Perhaps the most coveted effect in acrylic pouring, and one that tends to frustrate beginners because of the variety of conflicting information that's out there. In this article, I aim to give you a deeper understanding of the science at work when cells form in acrylic pouring.
The reality is, there are multiple methods to create cells. All of them look slightly different. All of them require different techniques and materials, and all of them do in fact work. They're just not interchangeable.
There are 3 things that have to be considered when looking to create cells:
Surface Tension
One of the really important things you should understand about paint is the concept of surface tension. This is the largest factor in determining how paints move, flow, and grip. When we think about cells, surface tension is the factor that determines how easy it is for a cell to reach to climb its way to the surface.
This is why sometimes you need heat to bring cells up, and sometimes you don't. It's usually the pouring medium that determines this. Retail pouring medium has lower surface tension than glue and varnish, but higher surface tension when compared to US Floetrol. You might have read that US Floetrol is a paint conditioner that's designed to help house paint self-level and hide brush strokes. It does so by reducing the paint's surface tension.
This is why people mistake US Floetrol for a component that creates cells. It doesn't directly, but it creates an environment of low surface tension where if you make the right movements, you can destabilize the paints enough to cause a reaction. (more on this shortly)
Before you can bring density or the hydrophobic effect into play, first you have to make sure the surface tension is actually being used through movement. Cells need movement. By movement, I'm referring to two things:
The paints have to be fluid enough for the pockets of paint we call cells to move through them. So, regardless of the method, if our paints are too thick, no cells can form.
The act of destabilizing the fluids by pouring them, flipping them, swiping them, or any other motions of that nature. Meaning, if nothing moves, nothing will happen.
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Density
Nine times out of ten, if you've gotten cells, and you can't explain how, it's because of density. If you've heard of creating cells using water, metallic paints, Floetrol, or a "cell activator" these are all methods of creating cells using density.
The idea of making and manipulating cells comes down to physics, specifically, fluid dynamics. More dense fluids sink, while less dense fluids float. If you were to compare 5 different colors from the same manufacturer, you will find that they weigh slightly differently. With all other components in a tube of paint being the same, the pigment is what determines the density of the paint.
With that being said - I invite you to skim through this quick video on the Rayleigh-Taylor Instability and observe how the tinted water sinks in a pattern.
Essentially, the tinted water is denser than the clear water because of the dye. In the experiment, the tinted water is also warmed up to temporarily reduce its density, so it sits on top of the clear water. This configuration is originally stable, but as the tinted water cools down, it begins to destabilize, and then interesting things begin to happen.
The Rayleigh-Taylor Instability "occurs when a heavy fluid overlies a lighter one, and the two fluids are separated by a horizontal interface. The configuration is unstable, and a small perturbation to the interface grows with time."
What you're seeing here is exactly like the cross-section of a painting where cells were formed using any density method. In practical terms, using density means choosing the order of your colors based on the density of their corresponding pigments, and making sure that high-density and low-density paints mingle somewhere in the painting.
The issue is that not all paint manufacturers release information about their pigments, so when working with this method, you will tend to gravitate to your own personal experience of which combinations of paint work best with each other. My friend LeftBrainedArtist has a very useful tool for finding out paint density based on the information that is available out there.
The Cell Activator and Lacing:
One of the most commonly seen ways to use the Rayleigh Taylor Instability for cells comes from the SheleeArt Bloom Technique's Cell Activator. You can learn more about exactly what's done in the bloom technique with this video.
The Cell Activator is the recipe that goes on last in the Bloom technique. When destabilized with a blow, it exhibits the effect seen in the Rayleigh-Taylor Instability, sinking through the colors underneath, creating a pattern we call "Lacing." With lacing, you'll notice a more geometric look to the patterns.
Let's also establish that the only thing that's supposed to be referred to as a "Cell Activator" is the Australian Floetrol and Paint recipe from the SheleeArt Bloom Technique.
Anything else which produces the lacing web-like effect is a "Lacing Recipe." Here is the original Cell Activator versus the easiest possible alternative Lacing Recipe to make in action:
The Hydrophobic Effect
If you've heard of creating cells by using enamel paint, a mix of matte and glossy paint, silicone oil, hair serum, or any other cell "additive" - this is the hydrophobic effect.
The hydrophobic effect refers to an inherent property of materials that repel or do not mix with water. You already know this as "Oil and water don't mix," but it's not exclusive to oils. The idea also refers to, comparatively, when one material is more or less resistant to water than another. Even if both are water-based materials.
The Cell Additive Method
We're going to split this category in two and start with the easier-to-grasp method. A cell additive (not to be confused with a cell activator) is usually an oil, like silicone oil. You may have also heard of people using treadmill oil or coconut hair serum. All of these are in fact silicone compounds. Silicone oil is non-toxic, non-flammable, and most importantly, clear-colored.
The idea with Silicone oil is very simple. The oil floats to the surface because oil is less dense than water. When the layers of paint are shallow enough after they've been spread around through tilting or swiping or another form of movement, the droplets of clear silicone oil reveal the colors underneath as they reach the surface. Spot heating with a torch or heat gun is often needed to bring the cells to the surface (but it's okay if you don't need heat!)
The Hydrophobic Method
There's another method that's specifically referred to as the "hydrophobic effect" and this one is a little more complicated to explain. Popularized by Gina DeLuca through her Straight Pour technique, the idea is that you mix up using paints with a gloss sheen and a matte sheen.
As I had mentioned, when comparing two materials, you can describe one as more hydrophobic than the other. Within the context of paint sheen, glossy paints tend to repel water and other liquids, while matte paints have a more absorbent nature.
If you want to dive deeper, this is because gloss paints are formulated to dry with a very smooth and reflective finish, which water slides right off of. Meanwhile, matte paints are formulated to dry with very small bumps and textures to scatter the light and create a matte effect. This is also why gloss finishes highlight defects, while matte finishes hide defects.
This difference in how the paint mixtures and their respective surface tensions interact with each other when destabilized through movement forms these distinct patterns. They kind of resemble boulders, which is what Gina calls them. Boulder Cells. This method doesn't exclusively create boulder cells (the pouring medium you use will also impact their shape), but it's usually the effect that's sought after when using this method.
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