUCTION Surface wettability and surface energy have been used in various industries to explore surface coating adhesion and surface cleanliness. In an attempt to understand these characteristics, the contact angle of a liquid on a surface may be measured to give wettability and surface energy information. Since Young’s first description of contact angles 1 in 1805, many have adapted his description and generated methods capable of measuring contact angles. Barth 2 measures the dynamic contact angle of a coating liquid while Sutton 3- 4 uses capillary pressure. Blitshteyn 5 , Friedrich 6, and Wright 7 project an image of the droplet so that the contact angle can be measured off the projection. Dumoulin 8, Martin 9, and Schneider 10 use reflected light or a laser beam to measure the contact angle. Poppe 11 uses a goniometer and Wapner 12 even uses folding metal plates to analyze the change of a droplet from wetting to non-wetting behavior. Many of these are primitive, but those developed by Irving Langmuir 13- 14 and J. J. Bikerman 15 may be combined with the improvements in digital imaging and computing to bring a contact angle measurement to the modern coating or adhesive production facility. In measuring the surface energy of a solid it is important to note that the results can vary greatly upon the choice of liquid and technique. Therefore, an attempt must be made to create a physical standard and method that can determine the surface energy of any surface. To reliably implement a standard contact angle technique, a solid standard reference material must be created which cannot be affected by environmental changes and has a known contact angle. By using a contact angle standard, any measurement technique, when preformed correctly, will produce a value that can be compared to the known angle. The standard is the best way to characterize human and mechanical uncertainty among labs or companies. This thesis describes the development and evaluation of the standard reference material and analysis procedure. The National Institute of Standards and Technology currently has not defined a contact angle standard. The American Society for Testing and Materials has developed many standard procedures relating to measuring the contact angle of a sessile drop 16- 17 but a calibration standard is not adequately defined which makes it impractical to discuss the accuracy of measurements between methods or between labs. By creating a welldefined and characterized calibration standard, this project fills that void. In developing this standard, three methods of measuring contact angle were compared; the side-on method as defined by Young, the top-down method as defined by Bikerman, and the reflected angle method as defined by Langmuir. Each method presents its own unique challenges and advantages. The standards explored include a ruby ball, pressed aluminum, and a steel ball all mounted differently and used in combination with one or more of the measurement methods. CHAPTER II CONTACT ANGLE THEORY Surface energy and surface tension are the two primary components in determining the characteristics of a solid-liquid interaction. The surface energy of a solid is the amount of energy required to create a certain area of surface, measured in Joules per meter squared 18. The energy is measured indirectly using wetting behavior of test liquids with known surface tensions. Surface tension is the surface energy of a liquid that it is expressed as a force in one dimension measured in newtons per meter 18. Mathematically, the surface tension γ23 is defined as the force vector between the liquid (2) and gas (3) phases of a drop. By multiplying the surface tension by meters over meters, thus giving a surface area, the surface energy can be measured in joules per meter. Equation 1 describes the mathematical relationship. ∙ (1) Zisman Plot In industry, a Zisman plot is used to examine the relationship between the surface tension of a liquid and the surface energy of a solid surface. W. A. Zisman developed a standard technique to determine the surface energy of a smooth, planar surface by studying the contact angle behavior of probe liquids of varying surface tension 19. The surface energy of the solid was defined to be equal to the surface tension of the liquid that wet the surface of the solid completely. This specific surface tension is defined as the critical surface tension. Zisman’s method requires multiple probe liquids and an accurate way to measure the contact angle of the liquid in question. Each liquid creates a unique contact angle with the surface. The definition of contact angle is the angle of the tangent of the drop curvature with the surface at the tri-phase point when the drop is viewed in profile and has come to equilibrium. Figure 1 illustrates the tangential line and the contact angle generated θ with the solid (1), liquid (2) and gas (3) phases. 