Minimal Invasiveness and Spectroscopy-Like Footprints for the Characterization of Heterogeneous Nanoscale Wetting in Ambient Conditions

Wetting at the nanoscale is investigated and discussed in relation to samples presenting amphiphilic domains and by employing dynamic atomic force microscopy (AFM) operating in the amplitude modulation (AM) mode. First, the capability of monitoring the growth and subsequent reduction of nanoscale water films on flat and homogeneous surfaces, i.e., graphite surfaces, with an AFM is demonstrated. The AM AFM data provide spectroscopy-like footprints of the presence and amount of water on the surface in terms of variations in nanoscale force profiles, i.e., conservative and dissipative interactions. These results are corroborated by means of attenuated total reflectance infrared spectroscopy. An important strength of the AFM technique presented here is that it allows controllably reducing invasiveness. This is particularly important in studies concerning soft matter systems. These capabilities are demonstrated on stearic acid monolayer films, soft nanoscale films that present low affinity to water, on a mica surface. The characteristic conservative and dissipative footprints of water are found on the hydrophilic mica surface only. When probing the stearic acid films, peak repulsive forces are controllably reduced until the stearic acid samples show no significant wear or damage (∼10 pN).