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

Abstract

A magnetic acoustic resonator sensor was silanized with octadecyltrichlorosilane in order to produce a hydrophobic surface. Confirmation of the presence of the silane film was obtained from quantitative X-ray photoelectron spectroscopy 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 analogous experiments conducted with the thickness-shear mode acoustic wave sensor. Atomic force microscopy showed that the cavities (depth, 2.9 nm; width, 11 nm) present on the bare surface numbered about 2860 per square micrometer. The calculated frequency shift associated with cavity-trapped water for the hydrophilic sensor was about half the value found by experimental measurement, assuming all similar-sized cavities on the hydrophobic device are filled with gas. Furthermore, since the cavities on the latter surface were largely filled by silane the level of supposed trapped gas was much reduced, leading to a gross overestimate of the possible air to water shift in frequency. The results of this work confirm that an alternative explanation for surface free energy effects connected to acoustic device responses is required.