3 2 θ 1 Figure 1. Contact Angle Definition By measuring the contact angle of each liquid and then plotting the cosine of that angle against the surface tension, a linear relationship can be extrapolated to the point where the cosine of the contact angle equals one. The plot of this relationship is called the Zisman plot. The Zisman plot and theory can be applied to the concept of energy minimization in industry by using surface coatings as the probe liquids 19. The best surface coatings must create the lowest surface energy and wet the entire surface completely. A Zisman plot can therefore be generated to compare a variety of coatings on the surface in question. Figure 2 is an example of the Zisman Plot: 1.0 0.9 Complete Wetting 0.8 0.7 Cos  0.6 0.5 0.4 0.3 0.2 0.1 0.0 60 62 64 γc66 68 700 72 74 76 78 8 80 n (23) / mN/m m Surrface Tension Figure F 2. Exam mple Zisman Plot In the plot, the cosine of the contact c anglee is plotted vversus the suurface tensionn of th he liquid, γ. A linear linee can be fitteed to the surfface tension measuremennts. A line iss th hen drawn frrom where cosine of the contact anglle is equal too one across to intersect that best fit line. At A the interseection of thee two lines liies the criticaal surface tension, γc, whhere an ny liquid witth a surface tension less than that wiill completelly wet the suurface in qu uestion. Thee Zisman plo ot method is only valid fo for when the contact angle can clearlly be seeen and the surface s tensiion of every test liquid iss known. Du Nouy Ring Tensiometer If one must determine the surface tension for a probe liquid, a du Nouy ring tensiometer can be used. The tensiometer uses a platinum-iridium ring suspended just below the surface level of the liquid. 20 21 22 23 The liquid adheres to the ring causing a cylinder of liquid to be lifted above the surface, as seen in Figure 3. The mass of the liquid lifted is proportional to the surface tension. The mass is measured by a digital scale attached to a computer which provides a digital output similar to Figure 4. Figure 3. Ring Tensiometer Example The force curve data in Figure 4 compares a solution of soapy water with a concentration of 159 parts per million dish soap by mass to deionized (DI) water. The curves effectively illustrate that by adding a surfactant to a solution, the force curve changes with respect to the maximum force on the ring. The y axis illustrates the mass of liquid on the ring while the x axis shows the time in seconds. The units of x and y can be multiplied by the distance the ring was pulled to give the newtons of force for the liquid, which is a measure of surface tension. Force Curve 1 0.8 Water Soap (159.92 ppm) Mass (g) 0.6 0.4 0.2 0 0 50 100 150 200 250 ‐0.2 ‐0.4 Time (sec) Figure 4. Ring Tensiometer Data Output Example The ring tensiometer is well-accepted as a method to determine surface properties of a liquid. The tensiometer cannot determine any properties of a surface. To measure the properties of a surface, the sessile drop technique may be used, just as Zisman did. Sessile Drop Technique The surface properties of a solid surface can be studied using the sessile drop technique. The sessile drop technique requires a known volume of liquid to be placed on a surface using a pipette in the range of 1 to 20 microliters. The characteristics of the drop formed describe the relationship of the surface and the liquid in question. The analysis of the contact angle of the droplet with the surface can then be used to characterize the surface energy of the surface. The half-angle method for measuring the contact angle of a drop, θ, on a surface is defined using the base, b, and the height of the drop, h, as seen in Figure 5 and Equation 2: 2 2 (2) 3 h 2 b θ 1 Figure 5. Half-Angle Method The half-angle method is only appropriate for perfect spheres, but since not all drops are perfect spheres, other theories must be explored to account for imperfections in drops. The half-angle method focuses on the base length and the height of the drop. Other theories, including the Young equation, focus on the forces generated along the solidliquid-gas interface. The Young equation (Eq. 3) describes the contact angle, between the surface (1), the liquid (2) and the surroundings (3) using the forces generated between each (γ) as measured in joules per meter 1: (3) 3 2 (a) θ 1 cosθ 3 (b) 2 1 θ cosθ Figure 6. Sessile Drop on a Surface: (a) Greater than 90°, (b) Less than 90° Figure 6 depicts a sessile droplet of liquid on a surface, with (a) showing a drop with a contact angle greater 90 degrees and (b) showing a contact angle of less than 90 degrees. By measuring the contact angle, , generated by the sessile drop, the tri-phase point, also known as the solid-liquid-air interaction point, can be analyzed. The contact angle is related to the amount of surface energy present. A low contact angle indicates a high surface energy of the solid and strong solid-liquid interactions. In such cases the liquid spreads extensively to generate the lowest overall surface energy of the system. A high contact angle value indicates a low surface energy of the solid with less solid-liquid interaction. By working towards a point at which the total surface energy of the surface is minimized, the equilibrium or minimum energy is achieved. The Young equation takes into account adhesive and cohesive forces while the Zisman method treats the surface energy as a single parameter. The single parameter approach, while simplistic, can be very precise so long as the surface energies of the probe liquids are well known. The sessile drop technique has been criticized because the contact angle results are often method and operator dependent. 24 25 26 Therefore, for the Zisman analysis to be successfully implemented, one must be able to refine the measurement techniques and operator skills. Method refinement and operator training is much more successful if one has a stable reference material or standard sample. Actual water drops are not suitable because the contact angle changes over time with vibration, evaporation, and contamination. The direct use of the Young equation (Eq. 3) requires viewing the droplet from the side, but not all drops can be viewed from the side. By exploring the different ways a drop can be viewed, an improvement in the Young and Zisman approach to viewing the drop from the side might be made.