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Molding samples from gels
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Photoelastic samples can also be cast from more common materials than [urethane](https://git-xen.lmgc.univ-montp2.fr/PhotoElasticity/Main/wikis/molding-urethane). For example gelatin is a very good photoelastic material, with a high photoelastic constant, which means a small stress induces a large photoelastic effect ([Kuske & Robertson, "Photoelastic Stress Analysis"](https://books.google.com/books?id=qsdRAAAAMAAJ&redir_esc)). Gelatine is most likely the most used photoelastic gel but many others like agar or konjac exist ([Tomlinson and Taylor](https://www.spiedigitallibrary.org/journals/Optical-Engineering/volume-54/issue-8/081208/Photoelastic-materials-and-methods-for-tissue-biomechanics-applications/10.1117/1.OE.54.8.081208.short?SSO=1)).
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Photoelastic samples can also be cast from more common materials than [urethane](https://git-xen.lmgc.univ-montp2.fr/PhotoElasticity/Main/wikis/molding-urethane). For example gelatin is a very good photoelastic material, with a high photoelastic constant, which means that a small stress induces a large photoelastic effect ([Kuske & Robertson, "Photoelastic Stress Analysis"](https://books.google.com/books?id=qsdRAAAAMAAJ&redir_esc)). Gelatin is most likely the most used photoelastic gel but many others like agar or konjac exist ([Tomlinson and Taylor](https://www.spiedigitallibrary.org/journals/Optical-Engineering/volume-54/issue-8/081208/Photoelastic-materials-and-methods-for-tissue-biomechanics-applications/10.1117/1.OE.54.8.081208.short?SSO=1)). In this section we focus on making samples out of gelatin but similar processes can be applied to other type of hydrogels.
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Making cross-linked gelatin discs
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Gelatin discs can be molded in the same way as [urethane](https://git-xen.lmgc.univ-montp2.fr/PhotoElasticity/Main/wikis/molding-urethane) discs. To make gelatin discs, first dissolve gelatin in water. Depending on the required rigidity, the concentration of gelatin can be tuned. A typical concentration is 10 weight percent. It helps to first heat the water (e.g. to about 60 degrees Celsius) before adding the gelatin, to help dissolve it. The gelatin will set at a temperature around 35 degrees. After pouring the warm gelatin solution in the rubber molds, store the molds in the refridgerator for around 3 hours, to make sure the gels are fully set.
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Gelatin samples can be molded in the same way as [urethane](https://git-xen.lmgc.univ-montp2.fr/PhotoElasticity/Main/wikis/molding-urethane) ones. To make gelatin discs, first dissolve gelatin in water. To do so let first the gelatin softness in cold water during few minutes. Then heating it up higher than 50 degrees but lower than 90 degrees. Depending on the required rigidity, the concentration of gelatin can be tuned. A typical concentration is 10 weight percent. The gelatin will set at a temperature around 35 degrees. After pouring the warm gelatin solution in the rubber molds, store the molds in the refridgerator for around 3 hours, to make sure the gels are fully set.
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After removing the gels from the molds, place them in a 10 weight percent glutaraldehyde solution for about 2 hours. This cross-links the gels, so that they will no longer dissolve in water ([Olde Damink et al](https://link.springer.com/article/10.1007/BF00123371)). Glutaraldehyde is toxic, so make sure to use gloves. Other cross-linkers do exist, but not all of them retain the photoelastic properties of the gelatin. Remove the gels from the glutaraldehyde solution, and wash them repeatedly with water, by leaving the gels in an excess amount of water for at least 12 hours. Repeat the washing procedure a few times to ensure all glutaraldehyde is washed away and diffused out of the gels.
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The samples can be used as such but they are not stable for long observation. If they stay in the air they will dry and solidify, and if they stay into water they will swell it and dissolve. In both cases this changes the photoelastic properties of the material. To stabilize them we can cross-link the gelatin. We also note that bacteria can develop in the gelatin ruining its properties.
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After removing the gels from the molds, place them in a 10 weight percent glutaraldehyde solution for about 2 hours (depending on the sample size). This cross-links the gels, so that they will no longer dissolve in water ([Olde Damink et al](https://link.springer.com/article/10.1007/BF00123371)). Glutaraldehyde is toxic, so make sure to use gloves. Other cross-linkers do exist, but not all of them retain the photoelastic properties of the gelatin. Remove the gels from the glutaraldehyde solution, and wash them repeatedly with water, by leaving the gels in an excess amount of water for at least 12 hours. Repeat the washing procedure a few times to ensure all glutaraldehyde is washed away and diffused out of the gels.
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![SqueezeGelDisk](uploads/9b696b8bdcf3fe7fa0ec8582629f9a53/SqueezeGelDisk.MP4)
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Video of the photoelastic response of a gelatin disk.
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![100_4740](uploads/ff9922c8d1ab580bdd5cc682e8ae927e/100_4740.JPG)
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Picture of the photoelastic response of a gelatin disks.
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We not that in the case of agar gel, this cross-linking process is not necessary since it does not dissolve in cold water and stays stable.
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Making cross-linked gelatin spheres
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... | ... | @@ -26,6 +35,6 @@ Gelatin spheres made using the technique published by ([Workamp et al](https://a |
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![PEResponseGelSuspension](uploads/dc8a7fd217686de833f13ea249ea1fcd/PEResponseGelSuspension.PNG)
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Photoelastic response of a suspension of bidisperse gelatin spheres in a Couette geometry. For details see [Workamp et al](https://doi.org/10.1051/epjconf/201714003020).
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Photoelastic response of a suspension of bidisperse gelatin spheres in a Couette geometry. For more details see [Workamp et al](https://doi.org/10.1051/epjconf/201714003020).
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[<go back to home](https://git-xen.lmgc.univ-montp2.fr/PhotoElasticity/Main/wikis/home) |
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