Atomic force microscopy for the characterisation of pinning effects of seawater micro-droplets in n-decane on a calcite surface.

Hypothesis: Roughness is an important parameter in applications where wetting needs to be characterized. Micro-computed tomography is commonly used to characterize wetting in porous media but the main limitation of this approach is the incapacity to identify nanoscale roughness. Atomic force microscopy, AFM, however, has been used to characterize the topography of surfaces down to the molecular scale. Here we investigate the potential of using AFM to characterize wetting behavior at the nanoscale.

Experiments: Droplets of water on cleaved calcite under decane were imaged using quantitative imaging QI atomic force microscopy where a force-distance curve is obtained at every pixel.

Findings: When the AFM tip passed through the water droplet surface, an attraction was observed due to capillary effects, such that the thickness of the water film was estimated and hence the profile of the droplet obtained. This enables parameters such as the contact angle and contact angle distribution to be obtained at a nanometer scale. The contact angles around the 3-phase contact line are found to be quasi-symmetrically distributed between 10-30°. A correlation between the height profile of the surface and contact angle distribution demonstrates a quasi-proportional relationship between roughness on the calcite surface and contact angle.