  We report on the first experimental study of quadrupole collectivity in the
very neutron-rich nuclei \nuc{47,48}{Ar} using intermediate-energy Coulomb
excitation. These nuclei are located along the path from doubly-magic Ca to
collective S and Si isotopes, a critical region of shell evolution and
structural change. The deduced $B(E2)$ transition strengths are confronted with
large-scale shell-model calculations in the $sdpf$ shell using the
state-of-the-art SDPF-U and EPQQM effective interactions. The comparison
between experiment and theory indicates that a shell-model description of Ar
isotopes around N=28 remains a challenge.
