An acoustic technique for the monitoring of dynamic wetting behavior

A small strips of a viscous fluid deposited on a high-energy surface spread s with an increasing contact width that follows a characteristic 1/7th powe r law with time. At any instant, the fluid has a spherical cap cross-sectio nal profile with well-defined values of the spherical radius and dynamic co ntact angle. The evolution of such stripes has been followed using optical interferometry. Simultaneously, the changing contact area has been monitore d using an alternative method based on high-frequency (similar to 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 mo nitoring 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 o ptically measured parameters and are interpreted using a viscoelastic model of the fluid.