Particle mixing in the islands. Each panel is a zoom around a magnetic island and shows particles initially outside of the current sheet in green and particles initially located inside the current sheet in red and shifted below for clarity. Each snapshot is taken at the end of the simulations, and these islands result from the merging of many small and then larger islands (from around 20 islands at the end of the linear phase of the tearing instability, to 3 big islands at the end), and yet the two particle populations remain separated. The mass ratio is 25 in all cases.