A surface acoustic wave technique for the observation of dynamic wetting

The spreading of small quantities of viscous oils has traditionally been studied using side views and plan views to obtain contact angles and profiles. The techniques employed have included optical profiling, interferometry, x-ray reflectivity and ellipsometry. In this work we report an alternative method, based on surface acoustic waves, which has the advantage of working directly within the spreading piane. In these experiments 150 ns pulses of 250 MHz surface acoustic waves are emitted along the solid-vapour interface towards a stripe of viscous oil. Both reflections from the advancing front edge of the oil and from a double transit signal which passes through the oil are observed. As the stripe becomes wider, changes in surface acoustic wave path lengths occur and these are measured using a phase comparison method. Results for the spreading of a small stripe of 100000 cS polydimethylsiloxane oil are presented. The results for the time dependence of the width of the stripe agree with both theory and optical experiments.