Fish cells swim circles around physics lab:
"... above this critical density the keratocytes moved in coherent groups. When the cell density was increased even further, the effect of collisions with the walls of the square-shaped incubator caused the cells to move in a whirl-like structure."
"The researchers were able to explain this phase transition by creating a simple model of the interaction between two keratocytes that is based on three forces acting at three different distances. At very short separations a repulsive force causes the keratocytes to move apart. At intermediate cell separations an attractive force causes the keratocytes to move together. At distances greater than about one cell diameter the force was set to zero. When used to simulate the behaviour of moving cells, this combination of simple forces caused the onset of collective motion at a critical density. This is unlike previous models, which assumed that the cells could respond to the motion of their neighbours."