Nicola Doy

Density-Viscosity Product of Small-Volume Ionic Liquid Samples Using Quartz Crystal Impedance Analysis

Quartz crystal impedance analysis has been developed as a technique to assess whether room-temperature ionic liquids are Newtonian fluids and as a small-volume method for determining the values of their viscosity-density product, rho eta. Changes in the impedance spectrum of a 5-MHz fundamental frequency quartz crystal induced by a water-miscible room-temperature ionic liquid, 1-butyl-3-methylimiclazolium trifluoromethylsulfonate ([C4mim][OTf]), were measured. From coupled frequency shift and bandwidth changes as the concentration was varied from 0 to 100% ionic liquid, it was determined that this liquid provided a Newtonian response. A second water-immiscible ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethanesulfonyl)imide [C4mim][NTf2], with concentration varied using methanol, was tested and also found to provide a Newtonian response. In both cases, the values of the square root of the viscosity-density product deduced from the small-volume quartz crystal technique were consistent with those measured using a viscometer and density meter. The third harmonic of the crystal was found to provide the closest agreement between the two measurement methods; the pure ionic liquids had the largest difference of approximately 10%. In addition, 18 pure ionic liquids were tested, and for 11 of these, good-quality frequency shift and bandwidth data were obtained; these 12 all had a Newtonian response. The frequency shift of the third harmonic was found to vary linearly with square root of viscosity-density product of the pure ionic liquids up to a value of square root(rho eta) approximately 18 kg m(-2) s(-1/2), but with a slope 10% smaller than that predicted by the Kanazawa and Gordon equation. It is envisaged that the quartz crystal technique could be used in a high-throughput microfluidic system for characterizing ionic liquids.

Small volume laboratory on a chip measurements incorporating the quartz crystal microbalance to measure the viscosity-density product of room temperature ionic liquids.

A microfluidic glass chip system incorporating a quartz crystal microbalance (QCM) to measure the square root of the viscosity-density product of room temperature ionic liquids (RTILs) is presented. The QCM covers a central recess on a glass chip, with a seal formed by tightly clamping from above outside the sensing region. The change in resonant frequency of the QCM allows for the determination of the square root viscosity-density product of RTILs to a limit of approximately 10 kg m(-2) s(-0.5). This method has reduced the sample size needed for characterization from 1.5 ml to only 30 mul and allows the measurement to be made in an enclosed system.

SU-8 Guiding Layer for Love Wave Devices

SU-8 is a technologically important photoresist used extensively for the fabrication of microfluidics and MEMS, allowing high aspect ratio structures to be produced. In this work we report the use of SU-8 as a Love wave sensor guiding layer which allows the possibility of integrating a guiding layer with flow cell during fabrication. Devices were fabricated on ST-cut quartz substrates with a single-single finger design such that a surface skimming bulk wave (SSBW) at 97.4 MHz was excited. SU-8 polymer layers were successively built up by spin coating and spectra recorded at each stage; showing a frequency decrease with increasing guiding layer thickness. The insertion loss and frequency dependence as a function of guiding layer thickness was investigated over the first Love wave mode. Mass loading sensitivity of the resultant Love wave devices was investigated by deposition of multiple gold layers. Liquid sensing using these devices was also demonstrated; water-glycerol mixtures were used to demonstrate sensing of density-viscosity and the physical adsorption and removal of protein was also assessed using albumin and fibrinogen as model proteins.

Separate density and viscosity determination of room temperature ionic liquids using dual Quartz Crystal Microbalances

The drive towards cleaner industrial processes has led to the development of room temperature ionic liquids (RTIL) as environmentally friendly solvents. They comprise solely of ions which are liquid at room temperature and with over one million simple RTIL alone it is important to characterize their physical properties using minimal sample volumes. Here we present a dual Quartz Crystal Microbalance (QCM) which allows separate determination of viscosity and density using a total sample volume of only 240µL. Liquid traps were fabricated on the sensing area of one QCM using SU-8 10 polymer with a second QCM having a flat surface. Changes in the resonant frequencies were used to extract separate values for viscosity and density. Measurements of a range of pure RTIL with minimal water content have been made on five different trap designs. The best agreement with measurements from the larger volume techniques was obtained for trap widths of around 50 µm thus opening up the possibility of integration into lab-on-a-chip systems.

Determination of the physical properties of room temperature ionic liquids using a love wave device

In this work, we have shown that a 100 MHz Love wave device can be used to determine whether room temperature ionic liquids (RTILs) are Newtonian fluids and have developed a technique that allows the determination of the density-viscosity product, ρη, of a Newtonian RTIL. In addition, a test for a Newtonian response was established by relating the phase change to insertion loss change. Five concentrations of a water-miscible RTIL and seven pure RTILs were measured. The changes in phase and insertion loss were found to vary linearly with the square root of the density-viscosity product for values up to (ρη)(1/2) ~ 10 kg m(-2) s(-1/2). The square root of the density-viscosity product was deduced from the changes in either phase or insertion loss using glycerol as a calibration liquid. In both cases, the deduced values of ρη agree well with those measured using viscosity and density meters. Miniaturization of the device, beyond that achievable with the lower-frequency quartz crystal microbalance approach, to measure smaller volumes is possible. The ability to fabricate Love wave and other surface acoustic wave sensors using planar metallization technologies gives potential for future integration into lab-on-a-chip analytical systems for characterizing ionic liquids.

Density and viscosity measurements of room temperature ionic liquids using patterned Quartz Crystal Microbalances

Ionic liquids are becoming of increasing interest for an extensive range of applications. Small scale characterization processes are being continually researched to find cheap and efficient methods for processing ever smaller sample volumes. This work presents a dual Quartz Crystal Microbalance (QCM) setup with one smooth, and one patterned surface using chemically compatible materials allowing separate viscosity and density measurements of room temperature ionic liquids. Measurements were corroborated with standard measurement techniques and show good agreement, demonstrating the merit of the dual QCM setup in determining the physical properties of these exciting new solvents.

Layer guided surface acoustic wave sensors using langasite substrates

The use of acoustic wave sensors for industrial applications is widespread. At present there are few sensors for assessing fluid properties which are capable of operating at temperatures in excess of 500°C. In this work we present surface acoustic wave devices fabricated on Langasite substrates as possible candidates for such sensors. Two port delay line devices are produced and investigated in terms of temperature and their ability to measure viscosity-density properties of liquids. Single port resonator devices are fabricated and a polymer guiding layer applied to enhance sensitivity. A sharp resonance is seen for a guiding layer thickness of 4.2µm and the mass sensitivity is assessed by depositing layers of gold onto its surface. This sensitivity is found to 749 Hz·ng−1·cm−2 which is several orders of magnitude higher that that for a thickness shear mode device produced on the same substrate. By further developing these devices with particular focus on the reflector arrangement on the single port resonator devices, highly sensitive sensors for temperatures in excess of 900°C may be produced which will be suitable for use with automated data processing.

Small volume determination of the viscosity-density product for ionic liquids using quartz crystal harmonics

Data for the physical properties of room temperature ionic liquids (RTIL) as a function of chemical composition is limited, owing to the expense and difficulty of producing large volumes of pure samples for characterization. In this work we demonstrate that the viscosity-density values, obtained using impedance analysis of a quartz crystal microbalance are consistent with those obtained using a viscometer and density meter, but only requires a sample volume two orders of magnitude smaller. We also demonstrate that the third harmonic yields closest correlation out of all the harmonics from the fundamental to the eleventh.