The aim of this work was to determine the phenomena affecting bubble size, according to process conditions (pressure, flow rates, whipping rotation speed) and formulation properties. Model fluids were formulated in order to get simple and well-defined rheology. An instrumented foaming pilot-scale line was built and allowed us to monitor the process and to characterise bubble size under pressure, at the exit of the mixer. Viscosity and rotation speed of the whipping head are the most influent parameters on foam morphology: interaction between these factors have been highlighted. Experimental measurements obtained were consistent with the critical Weber number since a corrected fluid viscosity is used. It seems indeed that the fluid viscosity is reduced in the whipping head, due to the presence of local heating. These temperature variations consequent to the shear of viscous fluids in a narrow-gap geometry were quantified and modelled as a function of shear rate and fluid viscosity. (c) 2005 Elsevier Ltd. All rights reserved.