  Possibility of the diverging gradient of the macroscopic quantity near the
boundary is investigated by a mono-speed Lorentz-gas model, with a special
attention to the regularizing effect of the grazing collision for the
infinite-range potential on the velocity distribution function (VDF) and its
influence on the macroscopic quantity. By careful numerical analyses of the
steady one-dimensional boundary-value problem, it is confirmed that the grazing
collision suppresses the occurrence of a jump discontinuity of the VDF on the
boundary. However, as the price for that regularization, the collision integral
becomes no longer finite in the direction of the molecular velocity parallel to
the boundary. Consequently, the gradient of the macroscopic quantity diverges,
even stronger than the case of the finite-range potential. A conjecture about
the diverging rate in approaching the boundary is made as well for a wide range
of the infinite-range potentials, accompanied by the numerical evidence.
