Oscillating repulsive force between macroscopic surfaces as a consequence of the fluctuation of the density in the gap region

We try to explain the short-range repulsive force between surfaces as a consequence of a decrease in the configurational entropy of liquid molecules in the gap region as the width of the gap decreases. A simple model shows that the density has an oscillating variation when the surfaces are pushed towards each other. As a consequence, an oscillating repulsive force appears between the macroscopic surfaces, which is the result of a loss of configurational freedom of the liquid molecules because of their orientation in the gap. Received: 14 August 2000 Accepted: 30 January 2001