Squeezing a Film

August 25, 2012

Films of nanoparticles at an interface exhibit rich behavior, determined by the material properties of the nanoparticles as well as the thermodynamic and mechanical responses of interfacial structures.

a) The gold nanoparticles initially form islands of monolayer on the water interface. The nanoparticle film is purple in the image; the surrounding air-water interface appears bright. Compressing in the x direction rearranges these islands into an optically continuous monolayer. Further compression forces the monolayer to S fold into a trilayer, appearing as a dark hash on the surface.

(b) As compression continues, more of the film undergoes the hash transition

(c) eventually bringing the film to a continuous trilayer.

(d) Compacting the trilayer causes it to wrinkle.

(e) Continued confinement increases wrinkle amplitudes until vertical folding starts to occur.

(f ) The trilayer when many wrinkle-to-fold transitions have occurred.  

We examined experimentally the mechanical response to compression of a self-assembled gold nanoparticle monolayer and trilayer at the air-liquid interface (Langmuir films of gold nanoparticles).  Analysis of the film’s buckling morphology under compression reveals an anomalously low bending rigidity for both the monolayer and the trilayer, in contrast with continuum elastic plates. We attribute this to the spherical geometry of the nanoparticles and poor coupling between layers, respectively. The elastic energy of the trilayers is first delocalized in wrinkles and then localized into folds, as predicted by linear and nonlinear elastic theory for an inextensible thin film supported on a fluid.

"Geometric Stability and Elastic Response of a Supported Nanoparticle Film", B. D. Leahy, L. Pocivavsek, M. Meron, K. L. Lam, D. Salas, P. J. Viccaro, K. Y. C. Lee, and B. Lin, Phys. Rev. Lett. 105, 058301 (2010).

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