We’ve all had the experience of unboxing a brand new cart and thinking “Hey, the cart isn’t filled! Is there really 0.5mg of oil in there? Am I getting what I paid for?” Well, guess what, it’s perfectly normal. The short answer is that some of the oil has been absorbed by the wicking material of the coil. For a more detailed explanation of the mechanics behind this absorption please read on.
The main culprit is capillary action
Even if you’ve never heard of capillary action, it is still important in our everyday life. Capillary action is important for moving a liquid around. It is defined as the movement of that liquid within the voids (spaces) of a porous material due to the forces of adhesion, cohesion, and surface tension.
Plants and trees couldn’t thrive without capillary action. It helps bring water up into the roots. With the help of adhesion and cohesion, water can work it’s way all the way up to the branches and leaves.
Coils and the wicking material
We all have a basic understanding of how vaping works: you fill your cartridge with cannabis oil, cap it, and then apply heat to it using a battery that turns the oil into an aerosol (the “vapor” that you inhale). Generally, by the time you purchase a cartridge, some of the oil has already been absorbed by the coil. Capillary action is when the oil is “sucked up” in the wicking material. Contrary to common belief, it happens even without using the device until a “stable” state is reached. Using the cartridge and pulling on it only accelerates capillary action.
Most wicking material is made of porous ceramic and in some cases, with a thin layer of organic cotton wrapped around the ceramic. The reason there is some cotton is to optimize the absorption rate of the wicking material and the vaping experience. Keep in mind that the oil never comes in contact with the heating element.
The wicking material of the coil is the main hardware component that influences the absorption rate and thus how full your cartridge looks when you buy it. That said, there are also other components and factors involved, such as the intake holes.
The forces at play in capillary action are adhesion, cohesion, and surface tension. Those forces are not only influenced by the wicking materials (ceramic, cotton, carbon, etc…) but also by the size of the intake holes as they clearly affect the surface of the wicking material exposed to the oil. Simply put, the larger and more numerous the intake holes are, the easier it is for the coil to absorb the oil.
A simple experiment you can try at home here is with two straws, a thick one and a thin one made with the same material. Fill a glass with water and insert both straws in it. You’ll find that the water level in the thinner one is higher than that of the thicker straw. The narrower the bore of the straw, the greater the extent of raising or lowering of the liquid.
In some cases, depending on the porosity of the wicking material, we’ve observed up to 0.125g get absorbed. In a 0.5mg cartridge, this means that 25% of the oil is absorbed in the coil, thus the cartridge looks 75% full. This same scenario in a 0.3mg cartridge with the same coil absorbing 0.125mg means that the cartridge will look roughly 58.3% full.
If you’re interested in learning more about capillary action, along with a more detailed explanation of the intermolecular forces involved, please view Professor Dave’s explanation.
Extract properties in relation to capillary action
Oil inclination angle with regard to the wicking material
Have you ever tried to siphon gas from a tank? If so, you know that the angle at which you set a tube has an influence on the speed due to gravity, adhesion, and cohesion forces. This siphon concept is the same here when we think about how the cartridges are stored after filling.
If they are stored vertically with the mouthpiece on top, it’s obvious that capillary action will happen faster and that the wicking material will be absorbing the oil, because of gravity. The level of filling will seem lower.
If, on the other hand, you store the cartridge upside down, the oil might not be in contact with the wicking material, and gravity will prevent capillary action. In this case, when you start using the cartridge, you’ll have to turn it upright and wait a while for the oil to reach the coil. If you don’t, you might find yourself pulling a “dry hit”. This is a safety issue as you would essentially be heating a coil that hasn’t absorbed enough oil, hence burning it and potentially inhaling toxic emissions. You would taste it immediately as burnt.
What if your cartridge is stored horizontally? Well, the answer is simple, you would get a mix between the two results explained above.
Type of extract – intermolecular forces
There is much to be said here given the different types of extracts and even different batches of the same extract, but for our purposes, we’re interested in the intermolecular forces responsible for cohesion and adhesion. This is the main reason that there is no one size fits all in terms of cartridges.
Capillary action is unique to each extract. For very low viscosity oils, you may experience faster absorption by the wicking material due to very high capillary action just as for very high viscosity oils, you may experience a slower absorption rate due to very low capillary action. When a processor fills a cartridge with a distillate, the “stable” state might be reachable in a few minutes or hours. If the same processor now fills the same cartridge with live resin, for instance, the “stable” state might be reachable in a few days.
These are also some of the reasons why it’s important to measure the viscosity of your oils to gain a perfect understanding of how they will react to hardware but also to help pinpoint the “sweet spot” in terms of flavor and effects.
