Assessment of three-dimensional large-scale deformations during drying of hydrogels via a structure from motion photogrammetry

A low-cost, efficient and accurate structure from motion (SfM) photogrammetry is used to reconstruct the complex morphologies of unevenly deforming hydrogels during the drying process. The primary setups comprise a camera and rotary platform, which are low-cost and works stably for most drying conditions. The dynamic three-dimensional (3D) reconstruction of cylindrical hydrogel samples is achieved by SfM through the generation of point clouds, and the 3D morphologies are calculated by a Poisson reconstruction algorithm based on these point clouds. The accuracy of SfM in quantitatively measuring large-scale deformation is validated, and its applications in drying studies are explored. The results show that as drying proceeds, the maximum measurement deviation of the hydrogel volume between SfM and the traditional buoyancy force method is 1.93% at a volume shrinkage of 69%, confirming the accuracy of the current method for measuring large-scale drying shrinkage. Based on SfM, the temporal evolution of the cross section of the hydrogel is obtained. Comparing to conventional drying experiments that assess the dryness of deformed materials based on overall moisture content changes, SfM enables the evaluation of localized complete drying by quantitative analysis of deformation parameter variations.