(207b) Liquid-to-Solid Phase Transition: Drainage of a Simple Nonpolar Fluid Under Nanometer Confinement

Ultrathin liquid films are ubiquitous in many technological problems, including wetting, adhesion, nanofluidics, and boundary lubrication of friction contacts. In surface force measurements, there are long-standing controversial debates concerning the thermodynamic properties of nanometer confined fluids. In this presentation, we report the dynamics thinning (squeeze-out) of a liquid argon between two solid boundaries. Using a liquid-vapor (LV) molecular ensemble and a steered molecular dynamics approach, we studied the mechanical properties of liquid argon during approach and retraction processes. Our simulation results show that when the gap distance is less than 2.5 nm (about 7 molecular diameters of argon), the solidification to a crystalline structure of the confined fluid is a spontaneous process and does not depend on whether the contact between the fluid molecules and the solid boundary is commensurate or not. The solvation force is calculated during the approach of the upper solid surface. The periodic disordered-liquid-to-ordered-solid phase transitions of the confined films are further reflected by the oscillations of solvation forces. The HCP compact structure of nonpolar argon in crystallized state was observed, accompanied by significant vacancy diffusion at larger distances. The non-symmetric force profiles during approach and retraction indicate that the compression strength is much larger than tensile strength of this HCP crystal. This study reveals a fundamental difference in the properties between aqueous and nonpolar fluids under nanometer confinement.