AFM Tip Effect on a Thin Liquid Film

作者：Ledesma Alonso R; Legendre D^*; Tordjeman Ph

来源：Langmuir, 2013, 29(25): 7749-7757.

DOI：10.1021/la4006086

We study the interaction between an AFM probe and a liquid film deposited over a flat substrate. We investigate the effects of the physical and geometrical parameters, with a special focus on the film thickness E, the probe radius R, and the distance D between the probe and the free surface. Deformation profiles have been calculated from the numerical simulations of the Young-Laplace equation by taking into account the probe/liquid and the liquid/substrate interactions, characterized by the Hamaker constants, H-pl and H-ls. We demonstrate that the deformation of a shallow film is determined by a particular characteristic length lambda(F) = (2 pi gamma E-4/H-ls)(1/2), resulting from the balance between the capillary force (gamma is the surface tension) and the van der Waals liquid/substrate attraction. For the case of a bulk liquid, the extent of the interface deformation is simply controlled by the capillary length lambda(C) = (gamma/Delta rho g)(1/2). These trends point out two asymptotic regimes, which in turn are bounded by two characteristic film thicknesses E-g = (H-ls/2 pi Delta rho g)(1/4) and E-gamma = ((RHls)-H-2/2 pi gamma)(1/4). For E > E-g, the bulk behavior is recovered, and for E < E gamma, we show the existence of a particular shallow film regime in which a localized tip effect is observed. This tip effect is characterized by the small magnitude of the deformation and an important restriction of its radial extent lambda(p) localized below the probe. In addition, we have found that the film thickness has a significant effect on the threshold separation distance D-min, below which the irreversible jump-to-contact process occurs: D-min is probe radius-dependent for the bulk whereas it is film-thickness-dependent for shallow films. These results have an important impact on the optimal AFM scanning conditions.

• 出版日期2013-6-25