Pattern Formation by Fracturing in Visco-elastic Materials

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A pattern growth of the injection fluid into visco-elastic materials was studied in two dimensional system. Ager gels were used as model materials of visco-elastic materials. A Hele-Shaw cell filled with the ager gel was used in the injection experiments. By changing ager contents of the ager solution, the visco-elastic property of the ager gel changes from one of viscous fluids to one of elastic solids. At the case of lower contents of ager solution (lower than 0.1 weight percent), ager gels behave like viscous fluids. The higher ager contents, the more elastic property ager gels will gain. Therefore, we can control visco-elastic property of model materials by changing the ager contents. Air is used as an injection fluid in our experiments. With changing the ager contents and the injection pressure, systematic pattern formation experiments were carried out under a constant injection pressure. Pattern growth during experiments was measured by CCD video camera. The pattern grows in the condition of outside equilibrium. Both viscous flow and elastic cracking are control mechanisms in our pattern growth. Viscous fingering is a well known phenomena : when a lower viscosity fluid is injected into Hele-Shaw cell filled with highly viscous fluid. The growth of viscous fingering is a quasi static growth. In the case of elastic solid, the pattern is controlled by fracturing. Hydro-fracturing, e.g.crack eruption in volcano, is one of the typical examples. In our system, a new type of pattern growth was observed. This pattern was generated under two competitive mechanisms ; cracking and viscous flow. We coined this new pattern as a visco-elastic fingering. Morphological phase diagram was obtained. At the lower injection pressure and lower ager contents, viscous fingering will appera. When we used higher contents ager gels, a single plane crack growth is observed. At the middle of the range, visco-elastic fingering is observed.