Surface tension and contact angle measurements can be used to determine the best wetting agent as well as optimize the amount of wetting agent in the formulation. Coatings are typically evaluated by methods such as laboratory drawdown where qualitative evaluation of the coating appearance are made. Surface tension and contact angle measurements can provide more quantitative approach. These measurements will not completely replace the visual evaluation but can be used to identify the best candidates.
Basics of contact angle
Contact angle is geometrically defined as the angle formed by a liquid at the three-phase boundary where a liquid, gas, and solid intersect. There are three different forces acting on this three-phase contact point between solid, fluid and fluid.
The γlv is the surface tension of a liquid, γsl is the interfacial tension between the solid and liquid and γsv is the surface tension of the solid i.e. surface free energy.
The well-known Young´s equation describes the balance at the three-phase contact.
γsv=γsl+ γlv cosθY
The interfacial tensions, γsv, γsl, and γlv, form the equilibrium contact angle of wetting, many times referred to Young´s contact angle θY.
Young’s equation assumes that the surface is ideal. This means that it is flat, rigid, perfectly smooth, and chemically homogeneous. Furthermore, it assumes that the system is stable i.e. there is no interaction between the liquid and the substrate. As neither of the above-mentioned criteria is fulfilled in real life, advancing and receding (i.e. dynamic) contact angle measurements are often performed. Surface roughness can also be corrected by measuring the roughness corrected contact angle. Contact angles are thus divided into three categories; static, dynamic and roughness corrected.
Contact angle to choose the best formulation for a particular substrate
Surface tension measurement alone is a great way to compare the general effectiveness of the additives. However, measuring the contact angle on the substrate to be coated will indicate which formulation will best coat the particular substrate. A lower contact angle indicates better wetting of the substrate. Change of contact angle as a function of time can also be measured.
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Contact lens should support a continuous tear film for optimum visual clarity. It should also resist the sorption of tear components as deposition can lead to decreased visual clarity and discomfort. Wettability of the surface affects both the tear film as well as biocompatibility of the lens.
In industry, wettability is important for example when manufacturing electronics or when planning how to extract oil from the oil reservoir. In all these applications, knowing the contact angle value gives a strong indication on the performance of the product or a successfulness of the process.
Contact angle is easy to understand visually. It is the angle a droplet forms in contact with a solid surface. Thermodynamically, it is a balance between different forces acting on a three-phase contact point where solid, liquid and air meet.