collisionlessly
Collisionlessly is an adverb describing actions or processes that occur without, or with negligible, particle-particle collisions. In physics, many systems are effectively collisionless when the mean free path of particles is large compared with the characteristic length scales, or when the collision frequency is much smaller than other dynamical frequencies. In such regimes the evolution is governed by collisionless dynamics, often modeled by the collisionless Boltzmann equation (the Vlasov equation in plasma contexts), in which binary collisions are neglected and collective fields drive behavior.
In plasma physics, collisionless behavior means electromagnetic forces and collective effects dominate over binary Coulomb collisions.
In astrophysics and gravitational dynamics, many systems are effectively collisionless on astronomical timescales, including dark matter
The term also appears in fluid and rarefied-gas contexts; when the Knudsen number is large, flows may
Collisionless regimes contrast with collisional regimes, where frequent particle interactions govern transport, relaxation, and thermalization. In