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Wednesday
Jun092010

The "Master of Disaster"

 “Bubble-logy”

      Understanding the tools used is the foundation from which the FLBSS builds his success. Mastering the foundation of how the Foam bubble behaves and what it will do, gives you the knowledge to anticipate every phase of a flammable liquid tank fire. Foam by chemical definition can be thought of as “limited solubility micellization”. Soap, like Foam, is a surfactant that is water miscible {soluble} when mixed in solution. Soaps, {surfactants} break down the surface tension of water to allow the removal of materials from any object that you want to decontaminate and clean. Its uses are limitless, and many other compounds can be added to the surfactant to enhance special contaminant removal needs. However, in the case of flammable liquid and the “Foams’” relationship, the reduction in surface tension may not exceed a certain point. It must have only limited solubility, or the Foam bubble will mix with the water and not form. If this happens, you may reduce surface tension but it will be at the expense of Foam bubble formation, vapor suppression, and no fire extinguishment. So, the most important measured property of firefighting surfactants in solution is “limited reduction of the interstitial interface tension” between the fuel surface and the Foam blanket.

     The schematic drawing demonstrates all the various phases of surface tension reduction. You can see that the area of concentration on water molecules is higher in the center than on the exterior edges. As they are packed closer together near the flattened surface of the “bubble”, this is the area of greater concentration and greater surface tension resistance. The function of this reaction depends on the replacement of fuel molecules by the Foam at the regions of lower surface tension {on the edges}. This reaction runs toward the center in increasing order, where the surface tension is at its greatest resistance. As the region of lower surface tension is released and begins to “free” its energy, the reaction works its way from both ends towards the bubbles center. When the surfactant adsorption is as close to complete as natural forces will allow, film drainage begins. This action allows the Foams flowing characteristic across the surface of the fuel. The accomplished result is vapor suppression and fire extinguishment, from cooling and smothering. Herein lays the challenge, creation of Foam that flows well across both polar and non-polar fuels, resists breakdown, yet still creates surface tension reduction.

     As the fuels surface constantly replenishes itself with fresh high tensioned molecules from underneath its’ surface, viscous drag is created by these newer untouched molecules rising to the surface interfacing between the Foam and the fuels body. This in turn replenishes the fuels characteristics of flammability and high surface tension. As this reaction continues to balance itself in normality, the Foam bubbles begin to ¼ drain time away, allowing replenishment of the system from the original higher surface tension fuel. This is why the Foam blanket frequently requires replenishment to maintain low surface tension, vapor suppression, smothering, cooling properties, and fire extinguishment. The greater the similarity of molecular characteristics, the greater the interactions, the greater absorption, resulting in lower surface tension. Now the problem becomes, create good surface tension reduction, with low absorption, in a wide variety of percentage polarity systems!

     For non-polar solvents that frequently exhibit high miscibility with water, the concept remains the same but slightly from the other side. In polar fuels our reactions all occurred at the surface between two opposing interstitial interfaces. For non-polar solvents we accomplish this ongoing molecular exchange through the process of Micellization.  Micelle formation is primarily a relationship most often seen in detergents. Micelle formation is the process of colloidal clusters forming “in solution”. This is the key to understanding because forming these clusters in solution reduces the Foams’ ability to remain on the fuels surface as these clusters are formed “under” the surface, not on or between the interfaces, with somewhat reasonable absorption capabilities. When hydrophobic materials dissolve in water they distort the structure of water by positioning themselves towards the “interior” of the cluster, while also creating absorption on the opposite side of the cluster in contact with the interface. This action, “from the other side” decreases the free energy of the system, and decreases surface tension. These are seen in short chain groups which aid in low micelle formulation resulting in surface tension reduction such as in the cases of MTBE or non-polar fuels. However, this function {underneath the surface} increases fuel absorption resulting in shorter lasting Foam blankets and larger volumes of Foam concentrate needed.  

 

     The final result of surface tension reduction for firefighting and Vapor suppressant Foam becomes a delicate balance of appreciable, but limited solubility in overall systems under the conditions to be used.

                                                                                                                                       Haz Mat Mike              



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