1) We have focused on several promising approaches to the control of flames: one, based on electric fields; the other, on powerful acoustic perturbations. The first approach relies on the fact that hydrocarbon flames are actually chemically driven, non-equilibrium plasmas. As such, they contain large concentrations of charged species (typically, ~1011 charges per cubic centimeter) that respond collectively to externally applied electric fields. Importantly, the movement of ions and electrons in the field can transfer momentum to the surrounding gas though frequent collisions with neutral species. For sufficiently large fields, this process can result in macroscopic gas flows – so-called ionic or electric wind – with speeds of up to ten meters per second. When placed in the proximity of a flame, the resulting gas flows act in a highly directional manner to rapidly displace the combustion zone from the fuel source. The second approach provides oscillatory perturbations to the flame with acoustic waves. We explore the required acoustic conditions necessary for extinction. Our goal is to understand the fundamental physics and potential applications of using sound to manipulate combustive processes.
2) We have constructed a model system to examine the eruption of small flames into intense, rapidly moving flames stabilized by feedback between wind and fire (i.e. “wind-fire coupling”—a mechanism of feedback particularly relevant to forest fires) (Ref 2). Using this model, we showed that slowly spreading flames can exhibit detectable symptoms of critical slowing down (i.e. the slowed recovery of multistable systems from perturbations as those systems approach tipping points) prior to such eruptions. This finding, which marks the first demonstration of critical slowing down in a combustion system of any kind, suggests that slowing responses of spreading flames to sudden changes in environment (e.g. wind, terrain, temperature) may anticipate the onset of intense, feedback-stabilized modes of propagation (e.g. “blowup” events in forest fires).
3) We are currently working on examining the role of spatial organization—and, more generally, the function of spatial heterogeneity—in guiding the propagation of signals through complex networks. It addresses the question: “how do spatial non-uniformities in a system of reactions operating out of equilibrium (e.g. a forest fire, the biological cell) dampen, amplify, or modify the signals to which spatially-localized reactions (or interactions) give rise?” This question is particularly relevant to forest fires—where fire propagates through varied, non-uniformly distributed fuels—and generally relevant to spatially biased networks (i.e. networks where elements, as a result of their spatial arrangements, have different probabilities of engaging in pairwise interactions).
|Figure 1. Experimental setup and transition between unstructured and structured flames|
1. Drews AM, Cademartiri L, Whitesides GM, Bishop KJM (2013), Electric winds driven by time oscillating corona discharges, J Appl Phys 114(14).
2. Fox J, Whitesides G (2015), Warning signals for eruptive events in spreading fires, Proc Natl Acad Sci 112(8):2378.