Experimental Test of the Border-Crossing Model of Diffusive Coarsening in Wet Foams

For dry foams, the transport of gas from small high-pressure bubbles to large low-pressure bubbles is dominated by diffusion across the thin soap films separating neighboring bubbles. For wetter foams, the film areas become smaller as the Plateau borders and vertices inflate with liquid. So-called ``border-blocking" models can explain some features of wet-foam coarsening based on the presumption that the inflated borders totally block the gas flux; however, this approximation dramatically fails in the wet/unjamming limit where the bubbles become close-packed spheres. Recently we accounted for the ever-present border-crossing flux by a new length scale defined by the average gradient of gas concentration inside the borders that is proportional to the geometric average of film and border thicknesses. We also showed how the well-known dA/dt=K(n−6) von Neumann law is modified by the appearance of terms that depend on bubble size and bubble shape as well as the concentration gradient length scale. Here we extend this theory and describe on-going experiments to test it by measurement of area changes for six-sided bubbles, which would neither grow nor shrink if perfectly dry. Indeed we find six-sided bubbles that change area with time, and we find that they do so in good accord with our prediction based on their size and shape and degree of wetness.