Ultrathin hydrated dextran films grafted on glass: Preparation and characterization of structural, viscous, and elastic properties by quantitative microinterferometry

We report the deposition and partial anchoring of ultrathin dextran films on glass surfaces and the characterization of their structural, viscous, and elastic properties by using latex beads as colloidal probes. The average distance between bead and surface was measured to an accuracy of ±0.2 nm by application of reflection interference contrast microscopy (RICM) and by application of an improved theory of image formation of this microinterferometric technique. The fluctuation of the height h(t) of the beads above the glass surface with time t was analyzed in terms of the theory of Brownian motion of particles in a potential V(h). The interaction potential V(h) of the bead was obtained by analyzing the height probability distribution of a bead in terms of the Boltzmann distribution law. It is shown that V(h) is determined essentially by the gravitational attraction and the polymer-induced disjoining pressure. The latter could be best interpreted in terms of a simple spring model, whereas the classical scaling theories of polymer-induced forces failed. An elastic constant of 300 N/m² was obtained. An apparent viscosity of the dextran film was determined by the evaluation of the damping constant of the height correlation function <h(t) h(0)>. Applying the Brinkmann model of flow in porous media, the damping constant is further used to evaluate a hydrodynamic decay length of the hydrodynamic field in the dextran film.