RECENT STUDIES OF SHORT RANGE FORCES

This paper describes recent studies of the forces between surfaces for separations of the order of tens to hundreds of angstroms. The first part deals with the direct measurement of forces between molecularly smooth mica sheets and shows that normal van der Waals' forces operate for separations less than 100 Å, retarded van der Waals' forces for separations greater than 200 Å. The second part describes a direct study of the repulsive forces exerted by charged double layers residing on solid surfaces. By using the contact between a soft optically smooth rubber hemisphere and a flat glass surface repulsive forces of the order of 105 dyne/cm-2 have been measured for film thicknesses of about 150 Å. The equilibrium film thickness decreases with increasing ion concentration and the results are in broad agreement with measurements by Mysels and others on black soap films. © 1969.