Bubbles and Black Branes in Grand Canonical Ensemble
  When the phase structure of the black brane in grand canonical ensemble is discussed, a new bubble phase with the same boundary data is found to exist in this structure. As such, the phase transitions among bubble, black brane and "hot flat space" are possible, therefore giving a much enriched phase structure. We argue that under some conditions, either the grand canonical ensemble itself is unstable or there are some unknown new phases. 
Chao Wu, Zhiguang Xiao and Jianfei Xu
hep-th