What is Surface Energy and Why Does It Matter In Manufacturing?

Introduction to Surface Energy

Surface energy, also known as surface free energy (SFE) and often called the dyne level of a material is the available energy at a material surface that can interact with the world around it. It can be measured in a number of units but most commonly mN/m (millinewtons per meter), dynes/cm or mJ/m² (millijoules per square meter).

A material such as PTFE with a surface energy of less than 20mN/m, for the chemists reading this, PTFE is a fluorine containing hydrocarbon and the bond length between the carbon and fluorine is very short. This holds the molecular structure together and interactions are favourable within the material itself, so it doesn’t interact well with the outside world.

A material such as Nylon has a much higher surface energy, usually around 40mN/m and when looking at the material structure there is significantly more available chemistry at the surface to interact with the world.

What is Surface Tension?

Liquid surface tension holds a liquid together and energetically the physical aim for a fluid is to minimise the surface area of the liquid to conserve energy. The higher the surface tension, the greater the ability make this happen; the smallest surface area shape that any fluid can make is a sphere, which is what the liquid will try and get to.

On Earth, with gravity and other interactions playing their part to change the pure shape of a water droplet, it’s difficult to get the perfect thermodynamically stable sphere shape, but water will try and do its best.

This is often seen, but perhaps a lot of the time not really thought about – we use the water on a waxed car bonnet example every time we start talking about this subject!

Surface Tension and Surface Energy Together

A solid surface must have some interactions with the world around it, whether this is very low and the material prefers to keep to itself, or high and the material readily sublimates and evaporates away.

Likewise a liquid is going to have either a high surface tension that holds itself together and aims for a perfect sphere, or it will have a low surface tension and not have enough energy to pull itself into a sphere.

Here are the possible combinations of these solids and liquids

Solid Surface Energy	Liquid Surface Tension	Observation
High	Low	There is more energy in the solid than the liquid, so the solid will attract and pull the liquid toward the surface. The liquid will readily spread and flow over the solid.
High	High	The solid has a high energy to pull a liquid toward itself, but the liquid has a high tendency to hold together. The liquid will flow and spread over the surface, but not as well as a low surface tension liquid.
Low	High	The liquid has a high surface tension and can hold itself together in a stable sphere like state. The solid cannot overcome this as it has a low surface energy and so the liquid will sit like a ball on the surface.
Low	Low	With both the surface energy and the surface tension low, the interactions will be minimal, but matched in a similar way to a high: high interaction. Medium flow and spread of the liquid will take place.

This is seen in many day to day applications and isn’t as trivial as it may seem. Here are a few examples of these situations.

Solid Surface Energy	Liquid Surface Tension	Example
High	Low	A high surface energy material could be a freshly abraded metal, like a sanded piece of steel. A low surface tension liquid could be a solvent based coating. The liquid will readily flow over the surface and give even coverage.
High	High	The same sanded steel, but with a water based coating will give a similar result, but the flow of the liquid is likely to be much trickier and not give as smooth of a finish.
Low	High	The common low surface energy solid with a high surface tension liquid is a waxed car bonnet being splashed with water. The liquid will bead up and roll away, not flowing or spreading at all and very little interaction between the two.
Low	Low	A Teflon coated pan is another common example of a low surface energy material and a low surface tension liquid could be melted butter than flows and spreads to cover the entire pan.

Industrial Applications of Surface Energy

Surface energy and surface tension interactions are important for a lot of industries. Anywhere that a solid and a liquid need to interact, there needs to be consideration. Inks, coatings, adhesives, tapes, material finishes and liquid flow equipment all need to understand this relationship and design the products sensibly.

Historically it was easy to overcome problems because there are a number of things you can do to move the interaction one way or another.

Adding water to a mixture would increase surface tension, while adding strong solvents like toluene and xylene would reduce the surface tension. A coating supplier would want to have good wetting and so high solvent content is sensible, however, these solvents often have many health implications and due to REACH restrictions, many of the solvents cannot be used in modern products.

