Particle packing structure in a rectangular micro-capillary

Packed beds of micro-sized particles may be used for enhancing heat and mass transfer in microfluidic devices where Reynolds numbers are small. By increasing the surface area per unit volume, such micro-packed beds can also be used to significantly enhance the functionality of such devices. As the effectiveness of a packed bed is dictated by its packing structure, it is of interest to understand this structure and how it is affected by system parameters such as the channel-to-particle ratio - this has received little attention for micro-packed beds. In the work reported here, X-ray micro-computed tomography and image analysis is used to determine the packing structure of micro-packed beds of glass particles for channel-to-particle ratios of 5.19, 5.80, 6.56 and 7.55 in a channel of D = 200 mum square cross-section. The bed-average porosity was found to be far higher than macroscale PBs, vary from approximately 67% to 60% as the channel-to-particle ratio increased, with the packing structure and porosity varying both across the bed width and along its length. The former clearly arises from the effect of the wall, whilst the latter arises from the sedimentation method used to create the packed bed.M. Navvab Kashani, V. Zivkovic, Z. Alwahabi and M.J. Bigg

Importance of surface forces in a micro-fluidized bed

Fluidized beds (FBs) potentially offer a means of significantly enhancing heat and mass transfer under the low Reynolds number flows that prevail in microfluidic devices. However, as surface forces start to dominate over gravity at the microscale, adhesion of particles to the walls of the micro-FB can prevent fluidization. We used the acid-base theory of van Oss, Chaudhury and Good to understand adhesion forces at the particlewall interface in a micro-FB. This approach, novel to the fluidization field, successfully predicts the adhesion problem in a micro-FB system as verified by our experimental results for liquid fluidization in a typical PDMS microfluidics channel. This demonstrates the importance of considering surface forces in the design of micro-FB, unlike for traditional large FBs where surface forces are often less of an issue.M.J. Biggs, V. Zivkovic, M.N. Kashani and Z. Alwahab

Design and application of near-field applicators for efficient microwave-assisted laser-induced breakdown spectroscopy

Laser-induced breakdown spectroscopy (LIBS) can benefit from sustaining laser generated plasma with microwaves to enhance elemental detection sensitivity. To achieve efficient microwave coupling, critical factors, such as the electromagnetic environment and reflection coefficient of the coupling device, need to be considered to quantitatively predict the electric-field strength in the plasma location. 3D full-wave electromagnetic simulations were used to design near-field microwave applicators suitable to maximize microwave coupling into the short-lived laser-induced plasmas. The simulations pointed out to four effective and practical designs containing varieties of isolation techniques. The four developed microwave applicators were then used to improve the detection of copper present in a mineral ore solid sample, using LIBS and imaging techniques simultaneously. It was found that, with 1.2 kW microwave power, an applicator design with a 30 mm diameter ground plane can significantly boost the signal of copper line 324.754 nm with a factor of 849, which is, to the authors' best knowledge, the highest reported value. Furthermore, an outstanding signal to noise ratio of 166 was recorded in a solid sample containing a certified 3.38 μg g⁻¹ copper concentration.Shengjian Jammy Chen, Adeel Iqbal, Matthew Wall, Christophe Fumeaux and Zeyad T. Alwahab

Mid-IR polarization spectroscopy applied for detection of methane at atmospheric pressure

Methane was studied with mid-infrared polarization spectroscopy (PS) in an atmospheric gas jet. Detection limit was investigated and laser induced fluorescence and PS spectra for the P, Q, and R branch are presented

High resolution polarization spectroscopy and laser induced fluorescence of CO(2) around 2 m micrometers

High resolution Infrared Polarisation Spectroscopy (IRPS) and Infrared Laser Induced Fluorescence (IRLIF) techniques were used to probe CO 2/N2 binary gas mixture at atmospheric pressure and ambient temperature. The probed CO2 molecules were prepared by laser excitation to an overtone and combination ro-vibrational state (1201, J=15) of CO2, centred at 4988.6612 cm-1. IRPS and IRLIF line profiles were recorded for several CO2/N2 binary mixtures. The observed IRLIF line shapes have the expected Lorentzian form while the observed IRPS line shapes are narrower by a factor of two than those recorded with the IRLIF and appear to have a Lorentzian-cubed profile. The recorded line profiles provide measurements of the pressure-broadening coefficient directly at atmospheric pressure. The Full-Width-Half-Maxima (FWHM) pressure broadening coefficients are measured, based on IRLIF, to be 0.2174±0.0092 cm-1atm-1 and 0.1327 ±0.0077 cm-1atm-1 for self- and N2 collision broadening, respectively. The broadening coefficients obtained based on IRPS were measured to be ~8% larger than those obtained with IRLIF. © EDP Sciences/Società Italiana di Fisica/Springer-Verlag 2007.Z.T. Alwahabi, J. Zetterberg, Z.S. Li and M. Aldé