Evolution of the $N=28$ shell closure: a test bench for nuclear forces

40 pages,15 figures, Procceedings Nobel Symposium 2012, accepted for publication in Physica ScriptaInternational audienceThe evolution of the $N=28$ shell closure is investigated far from stability. Using the latest results obtained from various experimental techniques, we discuss the main properties of the $N=28$ isotones, as well as those of the $N=27$ and $N=29$ isotones. Experimental results are confronted to various theoretical predictions. These studies pinpoint the effects of several terms of the nucleon-nucleon interaction, such as the central, the spin-orbit, the tensor and the three-body force components, to account for the modification of the $N=28$ shell gap and spin-orbit splittings. Analogies between the evolution of the $N=28$ shell closure and other magic numbers originating from the spin-orbit interaction are proposed ($N=14,50, 82$ and $90$). More generally, questions related to the evolution of nuclear forces towards the drip-line, in bubble nuclei, and for nuclei involved in the r-process nucleosynthesis are proposed and discussed