Forces and hydrodynamic interactions between polystyrene surfaces with adsorbed PEO-PPO-PEO

We are presenting the first measurements of purely steric levitation obtained with total internal reflection microscopy (TIRM), using F108 Pluronic triblock adsorbed to a polystyrene particle and a polystyrene film. The steric repulsion measured in these studies appears as a virtual hard wall. We report measurements which simultaneously detect both the "steric" and "hydrodynamic" edges of adsorbed polymer layers. The steric edge occurs at a few nanometers larger separations than the hydrodynamic edge. These measurements are also the first indication of the effects of adsorbed polymer on the net van der Waals attraction in good solvent conditions. The adsorbed polymer strengthens the net attraction for the same separation between the PS substrates but weakens the attraction for the same separation between the outer edges. It is also shown that attraction at small separations is dominated by the properties of the polymer layer, and attraction at large separations is dominated by the properties of the substrates. TIRM experiments are also reported which directly probe the adsorbed polymer layer thickness to establish substrate separation. The estimates from the TIRM experiments were within nanometers of the literature estimate of 15 nm.