Fast dynamic interfacial tension measurements and dilational rheology of interfacial layers by using the capillary pressure technique

The oscillating drop and bubble analyzer (ODBA) is an experimental set-up based on the measurement of capillary pressure under static and dynamic conditions. It allows studies of slow and fast dynamic surface and interfacial tensions, following different growing and oscillating drop or bubble protocols, as well as determination of the dilational interfacial visco-elasticity of liquid interfacial layers. For the visco-elasticity studies, drops or bubbles are subjected to harmonic oscillations of area or volume in a broad frequency range, and the resulting harmonic capillary pressure response is analyzed by Fourier analysis. Also, transient relaxations can be easily performed, which are of particular importance for isodense systems. The limits of applicability for highly dynamic conditions are given by the hydrodynamics of the fluid flow inside the capillary tip and the pressure cell which depend on fluid properties, capillary tip size, and geometry. For the growing drop protocol, additional phenomena during drop formation and detachment play a significant role. For oscillating drops or bubbles, the highest accessible frequencies also depend on the absolute drop size and applied oscillation amplitude. In this work, the capability of the technique for measurements in the frequency range between 1 and 100 Hz are discussed, and elasticity values at up to 80 Hz were measured for the liquid–liquid interface.