Viscoelasticity of a carbon nanotube-laden air-water interface.

The viscoelasticity of a carbon nanotube (CNT)-laden air-water interface was characterized using two different experimental methods. The first experimental method used a Langmuir-Pockels (LP) trough coupled with a pair of oscillating barriers. The second method is termed the Bicone-Trough (BT) method, where a LP trough was custom-built and fit onto a rheometer equipped with a bicone fixture to standardize the preparation and conditioning of a particle-laden interface especially at high particle coverages. The performance of both methods was evaluated by performing Fast Fourier Transform (FFT) analysis to calculate the signal-to-noise ratios (SNR). Overall, the rheometer-based BT method offered better strain control and considerably higher SNRs compared to the Oscillatory Barriers (OB) method that oscillated barriers with relatively limited positional and speed control. For a CNT surface coverage of 165 mg/m2 and a frequency of 100 mHz, the interfacial shear modulus obtained from the OB method increased from 39 to 57 mN/m as the normal strain amplitude increased from 1 to 3%. No linear viscoelastic regime was experimentally observed for a normal strain as small as 0.5%. In the BT method, a linear regime was observed below a shear strain of 0.1%. The interfacial shear modulus decreased significantly from 96 to 2 mN/m as the shear strain amplitude increased from 0.025 to 10%.

Surface pressure and microstructure of carbon nanotubes at an air-water interface.

This article reports the surface pressure and microstructure of two different types of carbon nanotubes (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 upon compression using a Langmuir-Pockels trough. However, f-CNTs showed a lower degree of aggregation compared with that of nf-CNTs. This is attributed to the deprotonation of the carboxyl groups within the water subphase, leading to additional electrostatic repulsion between f-CNTs. For the same initial amount of CNTs spread onto the interface, the actual coverage of f-CNTs was higher than that of nf-CNTs at a given trough area. 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 nf-CNTs, as calculated from the surface pressure isotherms. A Volmer equation of state was applied to model the measured surface pressure containing both thermodynamic and mechanical contributions. The Volmer model, however, did not consider the loss of CNTs from the interface due to 3D aggregation and consequently overestimated the surface pressure at high compression. The actual coverage of CNT during compression was back calculated from the model and was in agreement with the value obtained independently from optical micrographs. The findings of this work may have a broader impact on understanding the assembly and collective behavior of rod-like particles with a high aspect ratio at an air-water interface.