Scaling behavior of universal pinch-off in two-dimensional foam.

We study the power-law scaling behavior and pinch-off morphology of two-dimensional bubble rafts under tension. As a function of pulling speed, we observe two distinct pinch-off morphologies that have been observed in other fluid systems: long threads (LT) and double-cone (DC). At any given pulling speed, there is a nonzero probability of observing LT or DC, with the probability of observing LT modes increasing with pulling velocity. The bubble rafts are composed of millimeter scale bubbles, and we are able to directly observe pinch-off to the point of final separation and measure the scaling of the minimum width in time. For both the LT and DC modes, the final scaling regime before pinch-off exhibits a universal power-law scaling behavior, with power-law fitting exponents of 0.73 ± 0.01. However, the final cone angle is different for states that initially exhibit LT or DC pinch-off, and for the LT case, the final scaling is best described as a local double-cone mode.