(369a) Global Strain-Field Mapping of a Carbon Nanotube-Laden Interface Using Digital Image Correlation

Identifying the correct stress-strain relationship experimentally is important to understanding the mechanical response of an interface and provides the basis for the theoretical development and experimental validation of any constitutive models. Langmuir-Pockels (LP) trough is one of the most commonly used tools for studying an interface. In a typical LP trough experiment, as the interface is compressed by a pair of barriers, a Wilhelmy microbalance is used to measure the corresponding "surface pressure". However, the as-measured surface pressure is based on a vertical force balance and thus contains both surface energy and rheological contributions. Decoupling these contributions is non-trivial. Further, despite the relatively simple experimental setup, a mixed deformation field is created, further complicating the interpretation of the experimental results. Most, if not all, existing studies assume a 1D uniaxial compression during a LP-trough compression experiment. To examine this assumption, we custom-built a glass-bottomed LP trough equipped with a camera to capture a series of optical images as an interface is compressed. Carbon nanotubes (CNTs) were chosen as the model system as they formed a "speckle pattern" when spread onto an air-water interface. Based on the change in this speckle pattern, the displacement and strain fields were calculated using digital image correlation (DIC) analysis. Our experimental findings clearly show, for the first time, the development of a non-uniform and complex 2D strain field during compression. Although the compressive strain averaged over the whole trough area closely resembles the 1D uniaxial compression strain, the 1D compression assumption underestimates the local strain by about 36% at a compression area of 25 cm² at the center of the trough, where the surface stresses are measured. The DIC-corrected strain data were subsequently analyzed with the surface stress data to quantify the surface shear and dilatational moduli of the CNT-laden interface. This is the first study in applying the DIC technique to map out the global strain field as a particle-laden interface is compressed. The method may also be applicable to other systems with similar optical texture, allowing the correct identification of stress-strain relationship of an interface. This work is supported by NSF Career Award (# 1253613), GE fellowship, and Anton Paar fellowship.