Interfacial dilational rheology by oscillating bubble/drop methods

Dilational rheology represents a powerful tool to investigate equilibrium and dynamic properties of simple and more complex interfacial layers containing surfactants, proteins, polymers or micro-nano sized particles. Concerning the experimental techniques for dilational rheology, drop/bubble tensiometers based on the acquisition of the drop/bubble profile and capillary pressure tensiometers are especially effective. This article focuses on oscillating drop/bubble methodologies where harmonic variations of the interfacial area are utilized for the measurement of the dilational viscoelasticity in the frequency domain. The increasing efficiency of these techniques of the last ten years is due, from one side, to the implementation of advanced instrumentations which make faster the drop/bubble control and the data acquisition and, on the other side, to the application of new theoretical approaches for data acquisition and interpretation. A critical analysis of such drop/bubble instruments is presented where their potentialities and limitations are underlined. Moreover, recent improvements in the definition of calculation methods based on the modelling of the experimental set up are reviewed together with some examples of experimental studies based on the utilisation of such methodologies.