Density fluctuations under confinement: When is a fluid not a fluid?

Knowing the behavior of a fluid in small volumes is essential for the under standing of a vast array of common problems in science, such as biological interactions, fracture propagation, and molecular tribology and adhesion, a s well as pressure solvation and other geophysical processes. When a fluid is confined, its phase behavior is altered and excluded-volume effects beco me apparent. Pioneering measurements performed with the surface forces appa ratus 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 correla tion to gain insight into the behavior of a thin film of cyclohexane confin ed within attoliter volumes, with simultaneous measurement of film thicknes s 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 dens ity fluctuations of unexpected magnitude are observed.