Droplet impact on highly deformable substrate and three-dimensional stress fields measurement by photoelastic tomography

Understanding the droplet impact behavior on soft substrates is important in many industrial technologies, such as 3D inkjet bioprinters. When the droplet impacts on the deformable substrate, energy loss due to substrate deformation, which depends on the impact stress, must be considered. Therefore, measurement of the three-dimensional (3D) stress field within the soft substrate is essential to understand the droplet impact. To do so, we use photoelasticity. When circularly polarized light enters a material under a stressed state, it exits as elliptically polarized light with photoelastic parameters, which depend on the stress state. Photoelastic tomography can be used to reconstruct the 3D stress field from the measured photoelastic parameters. We developed a novel photoelastic tomography algorithm, combining vector tomography and conventional photoelastic tomography. Linear interpolation and matrix calculations are used to improve the conventional method. We applied this tomography algorithm for the droplet impact onto a soft substrate which is an unsteady phenomenon. We simultaneously measured droplet impact on a highly deformable substrate (gelatin gel) and the stress field in the substrate using a high-speed polarization camera.