Fig. 1: Schematic representation of a fluid of hard spheres confined in a slit pore formed by two hard walls.
From: Thermodynamics, statistical mechanics and the vanishing pore width limit of confined fluids

Hard sphere (blue) diameter: σ; Pore width: H; Accessible pore width: L = H − σ; Surface area of one wall: A; Pore walls: green; Inaccessible region of hard sphere center: light brown.