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 mu m polystyrene (PS) latex bead and eit her a bare glass microscope slide or a glass slide spin-coated with a 1 mu m 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 part icular, these are the first measurements of van der Waals interaction betwe en microscopic bodies of PS across water, for which the dielectric spectra are well-known. Under these conditions the bead is levitated above the slid e by double-layer repulsion. After the gravitational contribution is subtra cted, the potential energy profile displays a minimum of 0.5-2.3kT formed b y long-range van der Waals attraction and shorter-range double-layer repuls ion. The attraction was detectable at distances up to 200 nm. At separation distances greater than 100 nm (energy < 0.5kT), the measurements agree wel l with predictions using Lifshitz theory to predict the interaction of two PS half spaces, coupled with Derjaguin's approximation to account for the c urvature of the sphere. At all separations, both retardation and screening are very important to the van der Waals interaction. As the separation beco mes smaller than 100 nm, the measured interaction becomes weaker than predi cted. Using atomic force microscopy (AFM), we observed asperities with heig hts up to 10 nm on the spin-coated PS film and up to 30 nm on the latex bea d. If our experimental "zero" separation corresponds to contact of the larg est asperities, the separation distance used in the theory should be larger than that measured. Shifting the theoretical curve by the sum of the asper ity heights causes the theory to shift from overpredicting the van der Waal s attraction to underpredicting it. We suggest a new theory in which roughn ess is treated as a diffuse film whose composition varies from pure PS at t he inner surface to pure water at the outer surface. The composition profil e can be determined independently from the histogram of elevations measured with AFM.