A software-driven apparatus designed for measuring geometrical and physical properties of a large bubble formed at a capillary tip

We describe the construction of a software-driven instrument for the measurement of the properties of gas-liquid interfaces under not-far-from-equilibrium conditions. Instrumental design and operation are based on the formalism of systems' theory applied to surface processes (network surface thermodynamics). The apparatus forms a large bubble at a (circular) capillary tip immersed in a liquid solution. The bubble image, observed by a videocamera-microscope system, is acquired by a personal computer through a digitizing board. The computer controls the bubble size as a function of time by a feedback loop. In practice, the bubble volume is changed, according to a given functional form, by changing the wall curvature (and hence the volume) of an air chamber (made with piezoelectric discs) in pneumatic connection with the bubble (no mechanical part is in motion). Adopting a similar procedure to that already developed by Neumann et al. [14] for axisymmetric drop shape analysis, the observed geometrical profile (derived from the bubble image) is compared with the calculated profile from the Gauss-Laplace equation. The static and dynamic interfacial properties are determined from the fit parameters and from the bubble pressure. The experimental results, obtained by the instrument, appear to be useful for the physical characterization of marine waters.