Gas turbines, diesels and other heavy-machinery engines are omnipresent in today's society. Their efficiency and safety strongly depend on the condition of the lubricants and fuels that are necessary to operate the machinery. During storage, transportation and operations these - typically complex hydrocarbon - substances suffer from contamination and wear, i.e. chemical/molecular-structure transformation.
Even though the consequences of uneffective lubrication can be catastrophic , there currently exists not a single commercially available sensor that could reliably monitor in real time chemical or structural conditions of lubricants and fuels used in or supplied to the engines.
We use optical absorption, and excitation-emission matrix (EEM) fluorescence,to find a representative spectroscopic indicator of the degradation/contamination-caused chemical and structural changes in certain hydrocarbons. Our fiber-coupled sensor can be used in real-time in a running engine to determine the effectiveness of the lubricants. We use chemometric techniques, i.e. Parallel Factor Analysis (PARAFAC) and Principal Component Analysis and Regression (PCA\PCR), to assess the contamination and degradation level.
In a related project we were investigating the stability of the scintillator liquids used in one of the upcoming SNOLAB experiments for its stability against oxidation, thermal decomposition and photochemical degradation.
Ongoing research is directed towards the development of a much faster EEM spectrometer that allows for an acquisition of full 100 x 1000 pixel (= EEM wavelength pair) spectra at less than 10 seconds per EEM spectrum. this instrument will permit kinetic studies on liquids that degrade on the timescale of minutes.
A very smooth glass sphere with a diameter of a few tens to hundreds of microns is an excellent optical cavity. Light can be trapped in the "whispering gallery modes", i.e. modes that span the "equator" of such a sphere. The spectrum of such a cavity tends to be very sparse, since an integer number of wavelengths has to fit around the sphere for resonance to occur. On the other hand, the finesse of the microresonator cavity is excellent with Q-factors that frequently exceed 10,000,000 and are typically 100,000,000.
We have shown that measurements of the ring-down time using the phase shift cavity ring down technique are possible on these tiny cavities. We also demonstrated that one can measure the absolute absorption cross section of a vibrational overtone is possible on these cavities even if the molecules that are adsorbed from less than one monolayer.
With Gianluca Gagliardi (INO, Naples, IT) we published on the detection of analytes in liquid droplet resonators using similar detection principles.
Hollow-Core PCF Interferometry
We are interested in the use of hollow-core photonic crystal fibers (PCFs) for gas refractive index sensing. Traditional Mach-Zehnder interferometers consist of two separate waveguiding 'arms' of equal length, one reference and one sample arm. In an in-fiber MZ, these two arms exist in the same length of fiber, but occupy two different fiber modes. By filling the hollow core of a PCF with a specific gasses, it is possible to accurately measure the gas index to six decimal places through the interference of fiber modes.
Photoacoustic Detection of Trace Contaminants
When pulses of light energy absorbed by a sample are converted rapidly into heat, a sound wave can be produced through periodic thermal expansion. The intensity of the sound wave directly relates to the amount of absorber in the sample. Since the signal is produced directly from the absorber, as opposed to inferring it from the absence of signal (as in absorbance spectroscopy), it is well-suited for measuring trace contaminants.
Typically, this sound wave is recorded with a sensitive microphone. Impressive detection limits have been obtained, making photoacoustic techniques popular in trace gas detection. In our lab, we use a fiber Fabry-Perot (FFP) cavity in conjunction with the Pound-Drever-Hall (PDH) technique as an optical microphone to record the sound wave.
We have previously used this method to detect analytes in microfluidic devices and in capillary electrophoresis. Recently we use acoustic resonators such as a wineglass or cantilever beams to further amplify the signal.
Red wine excited with a 445 nm laser
Self-Assembly Kinetics using SPR
Surface plasmon resonance (SPR) is a popular label-free technique used to measure chemical binding, including kinetics and binding affinity. Surface binding causes a change in the local electronic environment, which affects the properties of nearby surface electrons (plasmons) and can be measured optically in real-time. SPR is ideally suited for studying surface reactions, for it is most sensitive to changes within 200 nm of the surface.
Using a lab built Kretschmann-style SPR instrument, we are investigating the model of N-heterocyclic carbenes binding to gold surfaces. An alternative to thiol functionalization, N-heterocyclic carbenes (NHCs) have the added benefit of resilience against harsh conditions when bound to a gold surface . However, their self-assembly on gold is not fully understood.
Kinetic studies of this nature help us understand the lifetime, useful conditions and limitations of NHC-based (bio)sensors.
 Crudden, C. et al. Nature Chemistry 2014, 6, 409.
Fiber-optic vibration sensors
Fiber Bragg gratings (FBGs) have previously found many applications as strain and vibration sensors and several commercial systems exist. Our group demonstrated that one can use an optical cavity written into a conventional single mode fiber to measure strain and vibration over a very large range of amplitudes and a massive frequency range - from static strain to medical ultrasound frequencies (over 6 MHz).
Their very high fidelity and extremely low noise permit their use as pickups for musical instruments and, specifically, for acoustic guitars . By fixing the fiber strain sensor to a vibrating part of the instrument's body, e.g., near the bridge of an acoustic guitar or on the headstock of a solid-body guitar, sound recordings were made and compared to those obtained with either piezoelectric pickups or with magnetic induction pickups. Paul Langlois of the Tragically Hip helped us with the demonstration of the prototype. (see also our youtube channel)
Ongoing research explores the use of these sensors in the characterization of acoustic properties of tissues and tissue phantoms in medical ultrasound imaging.