An automated system for performing continuous viscosity versus temperature measurements of fluids using an Ostwald viscometer.

An automated system was developed to measure the viscosity of fluids as a function of temperature using image analysis tracking software. An Ostwald viscometer was placed in a three-wall dewar in which ethylene glycol was circulated using a thermal bath. The system collected continuous measurements during both heating and cooling cycles exhibiting no hysteresis. The use of video tracking analysis software greatly reduced the measurement errors associated with measuring the time required for the meniscus to pass through the markings on the viscometer. The stability of the system was assessed by performing 38 consecutive measurements of water at 42.50 ± 0.05 °C giving an average flow time of 87.7 ± 0.3 s. A device was also implemented to repeatedly deliver a constant volume of liquid of 11.00 ± 0.03 ml leading to an average error in the viscosity of 0.04%. As an application, the system was used to measure the viscosity of two Li-ion battery electrolyte solvents from approximately 10 to 40 °C with results showing excellent agreement with viscosity values calculated using Gering's Advanced Electrolyte Model (AEM).

Cantilever-based sensing: the origin of surface stress and optimization strategies.

Many interactions drive the adsorption of molecules on surfaces, all of which can result in a measurable change in surface stress. This article compares the contributions of various possible interactions to the overall induced surface stress for cantilever-based sensing applications. The surface stress resulting from adsorption-induced changes in the electronic density of the underlying surface is up to 2-4 orders of magnitude larger than that resulting from intermolecular electrostatic or Lennard-Jones interactions. We reveal that the surface stress associated with the formation of high quality alkanethiol self-assembled monolayers on gold surfaces is independent of the molecular chain length, supporting our theoretical findings. This provides a foundation for the development of new strategies for increasing the sensitivity of cantilever-based sensors for various applications.

Measuring the cantilever-position-sensitive detector distance and cantilever curvature for cantilever sensor applications.

Measuring cantilever sensor deflections using an optical beam deflection system is more complicated than often assumed. The direction of the reflected beam is dependent on the surface normal of the cantilever, which in turn is dependent on the state of the cantilever. It is often assumed that the cantilever is both straight and perfectly level before the onset of sensing experiments although this assumption, especially the former, is rarely true. Failure to characterize the initial state of the cantilever can lead to irreproducibility in cantilever sensor measurements. We have developed three new methods for characterizing the initial state of the cantilever. In the first case we show how to define the initial angle of inclination beta of the chip on which the cantilever is attached. This method was tested using an aluminum block with a known angle of inclination. A new method for determining the initial distance L(o) between the cantilever and the position-sensitive detector (PSD) is also presented. This parameter which behaves as an amplification factor of the PSD signal is critical for obtaining precise cantilever sensor data. Lastly, we present a method for determining the initial curvature of the cantilever which often results from depositing the sensing platform on the lever. Experiments conducted using deflected cantilevers showed the model to be accurate. The characterization methods presented in this work are simple to use, easy to implement, and can be incorporated into most cantilever sensor setups.

A complete analysis of the laser beam deflection systems used in cantilever-based systems.

A working model has been developed which can be used to significantly increase the accuracy of cantilever deflection measurements using optical beam techniques (used in cantilever-based sensors and atomic force microscopes), while simultaneously simplifying their use. By using elementary geometric optics and standard vector analysis it is possible, without any fitted or adjustable parameters, to completely and accurately describe the relationship between the cantilever deflection and the signal measured by a position sensitive photo-detector. By arranging the geometry of the cantilever/optical beam, it is possible to tailor the detection system to make it more sensitive at different stages of the cantilever deflection or to simply linearize the relationship between the cantilever deflection and the measured detector signal. Supporting material and software has been made available for download at http://www.physics.mun.ca/beauliu_lab/papers/cantilever_analysis.htm so that the reader may take full advantage of the model presented herein with minimal effort.

Surface stress, kinetics, and structure of alkanethiol self-assembled monolayers.

The surface stress induced during the formation of alkanethiol self-assembled monolayers (SAMs) on gold from the vapor phase was measured using a micromechanical cantilever-based chemical sensor. Simultaneous in situ thickness measurements were carried out using ellipsometry. Ex situ scanning tunneling microscopy was performed in air to ascertain the final monolayer structure. The evolution of the surface stress induced during coverage-dependent structural phase transitions reveals features not apparent in average ellipsometric thickness measurements. These results show that both the kinetics of SAM formation and the resulting SAM structure are strongly influenced both by the surface structure of the underlying gold substrate and by the impingement rate of the alkanethiol onto the gold surface. In particular, the adsorption onto gold surfaces having large, flat grains produces high-quality self-assembled monolayers. An induced compressive surface stress of 15.9 +/- 0.6 N/m results when a c(4x2) dodecanethiol SAM forms on gold. However, the SAMs formed on small-grained gold are incomplete and an induced surface stress of only 0.51 +/- 0.02 N/m results. The progression to a fully formed SAM whose alkyl chains adopt a vertical (standing-up) orientation is clearly inhibited in the case of a small-grained gold substrate and is promoted in the case of a large-grained gold substrate.

Measuring thickness changes in thin films due to chemical reaction by monitoring the surface roughness with in situ atomic force microscopy.

Measuring the changing thickness of a thin film, without a reference, using an atomic force microscope (AFM) is problematic. Here, we report a method for measuring film thickness based on in situ monitoring of surface roughness of films as their thickness changes. For example, in situ AFM roughness measurements have been performed on alloy film electrodes on rigid substrates as they react with lithium electrochemically. The addition (or removal) of lithium to (or from) the alloy causes the latter to expand (or contract) reversibly in the direction perpendicular to the substrate and, in principle, the change in the overall height of these materials is directly proportional to the change in roughness. If the substrate on which the film is deposited is not perfectly smooth, a correction to the direct proportionality is needed and this is also discussed.