  It is known that internal energy flow in a light beam can be divided into the
orbital flow, associated with the macroscopic energy redistribution within the
beam, and the spin flow originating from instantaneous rotation of the field
vectors inherent in circular or elliptic polarization. In contrast to the
orbital one, experimental observation of the spin flow constituent seemed
problematic because (i) it does not manifest itself in the visible
transformation of the beam profile and (ii) it converts into the orbital flow
upon tight focusing of the beam, usually employed for the energy flow detection
by the mechanical action on probe particles. We propose a two-beam interference
technique that permits to obtain appreciable level of the spin flow in
moderately focused beams and to detect the orbital motion of probe particles
within a field where the transverse energy circulation is associated
exclusively with the spin flow. This result can be treated as the first
demonstration of mechanical action of the spin flow of a light field.
