Fire Ants as Active Matter

Caleb Anderson

Active matter systems are a class of systems that share striking universal features over broad range of length-scales. The field was originally founded to understand the behavior of biological crowds, such as flocks of birds, but as theory work has become more precise, many of its predictions have been primarily verified in synthetic systems, in which particles interact with each other in well-defined ways. We seek to use fire ants to return the field to its roots and see if the some of the most famous results in synthetic systems can be applied to the ants despite their extremely complicated, in fact social, interactions.

Ants can attract each other and form clusters in a manner called Motility-Induced Phase Separation (MIPS) because their social interactions cause them to slow down when they approach each other, like particles in several synthetic systems.

However, at very high densities, ants undergo “activity cycles”, in which long periods of relative low activity are punctuated by short bursts during which all of the ants attempt to move, regardless of the density. During these bursts, the ants don’t slow down when they approach each other, so they don’t undergo MIPS. Instead they, leading to another behavior famous in active matter systems – collective motion. In systems of tens of thousands of ants, this combination of varying contagious activity levels and the resulting collective motion leads to beautiful “activity waves”, that propagate towards the free surface!