Contact angle refers to a method of calculating surface free energy by evaluating the interface of a liquid and a solid surface.
A contact angle (also referred to as a wetting angle) is formed when a drop of liquid is placed on a material surface and the drop forms a dome shape on the surface. The angle formed between the surface and the line tangent to the edge of the drop of the water is called the contact angle.
Θ is the contact angle in the illustration above. As the drop of water spreads across a surface and the dome becomes flatter, the contact angle becomes smaller. If the drop of water beads up on the surface (as you might see on a water resistant article of clothing or a waxed car) the dome becomes taller and the angle becomes larger.
When the angle the drop makes with surface is measured, the resulting angle indicates whether the drop of water is more attracted to itself or to the surface it is on. Unseen forces on the surface of the material are acting on the water drop as soon as contact is made.
If these forces are strong, their pull on the water drop will cause the drop to “wet out” or spread further over the surface. If the forces are not stronger than the attractive forces the drop has for itself, then the drop will constrict into a shape as close to a sphere as it can. These two forces are working in tandem on the drop creating an angle that can be measured. This measurement allows us to understand the relationship between the liquid drop and the surface.
Young's equation mathematically expresses the relationship between the surface energy of the solid surface (ϒs), the surface tension (which is a term for surface energy of liquid phase substances) of the liquid (ϒl) and the surface energy of the interface (ϒsl).
Young's Equation: ϒs = ϒsl + ϒl cosθ
The forces acting on the drop of water from the surface are referred to as the surface energy of the material. If the forces are strong, the surface has high surface energy or the surface can be said to be very energetic. This high energy pulls hard on the water, causing it to wet out. This wetting phenomenon simulates how an adhesive, paint, ink or coating acts when placed on a high energy surface.
Determining the contact angle will allow you to know the quality of a material surface prior to an adhesion process such as coating, sealing, soldering, adhesive bonding, printing or painting. The contact angle can also be used as a surface quality check after cleaning processes like solvent cleaning, parts washing, ultrasonic cleaning, and more. This angle provides quantitative data about the cleanliness of a surface at a molecular level.
Sources of contamination that can create a low energy surface are oils, silicones, grease, debris, or anything that might be in a manufacturing environment that could potentially find its way onto a material surface. These contaminants are detrimental to the adhesion processes. The water drop is very sensitive to molecular differences in surface energy and, by measuring the contact angle, you can accurately predict if adhesion will occur between a coating, ink, paint, glue, primer or sealant and the surface of the material.
In many manufacturing and assembly processes a liquid or a melted material needs to spread out adequately on a surface. In brazing and soldering, a molten metal needs to spread out over a surface of another metal; in medical diagnostics, substances like blood need to flow on a testing strip; in electronics manufacturing, a coating needs to flow into small crevices and thoroughly cover the surface of circuit boards.
Wettability is the amount to which a liquid can spread on a surface determined by the intermolecular forces between the surface and the liquid. Controlling these forces allows manufacturers to control the processes relying on the intermolecular forces.
To learn more about the best way to measure surface quality in manufacturing, read this technical paper: Why One Contact Angle Fluid is All You Need to Control Your Process
The Surface Analyst technology uses an innovative approach to depositing the droplet of water and measuring contact angle that overcomes the variability that is often found in manufacturing environments.
Surface Analyst products use patented Ballistic Deposition™ to deposit a drop of water onto a surface. After the drop is deposited, this innovative method uses a top-down view to take a precise image of the drop to measure the contact angle.
By constructing a drop of highly purified water in this unique manner and measuring the drop top-down, the Surface Analyst is able to take measurements on a variety of surface angles—vertical, horizontal, convex and concave--as well as different surface finishes--smooth, textured and rough.
To learn more about the best ways to utilize water contact angle measurements in your process, download our eBook about verification techniques. The information in this book will put you on the right path to a predictable coating process that results in the highest quality diagnostic products. Download it now: Predictable Adhesion in Manufacturing Through Process Verification
A bench top contact angle goniometer is a commonly used laboratory instrument for measuring contact angles. Advanced handheld and automated contact angle meters (like the Surface Analyst family of products) calculate surface energy directly on actual parts as they proceed through production processes to predict adhesion outcomes.
For manufacturers who are concerned with bonding, sealing, coating, cleaning, painting or printing, a contact angle measurement instrument that rapidly quantifies surface cleanliness during production is the best way to monitor critical adhesion processes.