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 similarsized 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.

Contact angle-based predictive model for slip at the solid–liquid interface of a transverse-shear mode acoustic wave device

“…27 Molecular slip is a plausible alternative scenario. In the following section, we consider the situation of molecular mobility as it is related to slip at a smooth, partially wetting, TSM surface.…”

“…2,26 While the no-slip boundary condition has been applied in many cases, it may not represent an accurate description for complex biomolecular adsorption in liquids. 27 The motional resistance R m is another experimental value that can be measured along with the series resonant frequency, f s . It is a measure of the amount of energy dissipated by any adsorbed mass or liquid at the surface, and can be thought of as due to the internal friction of a viscous or viscoelastic layer.…”

Novel transducer configurations for bulk acoustic wave sensors

“…1c) has been developed to improve upon the best aspects of the standard QCM sensor (Fig. 1a) while integrating the positive features of other acoustic wave sensors [7,8]. The MSCAT utilizes a photolithographically deposited planar spiral coil antenna that radiates a time varying electric field which penetrates the substrate.…”

The application of the acoustic spectrophonometer to biomolecular spectrometry: a step towards acoustic ‘fingerprinting’

“…That acoustic devices can provide information beyond mass detection and sensitivity are gradually being recognized (Lucklum and Hauptmann, 2000;McHale et al, 2000a, b;Thompson et al, 2000;Urbakh and Daikhin, 1998). These additional effects relate to changes in the conformation of the adsorbed molecules (Furtado et al, 1999;Jayarajah and Thompson, 2002;Lyle et al, 2002) resulting from changes in hydration, accessibility, functionality, activity, orientation and cooperative interactions arising from direct adsorption to the surface or subsequent interactions with other related molecular species.…”