Molding samples from gels
Photoelastic samples can also be cast from more common materials than 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"). Gelatin is most likely the most used photoelastic gel but many others like agar or konjac exist (Tomlinson and Taylor). In this section we focus on making samples out of gelatin but similar processes can be applied to other types of hydrogels.
Making cross-linked gelatin discs
Gelatin samples can be molded in the same way as urethane ones. To make gelatin discs, first, dissolve gelatin in water. To do so let first the gelatin softness in cold water for a few minutes. Then heat 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 be set at a temperature of around 35 degrees. After pouring the warm gelatin solution into the rubber molds, store the molds in the refrigerator for around 3 hours, to make sure the gels are fully set.
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 in the water they will swell 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.
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). 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.
Video of the photoelastic response of a gelatin disk.
Picture of the photoelastic response of gelatin disks.
We note 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.
Making cross-linked gelatin spheres
Gelatin spheres can be made by dripping a hot gelatin solution in a cold oil (e.g. decane). To make sure the droplet remains spherical, it is important that the Bond number (the ratio of the weight of the droplet and surface tension) is low. This can be achieved by making sure the droplets are sufficiently small (by using a very small nozzle) and avoiding using surfactants. The droplet size can be made even smaller by applying a coaxial airflow. This airflow along the nozzle allows gelatin particles to be made with sizes ranging from 2.3 to 7 mm (Workamp et al). To ensure the droplets are fully solidified before touching each other (which could lead to merging), it is convenient to have a sufficiently long tube of cold oil in which the droplets slowly solidify while settling to the bottom.
Gelatin spheres are made using the technique published by (Workamp et al). Grid line spacing is 1 mm.
Photoelastic response of a suspension of bidisperse gelatin spheres in a Couette geometry. For more details see Workamp et al.