Foam starts as a turbulent mixture of gas and liquid.  Exactly the kind of environment we want in trayed gas/liquid contacting towers for heat and mass transfer.  This kind of foam is called froth.  The froth created in the tower lasts long enough for the transfer to take place, but breaks before being carried into an adjacent tray (above or below).  Froth breaks when the gas and liquid disengage.  Simply put, the bubbles break. When the froth fails to disengage fast enough, an hydraulic imbalance forms. Both gas and liquid flow through the column are restricted.  These are the first classic symptoms of tower foaming.  These symptoms can also be caused by changes in gas or liquid traffic through the column, fluid viscosity, or mechanical restriction due to fouled internals.  It does not mean that the solution is more foamy.  The solution's foaming properties may not be seen to change despite the column showing foaming symptoms.  These symptoms will also react to antifoam injections. (see Antifoam). Solution contamination can inhibit gas/liquid disengagement.  The most common contaminants seen in amine systems that inhibit froth breaking include solid particles, and soluble surfactants (detergents).  Suspended solids do not actually cause foaming, but rather amplify the solution's tendency to foam.  They affect the drainage of liquid from the bubble wall by affecting local fluid viscosity and pooling.  The bubbles remain wet and flexible, therefore resist breaking. Surfactants (surface active agents) stabilize froth to create foam.  They are soluble in aqueous solutions like amines, and are not filterable.  Activated carbon does remove some foam causing surfactants by adsorption, but unreliably. (see Activated Carbon).  A description of how surfactants cause foaming is given below.

Normal Froth

Surfactant molecules love water on one end, and hate it on the other.

Foam

They adsorb to interfaces like gas or oil bubbles so that their water loving ends face the aqueous side of the interface, and their non-polar ends in the non-aqueous side.  This is really where foam starts to form.

An interfacial coating of adsorbed surfactant molecules inhibits gas to liquid mass transfer, and stabilizes the gas/liquid interface to form foam.

The gas bubbles carry the surfactants reinforcing the interface out of the liquid as it is displaced through the bulk liquid surface.

The surfactants incorporated in the foam structure stabilize it by increasing its flexibility, and inhibiting the bulk liquid drainage. Liquid drainage takes place through the cell walls (lamellae), like tiny pipes.

The structure of young, wet foam has round cells.  The cells begin to take on a more polygon shape as they drain and become more rigid.

There is always some free liquid held in the foam structure due to capillary action between the cells, and the hydrophilic nature of the surfactants in the cell wall.