When an improvised explosive device is found lying on the ground, there are several ways to neutralize it. Perhaps the most effective method is to pop a tent over the explosive, then pump the tent full of an aqueous foam. When the explosive is subsequently exploded, the foam will take all of the hit. The shock wave will reduce the foam to nothing, and not go beyond the tent.
How does foam manage to suck up so much energy? Is it possible to make an even superior foam that could handle greater blasts? These are the questions that Ashwin Chinnayya, a professor at the Institut Pprime in Futuroscope, France, has been trying to answer. In his paper for Shock Waves this year, “Macro-Mechanical Modeling of Blast Wave Mitigation in Foams,” he’s gone a long way toward answering them.
An aqueous foam, as Chinnayya explains, is a two-phase cellular system. Very thin liquid films surround gaseous cells. The liquid is interconnected, or continuous, separating the gas into discrete cells. “The bubbles take the form of polyhedral cells with liquid surfaces meeting in lines and lines merging at vertices.” These lines are called, after their discoverer, Joseph Antoine Ferdinand Plateau, Plateau Borders. There is more liquid in these lines at the edge of a bubble’s shape than there is in the thin walls between bubbles.
Bubbles in a foam of soapy water obey Plateau’s laws. Image: Wikimedia Commons
Any force applied to the foam will make Plateau Borders, cell walls, size and shape all go dynamic. “As liquid volume fraction increases, Plateau Borders swell and bubbles progressively recover their spherical shapes and the foam evolves into a wet foam,” says Chinnayya. As the liquid increases, the bubbles come apart and the foam becomes a mere bubbly liquid.
But when a shock from an explosion hits the foam, the films between cells are immediately broken. Those films are what ensure the stability of each cell, and the stability of the foam. Once the films are broken, the Plateau Borders lose their stability as well. “The aqueous foam then becomes a spray of ligaments, flowing and deforming under gas dynamics,” says Chinnayya. “The two-phase mixture becomes a relaxing media, where multiphase momentum and energy exchanges take place between the gaseous and the liquid phases.”
Just how the foam unfolds—or unfoams—under pressure is one of the chief difficulties of modeling it. But both modeling and experiments blasting various foams have explained much of the how foam subdues a blast.
An explosion’s initial shock wave is followed by a rarefaction waves, waves of low pressure. Since the “speed of sound decreases as the porosity (volume fraction of gas) decreases,” the rarefaction waves catch up to the blast wave, cutting its intensity. “This is one of the main reasons why blasts are mitigated by the presence of foams,” says Chinnayya. Other mitigating factors, as it were, are energy exchanges, drag, and heat transfer. Chinnayya and his fellow foam investigators once thought evaporation may have contributed to lessening the force of an explosion as well. But models show that the expanding fireball quickly lowers the temperature of the foam, effectively eliminating the possibility of evaporation. In certain situations foam can help a blast more than hinder it. “We have to be very cautious about the mitigation process. In some cases, for instance, the foam will create overpressures higher than in air,” says Chinnayya. This is the case when we are not far enough from the charge, and when a wall is too near from the charge also.”
Now, thanks to developments in the field of surfactants, which lower surface tension, foams can bubble up more quickly, more stably, and more homogenous. These factors help increase the speed with which a foam is deployed and the effectiveness of a foam surrounding an explosive, to be sure, but they also help make the modeling of foam more accurate. Chinnayya has determined that the ideal expansion ratio (the volume of the foam before foaming compared to the size of the foam once foamed) is 60. That ratio is easily dialed in on a model, but trying to create bubble cells of a specific size in the real world is another matter.
The models also show that for maximum mitigation, bubbles and their barriers must be allowed just the right amount of freedom. “If both phases are not able to have their own motion, the mitigation is lessened,” says Chinnayya. “But if they behave independently, no interphase exchange will occur and the mitigation will also be lessened.”
With a better bubble of the right size, allowed the right amount of freedom, Chinnayya may make it easier to disarm stray IEDs. Now, if he would just turn his mind to shaving cream, perhaps we could prevent widespread injuries of another kind.
Michael Abrams on September 2013