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Here we report a new study on the boundary conditions for the flow of a simple liquid in a confined geometry obtained by measuring hydrodynamic drainage forces with colloid probe atomic force microscopy (AFM). In this work, we provide experimental data obtained using a best practice experimental protocol and fitted with a new theoretical calculation (Zhu, L.; Attard, P.; Neto, C. Langmuir 2010, submitted for publication, preceding paper). We investigated the hydrodynamic forces acting on a silica colloid probe approaching a hydrophobized silicon surface in a single-component viscous Newtonian liquid (di-n-octylphthalate), a partially wetting system. The measured average slip lengths were in the range of 24-31 nm at approach velocities of between 10 and 80 μm/s. Using our experimental approach, the presence of nanoparticle contaminants in the system can be indentified, which is important because it has been shown that nanoparticles lead to a large apparent slip length. Under our stringent control of experimental conditions, the measurement of the slip length is reproducible and independent of the spring constant of the cantilever.

Interaction forces between a spherical silica probe and a cross-linked poly(dimethylsiloxane) (PDMS) colloidal droplet have been determined by atomic force microscopy (AFM). On approach of the surfaces, as a result of deformation of the PDMS, the repulsive force increases much less rapidly than expected for electrical double layer interaction of rigid particles. The amount of deformation is dependent on the loading force and the drive velocity and reflects the nanorheological response of the PDMS. On retraction of the surfaces, force curve hysteresis is observed and is dependent on the rate of approach/retraction. Hysteresis is due to the viscoelastic response of the PDMS droplet and can be described by theory (Attard, P. Phys. Rev. E 2001, 63, 061604). The moduli and the characteristic relaxation time of the PDMS colloid have been determined from the force measurements and offer a novel description of the nanorheological properties.