Behavior of a static bubble in a yield stress fluid
Résumé
Non-bearing materials currently used in building applications (walls or bulkheads) are generally formed by panels or blocks. Such materials require good thermal performances and minimal mechanical strength. Mineral foams are relevant solutions. We focus on calcium sulfate foams containing a large amount of air trapped in bubbles. The foam production starts with fluid suspension obtained by quick mixing of water, mineral binder and surfactants. Foaming is applied before the quick setting of the mineral matrix. To study the interaction between foaming and rheological properties evolution, we use a transparent fluid model (dispersion of carbopol 676 in water). The air bubbles stability in yield stress polymer gel is analyzed by picture processing. Carbopol gel is placed in plexiglas container. Bubbles are inserted with a variable volume using a syringe for injection controlling gas pressure. We start with the study of single bubble in different carbopols gels to model the effect of rheological properties evolution (setting effect). As expected, bubbles sizes and internal pressure evolve according to thermodynamical laws. During injection, bubble growing depends on various parameters: yield stress, injected volume and pressure, sphericity... Experiments are conducted in several conditions to determine the parameters influences on bubble shape and volume. This lead to the identification of rheological behavior linking pressure, bubble geometry and yield stress in static condition. The foam stability is mostly conditioned by coalescence phenomena. It consists on the fusion of two bubbles in contact after the separating diaphragm destruction. Experiments reveal that a small amount of surfactants is needed to preserve foam structure. Coalescence phenomenon is studied on Carbopol gel following the meet conditions of two injected bubbles. The type and the surfactants amount generate a wide range of configuration. Results leads to a better understand of the foams stability at fresh state. It gives us tools to optimize the formulation regarding mechanical and thermal properties targets.