Study of Liquid Crystal Prewetting Films by Atomic Force Microscopy in Tapping Mode

We demonstrate the use of tapping mode atomic force microscopy (TMAFM) for the study of liquid crystal prewetting films spreading on silicon wafers. With AFM, we are able to image the edge of the prewetting film when it spreads on the solid and to obtain images on its surface with a resolution in the nanometer range. As an example, we study the roughness of the film surface: we have observed, in agreement with theoretical predictions, that the undulations of the liquid films follow those of the solid surface (at least when the wavelength of the undulations is large compared to the film thickness (∼5 nm)). Further, we show that when the drive amplitude or the set point is below a threshold, a liquid crystal micromeniscus forms between the tip and the sample, causing an instability of the tip. Finally, we have also observed that the apparent thickness of the film depends on the imaging parameters in a way that cannot be described by the models that are generally used to explain tapping mode operations.