On volume and surface area of parallel sets. II. Surface measures and (non)differentiability of the volume

We prove that at differentiability points $r_0>0$ of the volume function of a compact set $A\subset R^d$ (associating to $r$ the volume of the $r$-parallel set of $A$), the surface area measures of $r$-parallel sets of $A$ converge weakly to the surface area measure of the $r_0$-parallel set as $r\to r_0$. We further study the question which sets of parallel radii can occur as sets of nondifferentiability points of the volume function of some compact set. We provide a full characterization for dimensions $d=1$ and 2.