When it comes down to it, this is another purely semantic question, much like the one we dealt with in another article comparing the terms “surface tension” and “surface energy.” Surface-free energy is free energy in a particular space - the surfaces of materials. Free energy, in its most thermodynamically pure sense, refers to the energy that is available to do work, cause action, and make something happen. Surface-free energy is concerned with the energy available to do work at the surface of materials.
For manufacturers and anyone concerned with adhesion, cleaning, bonding, coating, ink and paint formulation, sealing, or any other process that involves surfaces interacting with other surfaces or their environment, surface-free energy is typically shortened to just surface energy.
Surfaces are critical to all of the processes listed above, and they are often left unmeasured and, therefore, uncontrolled even though they have a direct impact on the performance of products manufacturers in all industries build.
Controlling surfaces in manufacturing means controlling the surface energy of the materials being used.
Surfaces are made up of molecules that interact chemically with each other as well as the molecules that make up the surfaces of other materials they come in contact with. To change surface energy, it must be understood that those molecules can be removed, replaced, or otherwise manipulated through cleaning and treatment to create a different level of surface energy and get the desired result. To control surface energy, it must be measured throughout the process of changing the chemistry of the surface to determine when it changes and how much. That way, the precise amount of necessary surface energy can be attained at the proper time in the adhesion or cleaning process.
To understand how molecules do the work of creating strong bonds and chemically clean surfaces, we need to understand the attractive forces that pull them together and make up the total surface-free energy available.
Dispersive vs. Polar Surface Energy
When we talk about the energy of a surface, we’re talking about the ability of that surface to do work. It’s literally the ability of a surface to move molecules around - this movement requires energy. It’s important to remember that a surface and the molecules that make up that surface are one-in-the-same. Without the molecules, there is no surface. And without energy making it possible for those molecules to do the work of grabbing onto an adhesive, there is no bonding.
In physics, energy equals work. It’s the same formula for both:
Force x Distance = Work
Force x Distance = Energy
Ergo:
Work = Energy
So, work and energy are directly proportional to each other. More work requires more energy. And if you have more energy, work increases right along with it. The ability of a molecule to do work comes from the molecule’s attraction to other molecules. Those attractions arise from a couple of different ways that molecules can interact.
Fundamentally, molecules interact because they have positive and negative charges that attract the opposite charges between molecules. Molecules have a cloud of electrons that float around their exterior. It’s because of these continuously moving electrons that molecules have variability in the charges of a given area of the molecule. If all molecules had uniform charges all around them, no molecules would attract each other. Imagine two ball bearings, each with a uniform distribution of electrons across their surfaces. Neither would be attracted to the other because they would both be negatively charged and have no positive charge to be attracted to.
Luckily, in the real world, these electron clouds are in constant motion; at any instant in time, there are areas that are more positive or more negatively charged. If you have two molecules that have randomly charged distributions of electrons around them at any point in time they will feel a little bit of an attraction to each other. The forces that result from the random redistribution of positive and negative charges in the electron cloud around the molecule are called dispersion forces.
These forces are very weak. There are dispersion forces between all molecules, regardless of the structure or composition of the molecule which is in direct contrast with polar forces which result from the structure of the molecule.
For example, dispersion forces are the only forces that exist between nitrogen molecules. At room temperature, nitrogen is a gas because the dispersion forces are too weak to resist thermal vibrations, even at the most moderate temperatures, and hold the nitrogen molecules together. Nitrogen gas becomes a liquid only if we remove almost all of the thermal energy by cooling it down below -195°C. Once the thermal energy is decreased enough, the weak dispersive forces are capable of overcoming the thermal vibrations and can pull the nitrogen molecules together to form a liquid.
If we look at water, it has a molecule with a similar size and mass to nitrogen, but water molecules have a different structure and composition than nitrogen molecules. Because water is a very polar molecule, the molecules are attracted very strongly to each other, and water remains a liquid until it reaches above 100°C, where the thermal energy overcomes the polar forces holding the molecules together, and the water becomes a gas.
The key point to understand is the difference in strength between dispersive and polar forces that attract molecules to each other. Keep this in mind as we talk about the surface energy that results from these attractive forces.
Dispersive Surface Energy
Dispersive surface energy is the component of surface energy that results from the dispersion of the electron clouds in the molecules on the surface of the material. Total surface energy is an expression of all the attractive forces that attract molecules to each other. Dispersive surface energy is a component of that total, even though they are a weak and undulating ingredient.
When Should You Pay Attention to Dispersive Surface Energy?
Dispersive surface energy differs for different materials. Highly aromatic polymers like polystyrene have a lot of benzene rings and relatively large dispersive components of their surface energy. Similarly, PVC also has a relatively large dispersive surface energy component to its total surface energy due to containing large heteroatoms such as chlorine.
