A `liquid-solid' phase transition in a simple model for swarming, based   on the `no flat-spots' theorem for subharmonic functions
  We consider a non-local shape optimization problem, which is motivated by a simple model for swarming and other self-assembly/aggregation models, and prove the existence of different phases. A technical key ingredient, which we establish, is that a strictly subharmonic function cannot be constant on a set of positive measure. 
Rupert L. Frank, Elliott H. Lieb
math.AP math-ph math.MP