INTRODUCTION

FLUIDS’ SURFACE TENSION

The surface tension of a fluid is the particles’ cohesive force that acts on its superficial area.

Thanks to the cohesive force, all particles in the inter-phase*** between the liquid and the air are linked together, creating an invisible film. This external cohesive force is precisely called surface tension.

***Interphase: boundary between two spatial regions occupied by different matter, or by matter in different physical states.

A PROPER MEDIUM

In general, in order to get the proper glaze application, it is always important to work on the fluid's surface tension.

To do that, a huge range of medium can act as levelling agents for glazes (wetting agents or surfactants) or defoamers, depending on their formula and chemical nature.
They are designed to solve problems regarding excessive foaming, issue caused by water repellence or improper levelling of the glaze.

THEY ALL OPERATE, DIRECTLY OR INDIRECTLY, ON THE SURFACE TENSION.

Nevertheless, it’s important to remember that also organic molecules in aqueous solution are generally able to decrease the water’s surface tension (such as some fluidifying for glazes that act also as wetting agents).

Let's see these medium by category.

WETTING AGENTS

From a chemical point of view, wetting agents are organic molecules (block copolymers, acids, modified natural fats, other) partially or slightly water-soluble.

This range of additives are able to act both in the interphase between the ceramic support and the glaze as well as between the glaze and the air: 
the levelling action is specifically provided by decreasing the surface tension in the glaze/air interphase.

As a final result, wetting agents facilitate the de-agglomeration of the solid parts in solution enhancing their homogeneous dispersion within the fluid, therefore providing the glaze with an excellent levelling application.

How do they do it?

The water surface tension's decrease helps the water penetration between the solid agglomerated particles in suspension that are holding air between one and the other. The high water surface tension, in fact, tends to avoid the water flow within the agglomerations, not allowing the particles to be hydrated and so wetted.
► IMAGE 1 

Being partially water-soluble, wetting agents give levelling/wetting properties to the glaze without causing  foam formation.

On the other side, the low solubility features of the aqueous system can sometimes lead to problems such as dimples or pinholes ( → the medium's oily micro drops leans on the ceramic support replacing the water).

SURFACTANTS & DEFOAMERS

When talking about surfactants, we generally refer to water-soluble products with high-performance properties that however can produce foaming side effect.

They are, by definition, products able to decrease the water surface tension thanks to their specific molecular structure: 
Hydrophilic head + Hydrophobic tail.
► IMAGE 2

This is what happens on the fluid's surface when suerfactants are added to a solution: 
while the tail remains out of water because of its hydrophic properties, the head in the solution separates the water's molecules.
This sort of action decreases the water surface tension leading to a proper glaze application.
► IMAGE 3

Being completely water-soluble, surfactants do not usually produce pinholes or dimples.
On the other side, as we already said, they are highly foaming and difficult to control: this is the reason why they are not often involved in ceramic production processes and they are preferably replaced with wetting agents that can be more easily managed according to the production needs.
Nearly all glazes and ceramic suspensions are, in fact, constantly stirred with systems that tent to incorporate air within the fluid, therefore leading to foam.

How surfactants produce foam phenomena?
What is the structure of a bubble of foam?

Foam bubbles generated by surfactants are nothing more than spherical films of water molecules held together by the surfactant’s action. In ceramic, when the aqueous glaze suspension is stirred, the air starts moving within the glaze and bubbles rise to the surface.
► IMAGE 4

The use of defoamers can work against the surfactants action, partially or totally solving the problem.
In any case the final result is very delicate and unstable:
while defoamers fight against surfactant's side effect it is important, in fact, to simultaneously control the possible defect deriving from and excessive use of defoamers (which are water insoluble) in order to avoid pinholes or dimples.
Like a dog chasing its tail!

This scenario does not mean that defoamers and surfactants cannot be used in combination.
On the contrary, they are often applied together but they must be carefully managed, reaching a perfect balance and so creating tailor-made solutions.

How to eliminate foaming phenomena?

All defoamers, unlike water, are non-polar substances and so they are water insoluble.
They are able to break the bubble’s stability, acting on the surfactant’s molecules and breaking the their tails. This is the way that all bubbles disappear.
Their intervenction also destibilizes all that organic molecules that, while not being surfactants, are able to promote foaming phenomena.

It is finally important to underline that some defoamers can be more or less effective according to the kind of foam, whose origin is sometimes difficult to detect.