  We analytically and numerically investigated the internal optical forces
exerted by an electromagnetic wave inside an amorphous metamaterial medium. We
derived, by using the principle of virtual work, the Helmholtz stress tensor,
which takes into account the electrostiction effect. Several examples of
amorphous media are considered, and different electromagnetic stress tensors,
such as the Einstein-Laub tensor and Minkowski tensor, are also compared. It is
concluded that the Helmholtz stress tensor is the appropriate tensor for such
systems.