Environmental conditions influence capillary action
It’s important to keep in mind that everything we have reviewed previously is based on theory. In that theory, we “assume” that outside conditions are constant. In the real world, our cartridges are submitted to the same environmental conditions that we are. The two main ones are temperature and pressure, and they both influence the capillary action explained above.
We all understand by now that temperature influences viscosity. Who hasn’t tried to heat a cartridge with a blow dryer if it was clogged? This is also why some push-button batteries have a preheat feature, which is very useful for instance in cold weather. The higher the temperature, the less viscous the oil is thus influencing its cohesion and adhesion which we’ve already established are the main factors in capillary action.
What this means for your cartridge is if for some reason, the cartridge has been left on your dashboard for a while in Florida or California weather, the coil will absorb more oil and the cartridge will look emptier. With prolonged exposure to heat, the cartridge can also start to leak because of the modifications to cohesive and adhesive forces in the oil.
In some cases, depending on the extract, an incubation period is recommended (so as not to degrade the constituents of the oil, like terpenes) in order to reach the “stable” state before dispensing the cartridges.
Another important force is pressure. You experience this often if you fly a lot with a water bottle. The density of water increases with increasing pressure and decreases with increasing temperature. This is why your bottle seems emptier (volume decrease) when up in the air (and the bottle crumples up as well).
If your cartridge is submitted to intense pressure changes at any time in the manufacturing or distribution process, it will influence how full the cartridge seems. A frequent variance of pressure can also have a negative effect on capillary action accelerating it in some instances then slowing it down in others. Variance in pressure can also cause leakage of the cartridge.
Additionally, if you try to “top up” a cartridge afterward, once the wick has already absorbed a lot of it and you don’t leave any air in the tank, you will not be able to vape as the surface tension will be impacted thus impacting the capillary action or you will experience leakage (no matter what the cartridge brand is).
Finally, the last environmental factor is humidity, the higher the humidity level is, the more capillary action will happen, as humidity also affects cohesive and adhesive forces.
Environmental factors like temperature, pressure, and humidity have an impact on capillary action. Going back to the example of capillary action in plants and trees, you now clearly understand why vegetation is more abundant in the rainforest than in a desert.
The filling process impacts capillary action speed
The immediate reaction to a cartridge that is not full is, “I’ve been cheated”. This is a natural reaction but as you’ve understood by now, it is completely normal and depends on a lot of factors. Nevertheless, there are steps that a processor can take in order to ensure that it doesn’t happen too much.
Capping and sealing
One important aspect during the filling process, whether you’re filling cartridges manually or automatically, is capping and sealing. Capping is the process of inserting the mouthpiece onto the filled cartridge and sealing is the process of inserting a silicone cap or plug on the mouthpiece as well as on the base of the cartridge (the 510 thread).
You may have heard about the need to cap and seal within minutes after filling. One of the main reasons this needs to be done is to stabilize the pressure inside the cartridge and slow down capillary action. This is also why we do not recommend filling equipment that fills X amount of cartridges and caps them only after all of them have been filled. You can see for yourself, the level of the first filled cartridge seems lower than the last one while the filling machine has dispensed the same amount of oil (provided that it was well calibrated).
Critical control points
In quality control, we use “critical control points” throughout the manufacturing process to make sure that all products are equivalent in volume. There are a few of these control points that the processor can establish during the filling process to make sure that the same amount like weighing cartridges before and after filling (randomly or systematically).
These critical control points are what ensure you, as the consumer, that you are getting the right amount of oil which you’ve paid for. Cannabis regulations in US states and in countries like Canada are very strict when it comes to the quantity and constituents of oil in cartridges and the corresponding labels. When a brand sold on the legal market states 0.5mg of oil, even if the cartridge doesn’t seem full enough, you can be sure that there is 0.5mg of oil in there.
A vaping hardware supplier should work with its clients (MSOs, LPs, Processors and Brands) to make sure these critical control points are in place and help with the integration of these into standard operating procedures.
There have been multiple tests with various types of oil (live resin, distillate, etc…) over the years and in some cases, depending on all the factors above, we’ve witnessed the wicking material absorbing up to 25% of the oil. In a 0.5g cartridge, that’s 0.125g that ends up in the coil. All in all, this means that you’re getting your best hits out of the box.
If you’ve been through this entire explanation, you now understand why cartridges will never seem full. There are some manufacturers or brands that will use silicone or will make some design changes like silk screen printing at the top of cartridges in order to mask that area where the cartridge does not contain oil. Be aware that this is just aesthetics and that your cartridge, when purchased on the legal market, contains exactly what the label says.