High-Resolution Imaging of Liquid Structures: Wetting and Capillary Phenomena at the Nanometer Scale

Abstract

Scanning-probe microscopies (SPMs) have experienced an impressive development in the last 10 years, such that obtaining atomically resolved images of surfaces is now routine. The scanning tunneling and atomic force microscopes (STM and AFM) are the most widely known representatives of the growing family of SPMs. With these two instruments, which were invented more than 15 years ago, the atomic structure of the surfaces of solid materials can be imaged in real space and, very importantly, in virtually any environment: vacuum, air, reactive gas atmospheres, and under liquid. These two circumstances (atomic resolution and lack of environmental constraints) make SPM unique among microscopies. One frontier remains unconquered however: the imaging of liquid surfaces. Even though liquids constitute a very large fraction of all materials, the study of their microscopic morphology has been limited to the diffraction limit of visible optical microscopy, a situation that has remained unchanged for decades. Techniques capable of breaking this resolution limit are welcomed. One such technique was recently developed in our laboratory, which we call scanning-polarization-force microscopy (SPFM). It did not result from the discovery of any new physical principle but from the simple application of well-known methods and techniques, in combination with the AFM, to the field of liquid surface structures. In this article, we will first summarize the working principle of SPFM. Then we will show how we have applied it to the investigation of liquid films and droplets of nanometer dimensions and their interactions with solid supports.