Basic Defoaming Principles and Mechanisms

What makes a bubble pop?

Ever wonder what makes that bubble in your process pop when you add the defoamer? How come one product works, but another doesn’t? Have you ever experienced differences in performance when the temperature changes? Here we hope to answer some of these questions for you! A basic understanding of how defoamers function on the surface of a bubble will be key to understanding why they may fail, why one is favored over another, and how that pesky foam can be best controlled in your process!

Foam should be thought of as a dispersion of gas in a liquid. If you were to blow bubbles in pure water, the bubbles would rise through the water and burst at the surface. The rise through pure liquid water is due to the low density of the air and the higher density of water. When it reaches the surface, there is nothing to stabilize the air bubble, so it bursts. If you add a surfactant to the water and blow a bubble through, you will see stabilization of the bubble as it gets to the surface and begin to layer more and more bubbles as the generation of air in the system increases.

A surfactant is a ‘surface active agent’. It is a molecule which has a head and tail, one being hydrophobic and one hydrophillic. When added to water and mixed with air, the surfactant molecule will orient itself so that hydrophillic end of the molecule is in the water and the hydrophobic end is in the air bubble. This orientation creates a ‘sandwich’ of water that is stabilized by the surfactant.

It is easy to notice the structure of the foam change as it stacks upon itself. Toward the bottom of the foam, there are spherical bubbles packed together, and as you rise upward it begins to take a polyhedral shape. This transition from spherical bubbles to a polyhedren occurs when the percentage of air to water is >72%. As the spherical bubbles stack on each other, gravity drains the liquid down through the bubble walls. Bubble walls are referred to as lamella. A plateau border is where multiple lamellas join together. As the drainage of water decreases the ratio of water to air in the spherical stacked foam, the structure adapts to remain stable.

Popping a bubble then involves destabalizing the surfactants which are holding the bubble together. So how can you do that? Introducing a fluid with a lower surface tension than the foaming medium creates an interesting effect. Liquids with relatively high surface tension will create contact angles that are quite high on a standard tabletop surface, like forming a droplet of water. Lower surface tension fluids will create a lower contact angle on the same surface, appearing flatter and more spread out. This spreading observation is conveniently named, “spreading”. Introducing an insoluble liquid of low surface tension into a liquid of high surface tension creates the same phenomena! An antifoam (of low surface tension) on the surface of the lamella (of high surface tension) will spread and wet the surface of the lamella. But how does the bubble pop?

There are 3 primary understood mechanisms of foam destruction: dewetting, stretching/bridging, and destabilization.

Dewetting occurs when a hydrophobic particle comes into contact with a lamella or plateau border. A particle that comes into contact with both sides of a lamella (forming a bridge), and is sufficiently hydrophobic, yields a contact angle >90*. In other words, the liquid in contact with the particle will bend inward until both sides of the film touch and rupture. A particle not sufficiently hydrophobic, or incompatible with the foaming medium, will form a stable bridge around the particle with a contact angle <90*.

Stretching and bridging is the most understood mechanism and it occurs when the antifoam droplet of low surface tension is able to stretch across the lamella and form an unstable bridge. A phenomena called the Marangoni effect states that fluids of high surface tension will pull fluids with low surface tension because the high surface tension has stronger inter-molecular forces on the surrounding fluids. The presence of a low surface tension antifoam droplet kick starts the Marangoni effect by pulling fluid in the antifoam droplet across the lamella. The lamella will drain as the hydrophobic defoamer forms its bridge. At the critical point, the lamella will break in two and release the air which was trapped inside. In a bubble without antifoam present, the Marangoni effect is a stabilizing property for the bubble. As the force of gravity drives the fluid from the top of the bubble to the base, the surfactant concentration at the top of the bubble decreases and the increased surface tension pulls fluid back up to the top of the lamella.

Destabilization occurs when the hydrophobic particle in the antifoam droplet on the surface of the lamella attracts the hydrophobic tail of the surfactant. Surfactant stabilizing the bubble are drawn to the hydrophobic particle and can adsorb to the particle, removing the key stabilizing property of the foaming solution and popping the bubble.

If the equation conditions regarding surface tension are met, and the antifoam is insoluble in the foaming medium, then it will act as a defoamer. Creating the right defoamer from the right base chemistry is important for all applications. For some ingredients, temperature can cause decomposition of the active materials. High or low pH environments can act as a catalyst for key ingredient degradation. Antifoam added as a raw material to a liquid product which must remain stable and without turbidity issues can pose challenges regarding ingredients and particle size. There is no one size fits all product. A silicone product from Supplier A may not work the same as one from Supplier B. There are many factors to take into account and they all must be understood before a qualified defoamer rep can make a recommendation.

Lucky for you, we have that expertise. Contact our sales staff and we can get you a product which will best suite your needs!