Abstract
Coalescence is the rupture of a film between two adjacent bubbles in any type of liquid foam and has pronounced influence on the development of its macrostructure after solidification, mostly leading to larger pores and a wider size distribution. Foamable AlSi6Cu4 + 0.5 wt % TiH2 precursors made following the powder metallurgical route were foamed in a gas-tight X-ray transparent furnace under 500 kPa (5 bar) pressure with subsequent pressure release. Coalescence was investigated by fast X-ray radioscopy at a frame rate of 1250 Hz and an effective pixel size of 9 µm at the beamline ID-19, European Synchrotron Radiation Facility (ESRF), Grenoble, France. A self-developed program yielded the temporal and spatial detection of coalescence events in the acquired radioscopies. We observed the occurrence of film ruptures in a short temporal and spatial distance and we will refer to these phenomena as “avalanches”.
Highlights
During metallic foam production, the liquid foam suffers morphological changes by effects such as drainage, flow, coarsening, and coalescence, which clearly influence the inner structure of the resulting solid foam [1]
A high temporal resolution (>10,000 Hz), is mandatory for resolving coalescence, which starts with the rupture of the film and ends with the merging of the two adjacent bubbles, taking place in less than 1 ms [2,3,4]
The high frame rates of these radioscopies are achieved by the application of synchrotron radiation and high-speed cameras [5]
Summary
The liquid foam suffers morphological changes by effects such as drainage, flow, coarsening, and coalescence, which clearly influence the inner structure of the resulting solid foam [1]. Since the processes acting on the structure of a foam take place at different time scales, a careful choice of diverse time resolutions is required. A frame rate of about 1 Hz is sufficient for the investigation of, for instance, drainage or expansion. A high temporal resolution (>10,000 Hz), is mandatory for resolving coalescence, which starts with the rupture of the film and ends with the merging of the two adjacent bubbles, taking place in less than 1 ms [2,3,4]. The high frame rates of these radioscopies are achieved by the application of synchrotron radiation and high-speed cameras [5]
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