Gordon L. Hayward

A transverse shear model of a piezoelectric chemical sensor

This model of a transverse shear-mode piezoelectric quartz-crystal chemical sensor is based on the fundamental equations of motion and piezoelectricity. The geometry includes the crystal as well as a metal electrode and a polymeric sensing layer immersed in a fluid medium. All of these are considered to be viscoelastic materials, and slip at the three interfaces is included. The model is shown to agree well with both experimental data and published correlations of the series resonant frequency and equivalent series resistance under both mass and liquid loading. In addition, the mechanical resonant behavior is also calculated. The interfacial slip parameter and the complex shear viscosity are shown to be mathematically interchangeable but not physically equivalent. Negative elastic coefficients obtained from the slip parameters for less viscous fluids suggests that interfacial slip is an important process, which dominates viscoelastic effects for these fluids.

Fuzzy logic applications

Fuzzy logic is a modeling method well suited for the control of complex and non-linear systems. This paper illustrates some of the power of fuzzy logic through a simple control example. For the analytical chemist, fuzzy logic incorporates imprecision from measurement noise as well as from linguistic process descriptions to produce operational control systems.

Simultaneous measurement of mass and viscosity using piezoelectric quartz crystals in liquid media

Abstract An inexpensive mass and viscosity sensor can be constructed from a quartz crystal microbalance using an automatic gain control oscillator. The gain control voltage, which maintains a constant oscillation amplitude, is shown to be affected only by the energy loss to the liquid medium and the crystal mounting. The frequency shift between a loaded and an unloaded crystal operating in the same medium is shown to closely follow the Sauerbrey equation. A viscosity or density correction based on the gain control voltage can be applied to the measured frequency shift to obtain the mass loading. Errors due to the oscillator phase shift and the crystal mounting losses can be removed by calibration.

Interfacial slip on a transverse-shear mode acoustic wave device

This article describes a mathematical relationship between the slip parameter α and the slip length b for a slip boundary condition applied to the transverse-shear model for a quartz-crystal acoustic wave device. The theory presented here reduces empirical determination of slip to a one-parameter fit. It shows that the magnitude and phase of the slip parameter, which describes the relative motion of the surface and liquid in the transverse-shear model, can be linked to the slip length. Furthermore, the magnitude and phase of the slip parameter are shown to depend on one another. An experiment is described to compare the effects of liquid-surface affinity on the resonant properties of a transverse-shear mode wave device by applying different polar and nonpolar liquids to surfaces of different polarity. The theory is validated with slip values determined from the transverse-shear model and compared to slip length values from literature. Agreement with literature values of slip length is within one order of ...

Acoustic coupling of transverse waves as a mechanism for the label-free detection of protein–small molecule interactions

An on-line acoustic transverse wave device has been used to study the binding interactions of human serum albumin with the small molecule drug, warfarin. Four linking systems for the covalent attachment of the protein to the surface of the gold electrode of the sensor were employed, namely thioctic acid, cysteamine, an N-hydroxysuccinimide ester and 11-mercaptoundecanoic acid. All the attachment protocols involve the ability of thiols to form gold-sulfur bonds at the metal surface. The functional group present at the distal end of each thiol was chemically activated in order to facilitate covalent attachment of the protein. On-line sensor measurements of acoustic parameters show that the binding of warfarin to the protein can be detected, and depending on the linking monolayer used three of four possible combinations of changes in series resonance frequency and motional resistance are observed. Calculations of possible mass and thickness viscoelastic effects demonstrate that these conventional notions are invalid in terms of an explanation of the acoustic signals observed for the warfarin-protein interaction. The responses are ascribed to acoustic coupling phenomena.

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

We have revisited the Blake–Tolstoi theory [Coll. Surf. 47, 135 (1990)] for molecular and hydrodynamic slip and applied it to the fundamental description of acoustic wave devices coupled to a liquid of finite thickness. The aim is to provide a framework for a predictive model for slip, based on surface–liquid interactions and contact angle. This theory provides a description of slip that links hydrodynamic boundary slip to a schematic, molecular description involving the wettability of the liquid–solid interface. We redevelop the model, using current acoustic sensors notation, then evaluate its qualitative behavior as a predictive model for slip length in the context of acoustic wave devices. Finally, we discuss the limitations of the model and consider the advantages of a predictive model for boundary slip.

Measurement of solubilities in supercritical fluids using a piezoelectric quartz crystal

Abstract Solubility data is essential for any application of supercritical fluids. A new technique has been developed using a piezoelectric quartz crystal to measure solubilities. A small mass of solute is deposited on the crystal and solubility is measured by observing the crystal’s frequency change as this solute dissolves in the supercritical fluid. The technique is ideally suited to solutes which exhibit low solubilities. Solubility measurements of bis(acetylacetonato)copper(II) (Cu(acac) 2 ) in supercritical carbon dioxide were in good agreement with existing literature values. New solubility data were also measured for bis(thenoyltrifluoroacetonato)copper(II) (Cu(tta) 2 ).

Fuzzy predictor for fermentation time in a commercial brewery

Abstract A study was conducted at a commercial brewery to investigate the feasibility of an expert-system control strategy that used fuzzy logic to make inferences based on uncertain process information. The predictor made an initial estimate of fermentation time based on the yeast variables (pitching rate, pitched volume and viability). The predicted fermentation times were within 24 h of the actual fermentation time for 9 batches in the validation data set of 13 process histories. A second rule set followed the pH and specific gravity during fermentation and predicted the level of vicinal diketone (VDK). When the VDK level reached a threshold level, a third rule set updated the estimate of time required to complete the fermentation. Seven data sets were used to validate the predictions based on VDK level and 4 estimates were within 24 h of the recorded fermentation release times. All 7 predictions were within 32 h of the actual release time and 6 of the 7 estimates indicated that the batches were held longer than necessary.

Surface energy and the response of transverse acousticwave devices in liquids

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.

Simplex optimization of acoustic assay for plasminogen activators

This article discusses the optimization of a newly developed method for measuring the activity of plasminogen activators using a thickness-shear-mode acoustic sensor. A variable-size simplex algorithm was used for optimization. Preliminary tests were performed to design the first simplex. A desirability function was defined to translate each performance value to a membership value of 0 to 1. If there was more than one performance variable, their membership values were translated to an aggregated membership value using another function that considers their individual influence on sensor performance. Two rounds of optimization were carried out for streptokinase followed by a single optimization for tissue-type plasminogen activator. In the last optimization, ratios of control variables were used in order to reduce the number of parameters and to formulate easily adjustable assay conditions. The results showed the usefulness of the simplex method for optimizing this type of assay, and the importance of preliminary tests and prior knowledge in providing rapid convergence using fewer experiments. The optimized plasminogen activator assay can be considered a reference method for measurement of all members of this drug class.