  The work reported below is a first of its kind study of the properties of
turbulent flow without strong mean shear in a Newtonian fluid in proximity of
the turbulent/non-turbulent interface, with emphasis on the small scale
aspects. The main tools used are a three-dimensional particle tracking system
(3D-PTV) allowing to measure and follow in a Lagrangian manner the field of
velocity derivatives and direct numerical simulations (DNS). The comparison of
flow properties in the turbulent (A), intermediate (B) and non-turbulent (C)
regions in the proximity of the interface allows for direct observation of the
key physical processes underlying the entrainment phenomenon. The differences
between small scale strain and enstrophy are striking and point to the definite
scenario of turbulent entrainment via the viscous forces originating in strain.
