There is a growing interest in using unbaffled stirred tanks for addressing a number of processing needs such as low shear damage (sensitive biocultures), low attrition (solid-liquid applications), deep-cleaning/sterilization (pharmaceutical applications). The main feature of uncovered, unbaffled stirred tanks is highly swirling motion of the fluid that results in a deformation of the free liquid surface. At sufficiently high agitation speeds the resulting whirlpool reaches the impeller and gives rise to a gas-liquid dispersion, so leading to the formation of a dispersion without the use of gas-sparger; the so-called self-inducing operation of the vessel. In this work, digital image analysis coupled with a suitable shadowgraphy-based technique is used to investigate the shape of the free-surface that forms in uncovered unbaffled stirred tanks, when different stirrer geometries are considered. The technique is based on back-lighting the vessel and suitably averaging over time the recorded free surface shape. For each investigated geometry, the deformed free-surface was analyzed at different impeller speeds. Different geometries of the vessel were analyzed, by varying impeller distance from vessel bottom as well as agitator type (Rushton turbine, Lightnin A310, Pitched Blade Turbine). It is shown that impeller design strongly affects the free surface profile, and in turn the impeller speed at which the free surface reaches the impeller. A model was developed to fully describe free-surface profile at all agitation speeds and for all investigated geometries, suitable for being adapted to experiments by means of physically consistent parameters adjustment. (C) 2013 Elsevier Ltd. All rights reserved.