  In high energy heavy ion collisions and interacting cold atom systems, large
elliptic flow anisotropies have been observed. For the large opacity
($\rho\sigma L\sim 10^{3}$) of the latter hydrodynamics is a natural
consequence, but for the small opacity ($\rho\sigma L\sim 1$) of the former
hydrodynamic description is questionable. To shed light onto the situation, we
simulate the expansion of a low density Argon ion (or atom) system, initially
trapped in an elliptical region, under the Coulomb interaction (or elastic
scattering). Significant elliptic anisotropy is found in both cases, and the
anisotropy depends on the initial spatial eccentricity and the density of the
system. The results may provide insights into the physics of anisotropic flow
in high energy heavy ion collisions and its role in the study of quantum
chromodynamics.
