Surface energy and the response of transverse acoustic wave devices in liquids

A magnetic acoustic resonator sensor was silanized with octadecyltrichloros ilane in order to produce a hydrophobic surface. Confirmation of the presen ce of the silane film was obtained from quantitative X-ray photoelectron sp ectroscopy and from measurement of advancing water contact angle. Frequency shifts for operation of the device in water, compared with air, were much smaller than for bare, untreated sensors. This result is consistent with an alogous experiments conducted with the thickness-shear mode acoustic wave s ensor. Atomic force microscopy showed that the cavities (depth, 2.9 nm; wid th, 11 nm) present on the bare surface numbered about 2860 per square micro meter. The calculated frequency shift associated with cavity-trapped water for the hydrophilic sensor was about half the value found by experimental m easurement, assuming all similar-sized cavities on the hydrophobic device a re filled with gas. Furthermore, since the cavities on the latter surface w ere largely filled by silane the level of supposed trapped gas was much red uced, leading to a gross overestimate of the possible air to water shift in frequency. The results of this work confirm that an alternative explanatio n for surface free energy effects connected to acoustic device responses is required.