Determination of Wetting Velocities of Surfactant Superspreaders with the Quartz Crystal Microbalance

The radial sensitivity of the quartz crystal microbalance (QCM), comprising an AT-cut quartz crystal coated with gold electrodes, is exploited for the measurement of wetting rates of aqueous dispersions of a trisiloxane surfactant exhibiting “superspreading” behavior on different surfaces. The rate at which a droplet advances across the QCM surface is measured directly from the frequency transient accompanying radial spreading of the drop from the center of the quartz crystal to the periphery. This method provides for convenient measurement of spreading rates under a variety of conditions, allowing systematic investigations of the wetting behavior dependence on surfactant concentration, substrate surface energy, and humidity. The substrate surface energy is adjusted readily by the preparation, on the gold electrodes of the QCM, of self-assembled single-component or mixed organosulfur monolayers with judiciously chosen chemical functionalities at the termini of the alkyl chains. The surfactant concentration at which the maximal spreading velocity occurs is independent of substrate surface energy, indicating that this characteristic is dependent upon the microstructure of the surfactant dispersion rather than the energetics of the solution/substrate interface. Furthermore, these studies revealed a surprising dependence of the wetting velocity on surface energy in which the velocities were greatest on surfaces of moderate hydrophobicity. Previous work indicated that superspreading required a humid environment, suggesting that a thin precursor film of water on the hydrophobic surface was required. The studies described here revealed significant water condensation on rough hydrophobic QCM surfaces compared to smooth hydrophobic surfaces, which is attributable to capillary condensation on the rough surfaces. Consequently, superspreading is observed on rough surfaces of high hydrophobicity but is negligible on smooth ones. The resolution of the preexisting water film issue, and the heretofore undiscovered influence of surface energy on superspreading, demonstrate the utility of the QCM for measurements of dynamic wetting behavior.