  A thring is a recent addition to the zoo of spiral wave phenomena found in
excitable media and consists of a scroll ring that is threaded by a pair of
counter-rotating scroll waves. This arrangement behaves like a particle that
swims through the medium. Here, we present the first results on the dynamics,
interaction and collective behaviour of several thrings via numerical
simulation of the reaction-diffusion equations that model thrings created in
chemical experiments. We reveal an attraction between two thrings that leads to
a stable bound pair that thwarts their individual locomotion. Furthermore, such
a pair emits waves at a higher frequency than a single thring, which protects
the pair from the advances of any other thring and rules out the formation of a
triplet bound state. As a result, the long-term evolution of a colony of
thrings ultimately yields an unusual frozen nonequilibrium state consisting of
a collection of pairs accompanied by isolated thrings that are inhibited from
further motion by the waves emanating from the pairs.
