In situ monitoring of the morphology evolution of interfacially-formed conductive nanocomposite films and their use as strain sensors.

Hypothesis: Understanding and monitoring the film formation of interfacially formed layered films allows for the design of conductive nanocomposite films suitable for strain sensing.

Experiments: To understand the mechanism of interfacial film formation, the hexane/water interface was monitored during the evaporation process via confocal laser scanning microscopy. Scanning electron microscopy and atomic force microscopy were utilized to investigate final film morphology. Tensile testing was used to determine their mechanical properties under uniaxial strain.

Findings: Conductive nanocomposite films were formed at the hexane/water interface. Due to their low colloidal stability in hexane, the Vulcan carbon (VC) nanoparticles settled to the hexane/water interface prior to the onset of paraffin wax precipitation. Consequently, after the evaporation of hexane a two-layer structured film was formed. The bottom (water-facing, VC-rich) layer was conductive due to the existence of a percolated network of nanoparticle aggregates, while the top (hexane facing, paraffin-rich) layer was not conductive. The films showed high sensitivity for strains between 1% and 10%. We propose that the mechanism of strain sensing is similar to that of layer-structured sensors fabricated through embedding conductive nanofillers onto flexible polymeric substrates. The advantage of the films derived by the method proposed here is their ease of fabrication as well as their low cost.