So, the magnitude of the role that dispersive energy plays in manufacturing processes depend on the materials being used. However, because dispersive forces depend so little on specific molecular structures, we are very limited in ways to control them.
Polar Surface Energy
Interactions based on these fluctuating dispersive electron servings are not the only way molecules interact with each other. Molecules can interact with other molecules because of certain structural features which create other attractive forces between the molecules. There are many ways to classify these other forces, such as acid-base interactions, where molecules interact through their ability to accept or donate electrons.
Some molecules have structural features that create permanent dipoles - meaning that, in addition to the random dispersion of the electrons around the molecule, there are portions of the molecule that are always more positive or negative than the other portions. These permanent dipoles attract each other much more strongly than dispersive interactions.
Because of their structure, some molecules have areas that are permanently charged, either positively or negatively. Polar surface energy is the component of surface energy that results from the attraction of these charges between molecules.
We can conveniently lump all non-dispersive interactions together under the umbrella of polar interactions.
The dispersive character of a molecule is a function of the size of the molecule, specifically, how many electrons and protons are present. We don’t have much control over the number of electrons and protons, which limits our ability to control the dispersive component of surface energy.
The polar component, however, is determined by the location of the protons and electrons - the shape of the molecule. We can alter how the electrons and protons are distributed through treatment methods such as corona treatment and plasma treatment. It’s similar to how we can alter the shape of a lump of clay, but it will always maintain the same mass.
Why are Polar Forces Important?
Polar forces are very important because they are the component of surface energy we control when we perform surface treatments. The dipole-dipole attraction is responsible for the strong adhesion between most adhesives, paints, and inks to surfaces. We can increase the polar component of surface energy radically by cleaning, flame treatment, corona treatment, plasma treatment, or any other form of surface preparation and thereby improve adhesion performance.
We can inadvertently decrease the polar component of surface energy by introducing substances with low energy onto a surface simply by using the same side of an IPA wipe twice on the same surface. In addition, a surface can be overtreated, causing it to volatilize and reduce the surface energy. The polar component of surface energy also changes when a surface isn’t in production at all. A clean surface put into storage will attract molecules from the environment, including the packaging materials. This changes the molecular landscape of the surface and can decrease the surface energy.
We have little to no control over the magnitude of dispersive forces. These forces are essentially fixed and in manufacturing, there is little to no value in trying to change dispersive forces as a means to control surface quality to achieve reliable adhesion.
When we engineer or alter surfaces, we are engineering the properties of the polar components of surface energy. Therefore, if we want to develop a surface treatment process to control the surface of our materials, then we want to control the polar component of the surface.
How Do You Calculate Surface Free Energy?
Surface free energy is the sum of all of the individual forces that are acting between molecules. There are a few formulas for surface-free energy. If we decide to consider all non-dispersive forces as polar forces, the calculation for surface free energy is simple. The formula is:
γS = γSd + γSp
When Do You Need Surface Free Energy and When Do You Need Surface Energy?
When it comes to surface treatment, cleaning, and preparation in manufacturing reliable products, surface-free energy is the same as surface energy.
Since production requirements concerning a wide array of processes, such as bond performance of joints, proper adherence of inks on plastics, or coating performance of “self-cleaning” coatings on smartphone screens, rely on surface properties being controlled, it’s quite important to appreciate surface energy as a manufacturing concept of consequence.
Surface energy arises from the different ways molecules attract one another. The polar interactions between molecules are the most important for adhesion and cleaning processes because those molecular-level interactions are the ones we have the most control over through surface treatment, abrading, sanding, washing, wiping, or any other surface preparation method.
Knowledge of polar and dispersive components and surface tension can be important in the development of adhesives, inks and paints. But, for manufacturing products using adhesives, inks, paints and coatings, we generally only need to pay attention to the polar component of surface energy as it is the one being affected by the manufacturing process.
Measuring total surface energy is a relatively complex and error-prone process. However, the contact angle of a single liquid, like water, is determined almost entirely by the polar component of surface energy. So, by measuring the angle produced by the height of a drop of water on a surface, we can know, with incredible precision, how the polar component of surface energy is changing. Generally, the higher the surface energy, the smaller the angle will be due to the water drop being so attracted to the surface and spreading or wetting out. Low surface energy causes the water to bead up and constrict into a little bubble on the surface, creating a large contact angle. The consistency of this contact angle measurement correlating to surface energy and therefore correlating to adhesion performance gives manufacturers a reliable and repeatable way to guarantee the strength of their products.
To learn more about controlling manufacturing processes to achieve more predictable outcomes, download the eBook: Predictable Adhesion in Manufacturing Through Process Verification. 
