Today we’re addressing a topic that, as you know, is very close to my heart. It’s actually quite simple but often counterintuitive. Today, fewer mathematical formulas and a more streamlined session, but one that I hope will clarify one of the most important phenomena in our field: capillarity.

Capillarity, which primarily influences how we perform “wicking”—that is, the wicking or application of mesh—but also the behavior of complex and supercomplex coils, as well as the functioning of our beloved RDTA and Genesis atomizers.

As you may have realized, the goal of this Vape Tech segment is to eliminate all discretionary, personal, and debatable factors to provide absolute guidelines that allow people to make informed choices.

We have seen how the different parameters of a coil or atomizer contribute to a better flavor experience, bringing everything back to a single common denominator: temperature control. From this, we derive a good or bad flavor response, and capillarity, besides supplying our coils with liquid, contributes significantly, second only to airflow in moderating the temperature of the system.

It is known to everyone how water does not exceed 100°C when it boils. This is a physical phenomenon known as the boiling point: when our liquid is heated by the coil, the molecules gain kinetic energy, and at the boiling temperature, most of the molecules have enough energy to overcome cohesive forces (attraction between molecules) and evaporate. The energy supplied to the system is used to break intermolecular bonds rather than increase temperature. This is known as latent heat of vaporization.

It is interesting to note that our liquid is a mixture of various molecules, and its “flavor” component is not negligible in determining the boiling point, also because, generally, the boiling temperature of a mixture is not simply the arithmetic mean of the boiling temperatures of the components but depends on the proportions and chemical characteristics of each component. However, for our purposes and to avoid being too lengthy, we can estimate this boiling temperature of a 50/50 mixture without flavors to be around 200°C.

Before you ask: Yes, during the boiling of a mixture of propylene glycol and glycerin, separation of the two components can occur below 200°C. Propylene glycol will evaporate first, followed by glycerin at higher temperatures. However, for several reasons, which we won’t discuss, primarily the application of higher temperatures, this phenomenon is negligible.

Why this introduction if the focus is on capillarity? Because if, even for very brief fractions of a second or on some surface portions, our coil is not constantly moderated in temperature by the liquid, thermal equilibrium is immediately broken, exceeding the desired temperatures and compromising our beloved temperature control!

Capillarity is the physical phenomenon by which a liquid rises or falls within small spaces, without the influence of significant external forces such as gravity. This phenomenon is the result of interactions between cohesive forces (attraction between the liquid molecules) and adhesive forces (to the capillary walls).

If we see the formula that describes capillarity:

h = 2γ cos θ / ρgr

we can quickly discern what favors it and what hinders it:

Factors that favor it:

• γ (gamma) is the surface tension of the liquid (NB: glycerin has a higher γ than H2O).
• θ (theta) is the contact angle between the liquid and the capillary surface. When θ is 0°, its cosine takes on the maximum value (1), meaning the liquid adheres perfectly to the capillary walls, maximizing capillary rise due to maximum adhesion force efficiency compared to cohesion forces.
• ρ (rho) is the density of the liquid (not viscosity! Glycerin has a higher density).
• g is the acceleration due to gravity (if it has to rise, e.g., RDTA).
• r is the radius of the capillary.

Two points I would like to focus on: first, the issue of density. It is easy to be deceived by the viscosity of our liquid and assume it affects capillarity, which is not the case. As we see from the formula, viscosity has no effect on capillarity. Instead, density does—it simply weighs more! This phenomenon is simultaneously countered by higher surface tension, which is why the two liquids may exhibit similar capillary behavior. Obviously, the longer the vertical path, the harder it is for the liquid to rise, because at a certain point, the weight of the liquid (i.e., “g”) will be in equilibrium with γ, the surface tension.

Let’s get to the point, shall we?

This blessed formula, once and for all, tells us that the larger the capillary radius, the lower the height the liquid can reach! In other words, capillarity favors small channels over large ones:

How does this translate to our world?

For example, an RDTA works better if our “mesh” or wicks are made of very thin wires. A wrap or core of a complex coil made of very thin wires or that generates very narrow spaces (e.g., stacked ribbon) conducts liquid much more quickly (hence why they are generally more flavorful, even if the total capillary volume may be less than coils made of thicker wires). A very dense wicking cotton conducts our liquid BETTER!

However, a fundamental factor must be added to these analyses, which does not directly concern capillarity but drastically affects the performance of our atomizers and can easily negate all good practices: When liquid moves from the atomizer to the cotton, air must simultaneously enter the tank; otherwise, the vacuum inside would prevent further liquid from flowing out! This implies that while a good density is essential inside the coil, it is equally essential to leave some “breathing” space in the wells from which the liquid is drawn or in the drippers. In other words, completely clogging the point where the liquid flows would completely block the flow of liquid, far more severely than all the advantages obtained with good wicking!