The move to water based products lead to many headaches for manufacturers and the use of pretreatments, primers and physical abrasion and preparation became more important than ever. Water based coatings, inks and adhesives must have a higher surface tension than their solvent based alternatives, so the solid surface itself must be best prepared.

Wetting and Adhesion

Liquid flow and spread is an important and easy to observe interaction between a solid and a liquid and this is described as wetting or non wetting. Many industries use this to determine whether a liquid such as an ink will adhere to the solid. The theory that if the solid is strongly attracting the liquid, and the liquid is readily flowing on the material that it will also have strong adhesion properties.

This is likely, but it isn’t guaranteed and a high degree of wetting does not promise good adhesion. It is a good indicator and actually once determined, it is a repeatable property that can be used as a quality checking process.

Measuring Surface Energy

Measuring surface free energy is a simple test which is often done in industry for quality control, as well as in education to understand materials and interface interactions.

There are three key techniques, all need to have a liquid of known properties interact with a surface and the result observed to obtain a result.

The most technical technique is tensiometry using either contact angle or force tensiometers. The more standard, but usually quicker and cheaper option is using dyne pens or dyne inks.

There are other methods, such as theoretical calculations, which are usually the numbers found in textbooks. These results are useful for considering materials in general, however, real world readings on a specific material will take into account surface finish and texture, additive content, material age and any other factors that may have a drastic impact on the real world number.

Dyne Testing, Dyne Pens and Dyne Test Inks

Measuring surface energy or the dyne level of a material using a dyne pen or test ink is usually a 30 second test that can be done in a lab or most commonly in a quality area or production line.

Dyne pens and inks work the same, a pen or bottle of ink with specific number is used to apply a trail of ink across a material surface and the observation is observed in the first 3 seconds only.

The test is considered a pass if the ink flows and stays as a film, while a fail is when the ink beads up and draws back.

A pass means there is more surface energy available to pull the liquid toward the solid, and a fail means there’s more surface tension in the liquid trying to get away from the surface.

To get to the the final value of the material you will need to use a range of inks and determine where the highest passes and the next one fails. Dyne inks give a range for the surface energy, and they are best to test in slightly larger ranges.

For example, a material with surface energy 50 dynes/cm will pass with a 48 dyne pen, it will fail with a 52 dyne pen. trying to determine exactly the 50 pen would be very tricky. Therefore the test says the surface energy is 48-52 dynes.

For more information on this testing, you can read further information here for What is Dyne Testing?

You can also follow a usage guide for using either the test inks or pens in our How to Use Dyne Inks blog.

Contact Angle Meters

Contact angle is more technical, and can provide more information about the surface, but is usually a more costly method and while individual experiments are only moments, it normally takes longer than test inks and nearly always requires a trained operator.

Contact angle uses at least one known and ideally pure liquid to place a drop on a material surface. The droplet will sit on the surface and the edge of the drop will have an angle, this is known as the contact angle and for a low surface energy material, the droplet will sit proud and have a high contact angle, while a high surface energy material will pull the liquid closer and give a low contact angle.

A single drop is useful and the liquid usually used is pure water as it has a high surface tension so shows good results across a wide range of materials – a low surface tension liquid would likely wet even a low surface energy material and so the angle might be hard to determine.

Using two or more liquids adds more to the story and allows the contact angle number to be calculated in to full surface energy.

When using two liquids, you can use many different ones, however the most common method is to use one polar liquid, most often water and then one none polar liquid which is most commonly di-iodomethane.

Whether looking at contact angle only or performing a full surface energy calculation with multiple droplets and liquids, the method is usually accurate to several decimal places so much better than test inks for accuracy.

Polar and Dispersive Surface Energy

Diving deeper into surface energy by using the two liquids, we can break the total surface energy down in to polar and dispersive components. In theory the total surface energy is key to wetting and flow. A high surface energy will lead to good wetting, perfect for coatings to evenly coat the surface. But a high total surface energy made up entirely of dispersive surface energy is unlikely to give good adhesion. The finish might look perfect, but the coating could flake or be easily removed.

