Force oscillation and phase transition of simple fluids under confinement.

We use molecular dynamics simulations to explore the force oscillation mechanism of a simple fluid confined between two surfaces. Force profiles obtained through simulations are qualitatively similar to those in surface force measurements. Our results demonstrate that the layering transition is an abrupt, liquid-to-solid phase transition. The nucleation of solid phase starts at the central region of the confined film that can sustain a finite shear stress. The packing structure of the solid film is a hexagonal close-packed (hcp) crystalline structure, whose mechanical property is highly asymmetric. The elastic modulus and repulsive peaks of oscillatory forces during normal approach are always larger than those during retraction.