Oscillating bubble tensiomete

Static and dynamic surface tension of marine water onshore or platform-based measurements by the oscillating bubble tensiometer... 

Herein, our concern is to assess the suitability of the oscillating bubble tensiometer for marine purposes. Essentially, we conducted the present investigation with the aim of estimating the sensitivity of the measurement method, with respect to oceanographic studies and monitoring. In this paper we illustrate preliminary experimental results, obtained with a typical apparatus, working in an onshore laboratory. 

The measuring system is the oscillating bubble tensiometer, described in detail elsewhere (Loglio et al. 1996, 1998b, 2001, Kovalchuk et al. 2001, Miller et al. 2000). Figure 1 shows a schematic drawing of the measurement assembly, which is composed of three parts 1) the sample (20 mm x 20 mm x 40 mm) pyrex vessel, 2) the capillary (2.00 mm internal diameter stainless-steel tube with flat tip ), 3) an air chamber with deformable walls (made with piezoelectric discs). Possible fluidodynamic instability of the bubble has been taken into account in the design of the bubble pneumatic circuit. 

The oscillating bubble tensiometer, devised for marine studies, shows a particular feature, consisting of a couple of inlet and outlet tubes (Figure 1). Such conduit allows sea water to be fed either from the sampling bottle or directly from a continuously-flowing pumping system, submersed in the sea. Then, the tensiometer vessel can be initially flushed with sea water for... 

A technique has been developed for the continuous measurement of emulsion surface tension based on the pressure necessary to form a bubble in liquid. Details of the method may be found in Schork and Ray [24]. With a laboratory prototype of the bubble tensiometer, it has been possible to measure surface tensions continuously to within 1 to 2% [24]. A commercial instrument based on these principles is now available. Figures 5.5 and 5.7 demonstrate the use of the bubble tensiometer to monitor the surface tension of methyl methacrylate emulsion during continuous and batch polymerization. It will be noted that during conversion oscillation the surface tension oscillated as well, in accordance with the discrete initiation mechanism often postulated to explain this phenomenon. 

The design of a bubble pressure tensiometer changes from instrument to instrument, according to the specific measurement procedures. As an example, Fig. 14 illustrates the schematic diagram of the tensiometer BPA (SINTERFACE) equipped with a gas flow oscillation analyser to measure the bubble surface lifetime. 

Figure 14. Schematic diagram of maximum bubble pressure tensiometer using a gas flow oscillation analyser (BPA, SINTERFACE Technologies).

The drop and bubble shape tensiometer allows one to perform oscillation experiments. However, in contrast to similar studies with small spherical drops it is limited to slow oscillations. Depending on the hydrodynamic conditions, mainly the rheological behaviour of the bulk phases, oscillation from a certain frequency onwards do not provide drops or bubbles with a Laplacian shape, so that the data analysis will fail and yield unrealistic data. Even for liquids of high grade of purity interfacial tension changes are simulated and hence misinterpretations can be the consequence. Thus, the use of high speed video technique is not really relevant for shape analysis tensiometry (although provided by several companies), as the hydrodynamic relaxation can take much time to yield Laplacian menisci. 

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