An acoustic technique for the monitoring of dynamic wetting behavior

A small stripe of a viscous fluid deposited on a high-energy surface spreads with an increasing contact width that follows a characteristic 1/7th power law with time. At any instant, the fluid has a spherical cap cross-sectional profile with well-defined values of the spherical radius and dynamic contact angle. The evolution of such stripes has been followed using optical interferometry. Simultaneously, the changing contact area has been monitored using an alternative method based on high-frequency (∼ 170 MHz) surface acoustic waves. Such waves have their energy confined to within one wavelength of the surface and are potentially an in-plane technique for monitoring dynamic wetting. Acoustic signals that originate from reflections from the advancing oil, and from transit along the solid-liquid interface, are reported. The changes in these acoustic signals are compared with the optically measured parameters and are interpreted using a viscoelastic model of the fluid.