Phase Transitions in Nanoconfined Fluids: The Evidence from Simulation and Theory

N anoconfined fluids — that is, fluids confined between surfaces separated by nanometers — play important roles in many natural and man-made processes and products. One example is hard disk drive lubrication where, as data density has increased exponentially, the distance between the read head and rotating platen has been exponentially decreasing for several decades. This distance is now at 10–12 nm, and in the next generation of disk drives will be at 8 nm; currently, monolayers of lubricant are used to protect disk drives in abnormal situations (e.g., power loss), but in the future it is expected that they will be lubricated at all times, including during read/write operations. Additional examples include the lubrication of microelectromechanical systems (MEMS), and nanoelectromechanical systems (NEMS), and a model for the natural lubrication of synovial joints, all of which can involve moving surfaces separated by distances of the order of nm. The latter exhibit very low-sliding friction at normal pressures up to 5 MPa or more; the model system, consisting of polyzwitterionic brushes polymerized directly onto the mica sheets in a surface force balance (SFB), exhibits very similar low-sliding friction (within a factor of 2 of the natural synovial joints) at pressures as high as 7.55 MPa. These three examples highlight the desirability of being able to lubricate effectively between surfaces moving relative to each other while separated by distances on the order of nm. If the lubricant undergoes a fluid-solid phase transition under nanoconfinement, resulting in a many order of magnitude increase in the effective viscosity and the onset of a nonzero yield stress, then it is clearly not useful as a lubricant. In addition to lubrication at the nanoscale, phase transitions under nanoconfinement are also clearly important in industrial adsorption and catalytic processes (microand mesoporous adsorbents, with pore widths of under 2 nm and 2–50 nm, respectively, are widely used in the chemical, petrochemical, gas processing, and pharmaceutical industries for separations, pollution abatement, and as catalysts and catalyst supports). Additional application areas (e.g., in geology, oil recovery and nanofabrication, including nanotemplating through nanoconfinement) are described in the excellent review article by Gelb et al. Hence, understanding the phase behavior of nanoconfined fluids is key to the rational design and control of many processes and devices, both in the processing industries and in the emerging field of nanotechnology. Specifically, the change in melting temperature as a function of nanoconfinement is an important quantity to understand and predict. Gubbins and coworkers have been leaders in understanding these phenomena