  An experimental study of long interfacial gravity waves was conducted in a
closed wave tank containing two layers of viscous immiscible fluids. The study
focuses on the development in time of the mean particle drift that occurs close
to the interface where the two fluids meet. From a theoretical analysis by
Weber & Christensen (Eur. J. Mech.-B/Fluids, vol. 77, 2019, pp. 162-170) it is
predicted that the growing drift in this region is associated with the action
of the virtual wave stress. This effect has not been explored experimentally
before. Interfacial waves of different amplitudes and frequencies were produced
by a D-shaped paddle. Particle tracking velocimetry (PTV) was used to track the
time development of the Lagrangian mean drift. The finite geometry of the wave
tank causes a mean return flow that is resolved by mass transport
considerations. The measurements clearly demonstrate the importance of the
virtual wave stress mechanism in generating wave drift currents near the
interface.
