An interface obtained by collision of two independent rough surfaces: invasion by burning of paper

The growing surface at a fire front has the property of self-affinity. We start fires from two opposite edges of rectangular paper and obtain two independent rought surfaces with roughness exponent α = 0.73 and growth exponent β′ = 0.47. The two independent fronts later collide in the central region of the paper to form an interface with roughness exponent α′ = 0.67 and growth exponent β′ = 0.22.