A superhydrophilic AFM tip as a nanosensor of hydrophobicity: a rationale for a new analytical method

Abstract

Accessing surface hydrophobicity (SH) via water contact angle (θ w ) at a local (nm) scale is of obvious interest in virtually any field of the natural sciences. The method proposed exploits the nanoscale capillary formed between an AFM tip and any surface in ambient conditions, which depends on the value of θ w of both the tip (θ w-tip-apex ) and the probed (θ w-surface ), as well as on the tip radius.This capillary produces the pull-off force (F pull-off ) measured by the AFM. The key of this method is that θ w-tip-apex , a quantity not directly available, is well-defined. This may be achieved using a recently released tip material, TiN, bearing a superhydrophilic TiO 2 surface (θ w = 0°) after oxidation and UV-light illumination. This way, for each F pull-off measured, a value of θ w-surface can be derived. In summary, a lucky synergy between a newly introduced tip material and the easiness of rendering it superhydrophilic, makes it possible to obtain a map of SH (θ w-surface ) from a raw F pull-off map. In other words, it potentially turns the AFM int