Particles of appropriate size and wettability adsorb strongly at fluid-fluid interfaces, lowering the interfacial energy and thereby stabilizing emulsions and foams. The particles also form armor that prevents the droplets or bubbles from coalescing. Particle stabilized emulsions are commonly referred to as “Pickering emulsions”. Most existing studies focus on micron-sized particles with aspect ratios < 10, however, the physics is intriguingly complex due to a delicate balance between inter-particle forces, thermal motion, diffusion, and adsorption and desorption kinetics. The objective of the current research was to investigate and understand the interfacial rheology and microstructure of multi-walled carbon nanotubes (CNTs) having an aspect ratio ~ 40 at the air-water interface. To understand the collective behavior of CNTs, we report the surface pressure and microstructure of two different types of CNTs at an air-water interface; namely as produced CNTs (nf-CNTs) and CNTs functionalized with carboxyl groups (f-CNTs). Both types of CNTs formed 3D aggregates at the interface upon compression. However, f-CNTs showed less degree of aggregation compared with nf-CNTs. This is attributed to the deprotonation of the carboxyl groups within the water sub phase, leading to additional electrostatic repulsion between f-CNTs. At high compression, f-CNTs formed aligned CNT domains at the interface. These 2D domains resembled 3D liquid-crystalline structures formed by excluded volume interactions. The denser packing and orientational ordering of f-CNTs also contributed to a compressional modulus higher than that of nfCNTs. 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. Finally, we investigated the interfacial rheology of the CNT-laden interface by applying a sinusoidal oscillatory deformation to decouple the surface dilatational and shear elastic modulus and the surface dilatational and shear viscosities. The findings of this work will have a broader impact on understanding the assembly and collective behavior of rod-like nanoparticles with a high aspect ratio at an air-water interface. This work is supported by NSF Career Award (# 1253613), GE fellowship, and Anton Paar fellowship.
