AFM measurements of nanofluids/functional groups/mineral substrates systems

Abstract

The main hypothesis of this work is that silicon dioxide nanofluids can reduce the work of adhesion required to pull aromatic and alkane functional groups from quartz and feldspar mineral substrates. A secondary hypothesis is that the adhesion work is directly related to adhesion force - both of which are measured on AFM. The data collected shows that adhesion force decreases as nanofluid concentration increases up to 0.5 wt% after which it begins to decrease. Mean adhesion force (pN) with error margin was obtained from histograms generated during atomic force microscope (AFM) measurements. The measurements consist of 1024 force curves per test. Standard deviation values are also included. A 2-D plot can reveal the trend of adhesion force with nanofluid concentration. Similarly, a linear relationship seems to exist between adhesion force and adhesion work (aJ), but the latter seems to also depend on scan rate and displacement. Mean adhesion work measurements were obtained in similar manner to force curves. All associated data are available in the spreadsheet named" AFM data". All of the 1,024 force curves obtained per test were carefully analyzed and a single force curve was selected to describe the role of intermolecular forces. As presented in the document "AFM force curves", a plot consists of two portions: approach (red line) and retract (blue line). The vertical axis represents adhesion force (force, pN) and horizontal axis represents vertical displacement of probe/tip from substrate (z snr, microns). The adhesion histograms calculated via post-processors are more relevant for quantitative interpretations (adhesion work and force) than the single force curves, which are more useful for qualitative interpretations such as type of intermolecular forces governing substrate/tip/medium interaction. The approach line is used to predict the presence of Van Der Waal's force and electrostatic repulsion while the retract line is used to predict dominance of capillary adhesion and binding force; the reader is referred to the reference provided below for exhaustive details. Indeed, the force curves can be used to mathematically model these surface forces, but it will require more rigorous topographic imaging and careful examination of cantilever displacement as it approaches and retracts from the surface. This is beyond the scope of this work