Precise determination of the Poisson ratio in soft materials with 2D digital image correlation

In this article, we demonstrate the application of digital image correlation (DIC) in evaluating the strains and Poisson ratio of a range of soft materials in terms of their spatial and temporal resolutions. Homogeneous samples of polydimethylsiloxane (PDMS) elastomer were used as control substance and were measured to have Poisson ratios of 0.498, 0.503, 0.500, and 0.499, in agreement with the reported incompressible value of 0.50. Two carbon nanotube (CNT) elastomer composites of identical composition, but one of a homogeneous and the other of a highly inhomogeneous CNT distribution, were used to determine the spatial resolution with good results. The relaxation of a polydomain liquid crystal elastomer (3D-LCE), a cholesteric liquid crystal elastomer (CLCE), and a polyacrylamide gel (PAAm) in water, were used to determine the temporal resolution of the technique. A video at 25 fps was used to evaluate the time dependence of the 3D-LCE over which time an increase in the Poisson ratio was observed. The 3D-LCE relaxes from its initial state at 0.42 to 0.50, converging towards incompressibility at equilibrium. The CLCE was found to have a similar initial value of 0.44 but converged to similar to 0.60, a consequence of its anisotropic elastic nature. PAAm gel relaxation in water was studied over a time period of 7 hours with digital images taken periodically every minute. Its Poisson ratio was found to decrease smoothly from 0.50 to 0.26, with an accompanying reduction in force. The equilibrium result compares well to the 0.25 value predicted by a theory of the strain-induced swelling of dilute gels. In summary, we find DIC to be a powerful and easy to implement method of accurately measuring local strains in a range of soft materials.