Scanning polarization force microscopy investigation of contact angle and disjoining pressure of glycerol and sulfuric acid on highly oriented pyrolytic graphite and aluminum

For liquid droplets of sub-micrometer dimensions, the study of wetting properties (quantified by contact angle, disjoining pressure, spreading coefficient, etc.) is possible using the relatively new technique known as scanning polarization force microscopy (SPFM). This non-contact scanning probe microscopy technique was successfully implemented in our laboratory in order to study the wetting properties of glycerol and sulfuric acid on the surface of highly oriented pyrolytic graphite (HOPG) and glycerol on aluminum film deposited on mica. An AC polarization bias of 3 V at 3 kHz frequency was applied between a conductive atomic force microscope tip and the substrate. The resulting polarization force was measured with high accuracy, allowing non-contact topography profile measurements of liquid micro- and nanodroplets. The dependence of the contact angle on droplet height was determined in order to calculate the values of the spreading coefficient and the disjoining pressure between the liquid and substrates. The calculated potential energies give disjoining pressure values of ∼0.4 atm for glycerol on HOPG, ∼0.47 atm for glycerol on aluminum and ∼13 atm for H2SO4 on HOPG. In the case of H2SO4 on HOPG the strength of the force appears to be thirty times bigger than that for glycerol on HOPG and aluminum