What can prevent good skimming?

[SeanT]

Hi Randy,

I have a very vague, and possibly broad answered, question and I apologize.
Could you list some of the factors that could prevent a skimmer from getting a good frothy head of foam please?

I know of some of the simple; such as a low salinity.
And I know of the impossible; such as nothing to skim. ;)

But what other causes, temperature, pollutants, gasses (CO2 etc), that you can think of that may possibly effect a skimmer, which is 'bubbling' quite well but doesn't get a head on it to occur.

Thank you,
Sean

[Randy Holmes-Farley]

Hydrophobic immiscible liquids (oils) and solid particulates can lead to bubble popping and poor skimming. That's how antigas meds for people work. The article below lists some other aspects that impact skimming (like bubble residence times, draining, etc).

Here's a section on bubble popping from one of my articles:

Protein Skimming: How It Works
http://web.archive.org/web/20030415033358/http://home.mweb.co.za/jv/jv79/reef/skimmers2.html


"Bubble Popping

Other critical things can happen at this stage, and they usually impact skimming negatively. One is the addition of things that cause bubbles to pop prematurely. That is, things that cause bubbles to pop before they have drained and can form particulate organics or be removed. Oils, for example, cause this to happen. When oil droplets are added to a tank, they quickly arrive at the skimmer. An oil droplet is hydrophobic on all sides. Oil drops work their devilish tricks by spanning across the water between two air bubbles in a foam. Once an oil droplet completely spans the water, it causes an instability. Bubbles are a remarkable balancing act between surface tension and the "hydrophobic effect". All molecules have some attraction for each other, but water molecules form an especially strong interaction (hydrogen bonds). Water molecules at the air/water interface have nothing above them (only air) and are thus only able to hydrogen bond to things below them. Since they cannot form such good interactions, they are less "happy". Surface tension is thus the effect of all of these hydrogen bonds pulling at the water molecules on the surface. The net effect is that the water minimizes its surface area. Surface tension is why water drops are nearly always spherical: a sphere has the lowest surface area for a given water volume. So what does this have to do with a foam? A foam has a very high surface area, and the surface tension of the water is always trying to reduce the surface area. Popping of bubbles is one way to quickly reduce the surface area. This is the reason that bubbles formed in pure water pop almost instantly. Try it with tap water: your water will probably not be able to support bubbles for more than a second or two.
In water with organics, the organics greatly reduce the surface tension, and thus the tendency for popping, but do not eliminate it completely. [Note that a low surface tension is not the only requirement for stable bubbles. Pure organic solvents also do not form stable bubbles, even though they have very low surface tension. That's another story. In water with organics, the tendency for the organics to want to come out at the air/water interface opposes the surface tension, and bubbles become a balancing act between surface tension that wants to pop or otherwise decrease bubbles, and the spreading pressure of organics that want to spread out across the air/water interface. So back to the oil drops. Once an oil droplet spans the water gap between bubbles, its all over. What happens is that the amphipathic molecules on both of the bubble surfaces spread along the interface between the oil and the water (if they were not there already) and connect both of the air gaps with a continuous line of amphipathic molecules along this oil/water interface. Once these amphipathic molecules are in place, the system is unstable. The surface tension pulls at the oil drop, and it simply comes apart. The bubble ruptures from the site of the oil drop, and the effect is that the bubble pops. The reason that this does not happen in the absence of an oil drop is that to cause a rupture requires the water present between the air bubbles to become exposed as fresh air/water interface. In fact, it requires a continuous line of water molecules to become exposed all at once. Because that would require a large number of hydrogen bonds to be broken simultaneously, it simply requires too much energy to actually take place. When the oil drop is there, one is no longer exposing water molecules, but rather oil or amphipathic molecules, which are much "happier" to be exposed to air. In any case, if the oil drop explanation makes sense to you, great. If not, don't worry, as it is a very subtle and complicated concept.

Bubble Popping in Marine Tanks

In a real marine tank, many things have this bubble popping effect. One that most aquarists encounter is oil from their hands. On reaching into a tank, skimming action often comes to a near halt as bubble popping dominates over foam draining and collection. The popping will proceed until the oil is somehow removed. Among other ways, oil can be removed by splattering it above the foam height in the skimmer, being foamed out bit by bit, being emulsified into the general foam as very, very tiny droplets which no longer span air bubbles, becoming attached to solid objects and removed, being consumed by tank microorganisms, and by eventually dissolving into the bulk tank water. As an aside, the bubble popping action of hydrophobic oils is exactly how most antigas medications for humans function. Simethicone is really polydimethylsiloxane, which is a hydrophobic polymer liquid. It pops bubbles in your stomach or intestine, and permits the gas to be eliminated. Antifoaming agents are also the basis of a large number of industrial products that work by the same principle. Other things also cause bubble popping. One of these is the fatty acid supplement Selcon. It causes bubble popping in the same fashion as skin oil droplets. Hydrophobic solid objects can also cause popping. Sand coated with organics, inorganic precipitates from salt mixes covered with organics, food particles, etc. all function in a similar fashion. They cause bubble popping just like hydrophobic oils, except that they are solid. "