The polar component of surface energy is more reactive with the outside world and offers strong bonding capability. So even with a relatively low surface energy, if the polar component is high then you could get good adhesion.

This material property cannot be determined by using the inks and is one of the key strengths of using contact angle for measuring surface energy.

This is the most simple way of using contact angle and many options can be used which might include heated/ cooled environmental chambers, rotating and tilting cradle, material roughness calculations and others for more specific applications.

Force Tensiometry

A force tensiometer is a really useful tool, perhaps more useful than contact angle and test inks in many ways.

A force tensiometer uses a very accurate micro balance to measure the interaction of a solid probe as it is dipped into a known liquid. The opposite can also be done to determine surface tension, read about that later.

A high surface energy material will be attracted to the liquid and pulled closer, exerting more force on the balance. A low surface energy material, especially a small and light one like a fibre will be repelled by the liquid and a negative or low force will be exerted on the balance.

This system is very accurate and can be used to determine a calculated contact angle and surface energy. The fact that the droplet isn’t visible is often trickier to grasp so these systems aren’t always as intuitive as standard contact angle. However, because the material probe is being lowered in to the liquid it takes a lot of readings and takes in to account an average of the whole surface rather than a specific point.

Force tensiometers are also used for determining surface energy of materials that we can’t put a droplet on – fibres, powders and porous materials for example.

As with contact angle, there are quite a few extras that can be done to delve deeper in to the material characteristics, and all results are very accurate to several decimal places.

Which Method Is Best For Measuring Surface Energy?

Of the three main methods for measuring surface energy; test inks and dyne pens, contact angle and force tensiometers, you can get different information and they all have their own good and bad points. In a perfect world you would probably run all tests so you can get all the information and from different methods and points of view.

In reality, you’re likely to use one of the methods and here are a few great uses and poor uses for each.

Technique	Great Use	Poor Use
Dyne Test Pens	Dyne pens are quick and easy to use test pens that will tell you a quick pass fail in seconds and can be used by almost anyone. If your test is clear, for example testing a corona treated material for 38 dynes that started as 30 dynes then you’ll find the 38 test is clear and without doubt.	Test pens use a felt nib and can pick up dirt and contamination quickly, especially on greasy surfaces. This immediately ruins the pen so testing oily metals or greasy plastics is a bad idea with the pens and the inks are likely better.
Dyne Test Inks	Dyne Test Inks use a single use swab, perfect for measuring dirty surfaces or for getting into areas that other tests might not. They’re the most versatile for a wider range of surfaces and are usually as quick as test pens.	As dyne inks come in larger sized bottles than the pens, and the need additional test swabs, they’re not always as portable as the pens so while good line side, they really want to be at a quality control area where they can be used properly.
Contact Angle Meters	Contact angle is a fantastic tool for checking materials, the fact that software can record results for future evaluation makes it a strong quality lab tool that can be referred to even in years to come. The accuracy and repeatability also leaves very little doubt.	Contact angle machines are significantly more expensive than test inks and require trained personnel, so while their use on production lines is sensible, they are more suited to high value lines or labs rather than general production.
Force Tensiometers	Force tensiometers are great allrounders, but they really shine against other techniques when testing powders and fibres. They can measure the surface energy of nano powders or individual hairs. No other method can come close.	Using force tensiometry usually requires small samples and trying to test a large component can only be done by taking a piece away. A timely, damaging and costly method for material testing when compared with other methods.

For many, the day to day test is dyne test inks or pens, the in depth test is contact angle and the specialised technique is using a force tensiometer.

It’s worth knowing that while they all give a surface energy result, because they go about it in slightly different results its actually unlikely that you will get exactly the same results between any test method. They will all have their own variations.

This point is especially important when referencing or looking up values because one experiment could actually be different by 10mN/m. It’s not that either method is wrong, but the test method is just different.

Contact angle for example uses a small droplet and so can be heavily influenced by surface texture, force tensiometers require the material being dipped into the liquid to be pulled, so if the material is strongly repelled then it will give the wrong result. Test inks use a variety of chemicals and any of them can react with your material surface which would give skewed results.