title: Density fluctuations under confinement: when is a fluid not a fluid?
abstract: Knowing the behavior of a fluid in small volumes is essential for the understanding of a vast array of common problems in science, such as biological interactions, fracture propagation, and molecular tribology and adhesion, as well as pressure solvation and other geophysical processes. When a fluid is confined, its phase behavior is altered and excluded-volume effects become apparent. Pioneering measurements performed with the surface forces apparatus have revealed so-called structural or oscillatory solvation forces as well as the occurrence of a finite shear stress, which was interpreted as a solidification transition. Here, we report measurements obtained with an extended surface forces apparatus, which makes use of fast spectral correlation to gain insight into the behavior of a thin film of cyclohexane confined within attoliter volumes, with simultaneous measurement of film thickness and refractive index. With decreasing pore width, cyclohexane is found to undergo a drastic transition from a three-dimensional bulk fluid to a two-dimensional adsorbate with strikingly different properties. Long-range density fluctuations of unexpected magnitude are observed.
authors.affiliation: Laboratory for Surface Science and Technology, ETH Zentrum, CH-8092 Zürich, Switzerland. heuberger@surface.mat.ethz.ch
authors.forename: M
authors.initials: M
authors.lastname: Heuberger
authors.forename: M
authors.initials: M
authors.lastname: Zäch
authors.forename: N D
authors.initials: ND
authors.lastname: Spencer