The Van der Waals forces between a variety of macroscopic bodies have been measured with high accuracy by using a new dynamic method. Between samples of crystalline quartz and borosilicate glass separated by a distance $d$, the most important contribution to the Van der Waals forces was found to vary as $d^{-4\mathrm{}}$ in good agreement with theory. For two silicon samples, however, and for the sample combination borosilicateglass-silicon, the Van der Waals forces vary as $d^{-4\mathrm{}}$ only for large sample separations ($d>\mathrm{}0.25\mathrm{}$ μm). For smaller distances a marked deviation occurs which is not yet understood. Generation of free carriers by the illumination of the silicon samples with white light causes an increase of the Van der Waals force at large distances in qualitative agreement with an order-of-magnitude estimate.

• Received 22 January 1979

DOI:https://doi.org/10.1103/PhysRevB.19.6049