Gel-coated Lamb wave sensors

Abstract

We report on initial experiments and theory describing the operation of gel-coated Lamb wave sensors. The lowest-order flexural Lamb wave can propagate in gel-coated plates with low attenuation. This allows some new sensing approaches to be investigated, such as: (1) Using gels as filters for larger particles and molecules, while allowing smaller analytes to diffuse through to the sensor surface, e.g., for in vitro biosensing; (2) Realizing sensors based on gel density and viscoelastic changes. We develop two theories to predict the response of the sensor to the viscoelastic properties of the gel, and find excellent agreement between predictions from these theories and those from a numerical simulation program for waves in multi-layered media. One uses a Rayleigh-type solution with modified boundary conditions to derive the mechanical radiation impedance presented to the plate surface by the gel, and from this impedance calculates the phase velocity. The other uses transmission line theory originally developed by Oliner et al.

Monitoring of Soft Deposition Layers in Liquid-Filled Tubes with Guided Acoustic Waves Excited by Clamp-on Transducers

“…Considering the gelatin layer to be a viscoelastic layer, a larger damping of both wave packages compared to pure water and steel as media of sound transmission was to be expected [ 11 , 26 , 27 ], which explains the amplitude reduction observed in both cases. The retardation of the A 0 Lamb-type mode on the tube wall (component W) due to a viscoelastic layer is understandable as well from previous investigations [ 12 , 13 , 27 , 28 ] and from investigations with similar types of surface acoustic waves. With circumferential shear-horizontal type waves for example, Rose [ 17 ] reported a reduction of the velocity on a steel tube of 0.3% at a layer thickness of 180 µm, when a viscoelastic bitumenous layer was applied on the outer wall surface.…”

“…As outlined by Vellekoop [ 29 ], the mass increase resulting from the deposition of gelatin may contribute to the velocity change of component W. According to the formulas of the mass sensitivity of the Lamb wave velocity given therein, however, only a very small change of the velocity below 0.1% is expected for the system investigated in this study. This is mainly caused by the large thickness of the tube wall compared to the deposition layer in contrast to Lamb wave devices using flexural waves on very thin membranes (2 to 4 µm) covered with gel layers of 250 µm and which are operated at frequencies in the order of several MHz [ 28 ].…”

Frequency-independent and frequency-dependent polymer transitions observed on flexural plate wave ultrasonic sensors

“…(When discussing planar acoustic sensors, the modulus referred to is always the shear modulus.) The sensitivities of the FPW device to these properties in thick gels have been modeled theoretically (1). A decrease in modulus causes frequency to decrease.…”

“…Flexural plate wave (FPW) devices have demonstrated potential for a variety of sensing applications (1)(2)(3)(4)(5)(6)(7)(8)(9). As gravimetric sensors, these devices offer high mass sensitivity at low operating frequencies.…”

“…In this application a thin sorbent film, often a polymer, is applied to the device surface to collect and concentrate vapor molecules from the gas phase. Flexural plate wave devices also function in the liquid phase or with aqueous gels applied (1), and thus have clear potential for use as biosensors. In addition, physical properties of liquids such as density and viscosity can be sensed (4).…”