Nonlinear mechanical behaviors of a nanoparticle monolayer at the air-water interface

Nanoparticle can adsorb at the air-water interface and gives rise to the special interfacial mechanical properties. With the influence of external stimulus, the adsorption state of the particles may be changed and in turn the mechanical properties of the particle layer. In this work, we study the mechanical properties of a monolayer of silica nanoparticles deposited in the Langmuir trough. The area of the monolayer was varied sinusoidally by two oscillating barriers and the surface pressure was monitored by two orthogonal Wilhelmy plates. It has been found that the surface pressure of the particle layer exhibits a significant anisotropic effect. At the early stage of the oscillation, the surface pressure versus time is sinusoidal. However, with the increase of the oscillation time, the response of the particle layer significantly deviates the sinusoidal function, which implies that the response becomes nonlinear caused by a long-term oscillation. The fast Fourier Transformation (FFT) of the surface pressure data shows that the non-sinusoidal response is composed of several fundamental frequency responses. We eventually obtained the time variation of the compression modulus E and shear modulus G. A possible mechanism was proposed to account for the mechanical properties change and the nonlinear behavior of the particle monolayer.