We consider a ϕ-rigidity property for divergence-free vector fields in the Euclidean n-space, where ϕ(t) is a non-negative convex function vanishing only at t = 0. We show that this property is always satisfied in dimension n = 2, while in higher dimension it requires some further restriction on ϕ. In particular, we exhibit counterexamples to quadratic rigidity (i.e., when ϕ(t) = ct 2 ) in dimension n ≥ 4. The validity of the quadratic rigidity, which we prove in dimension n = 2, implies the existence of the trace of a divergence-measure vector field ξ on a H 1 -rectifiable set S, as soon as its weak normal trace [ξ · ν S ] is maximal on S. As an application, we deduce that the graph of an extremal solution to the prescribed mean curvature equation in a weakly-regular domain becomes vertical near the boundary in a pointwise sense.2010 Mathematics Subject Classification. Primary: 26B20, 28A75. Secondary: 35L65.Key words and phrases. divergence-measure vector field; weak normal trace; rigidity. G.P. Leonardi and G. Saracco have been partially supported by the GNAMPA project: Variational problems and geometric measure theory in metric spaces (2016).