DIRECT MEASUREMENT OF RETARDED VAN DER WAALS ATTRACTION

Total internal reflection microscopy is used to measure the total potential energy of interaction between a 6 μm polystyrene (PS) latex bead and either a bare glass microscope slide or a glass slide spin-coated with a 1 μm thick PS film, when the two interacting bodies are separated by 10−300 nm of aqueous solution having an ionic strength between 0.5 and 3 mM. In particular, these are the first measurements of van der Waals interaction between microscopic bodies of PS across water, for which the dielectric spectra are well-known. Under these conditions the bead is levitated above the slide by double-layer repulsion. After the gravitational contribution is subtracted, the potential energy profile displays a minimum of 0.5−2.3kT formed by long-range van der Waals attraction and shorter-range double-layer repulsion. The attraction was detectable at distances up to 200 nm. At separation distances greater than 100 nm (energy < 0.5kT), the measurements agree well with predictions using Lifshitz theory to predi...