Determination of the dilational viscoelasticity by the oscillating drop/bubble method in a capillary pressure tensiometer

The Capillary Pressure Tensiometry is an effective technique for dilational rheological investigations when it is used according to the oscillating drop/bubble method. This method allows for an indirect determination of the dilational viscoelasticity versus frequency through the acquisition of the pressure signal obtained as response to harmonic perturbations of the drop/bubble interfacial area. Acritical aspect of this technique concerns the calculation of the real and imaginary part of the dilational viscoelasticity from the directly acquired experimental data. Such calculation requires a specific experiment theory accounting also for the compressibility of the closed part of the experimental cell and for the fact that the acquired pressure is a superposition of different fluid dynamic contributions. Here we present an extension of previously proposed approaches, considering two different CPT configurations which refer to some already in use. The pressure inside the measurement cell is theoretically calculated considering the contributions, not only of the capillary pressure, but also of the Poiseuille pressure due to the fluid motion through the capillary and of hydrodynamic inertial terms. Simulations of experimental pressure signal are calculated as a function of specific experimental parameters in order to evaluate their influence on the measurement and to improve the effectiveness of the apparatus and the experimental methodology. Moreover in order to validate the assumptions of this approach, specifically acquired experimental data are analysed.