Design, modeling and characterization of microfluidic architectures for high flow rate, small footprint microfluidic systems

We propose a strategy for optimizing distribution of flow in a microfluidic chamber for microreactor, lateral flow assay and immunocapture applications. It is aimed at maximizing flow throughput, while keeping footprint, cell thickness, and shear stress in the distribution channels at a minimum, and offering a uniform flow field along the whole analysis chamber. In order to minimize footprint, the traditional tree-like or "rhombus" design, in which distribution microchannels undergo a series of splittings into two subchannels with equal lengths and widths, was replaced by a design in which subchannel lengths are unequal, and widths are analytically adapted within the Hele-Shaw approximation, in order to keep the flow resistance uniform along all flow paths. The design was validated by hydrodynamic flow simulation using COMSOL finite element software. Simulations show that, if the channel is too narrow, the Hele-Shaw approximation loses accuracy, and the flow velocity in the chamber can fluctuate by up to 20%. We thus used COMSOL simulation to fine-tune the channel parameters, and obtained a fluctuation of flow velocity across the whole chamber below 10%. The design was then implemented into a PDMS device, and flow profiles were measured experimentally using particle tracking. Finally, we show that this system can be applied to cell sorting in self-assembling magnetic arrays, increasing flow throughput by a factor 100 as compared to earlier reported designs.     

Depolarized light scattering,flow birefringence and local order in molecular liquids

We propose a generalization of De Gennes' theory of flow birefringence [3] to the case of two distinct local anisotropy variables and also some extensions to the existing three variable theories of light scattering spectra. We show that the application of the theories with one or with two local anisotropy variables to analyse flow birefringence and light scattering spectra give numerically different results. We also show that it is possible to determinewhether the second local anisotropy variable is a primary or a secondary variable.

The VH and HH light scattering spectra of a number of symmetric top molecules are analysed. They show an unambiguous ‘shear mode coupling’ down to very small values of (q ²η/ρΦ). These results are discussed and compared to flow birefringence experiments in the light of our theoretical considerations. The three variable correction is shown to be small and within experimental error for all liquids studied except one (pyridine). The flow birefringence coefficient for CCl₄ is measured and found to be negative as predicted by our theory. Limitations to the formalism of generalized hydrodynamics are also discussed briefly.     

A mathematical model of aging-related and cortisol induced hippocampal dysfunction

Background  

The hippocampus is essential for declarative memory synthesis and is a core pathological substrate for Alzheimer's disease (AD), the most common aging-related dementing disease. Acute increases in plasma cortisol are associated with transient hippocampal inhibition and retrograde amnesia, while chronic cortisol elevation is associated with hippocampal atrophy. Thus, cortisol levels could be monitored and managed in older people, to decrease their risk of AD type hippocampal dysfunction. We generated an in silicomodel of the chronic effects of elevated plasma cortisol on hippocampal activity and atrophy, using the systems biology mark-up language (SBML). We further challenged the model with biologically based interventions to ascertain if cortisol associated hippocampal dysfunction could be abrogated.     

Versatile method for electroosmotic flow measurements in microchip electrophoresis

A novel versatile method for the determination of low or high electroosmotic mobility values in microdevices of variable microchannel design is presented. The electroosmotic flow (EOF) calculation is based on the difference between the apparent and effective mobilities of a reference compound. The proposed method uses microchip frontal electrophoresis for the determination of these mobilities. This requires simple monochannel microchip design and demonstrates versatile and time-saving procedure when compared to conventional current monitoring method when measuring low EOF. It has been applied successfully to the characterization of different coating procedure in glass and poly(dimethylsiloxane) microchips.     

Chain retraction,length fluctuations,and small-angle neutron scattering in strained polymer melts

We propose a possible explanation for an apparent contradiction between the Doi–Edwards (DE) theory and time-resolved small-angle neutron scattering experiments. The original DE theory predicts that a chain in a melt subjected to a step-elongation undergoes a reduction of its dimensions in both the parallel and perpendicular directions to the strain axis, before returning to an isotropic conformation by reptation. In the present paper, we propose a crude model to compare the relative effect of retraction and of the chain length fluctuations, introduced later by Doi to explain the M power law of viscosity of polymer melts. We show that length fluctuations are able to screen retraction in most experimental situations available to small-angle neutron scattering, whereas that is not the case for viscoelasticity. Quantitative predictions are consistent with the experimental data presently available.     

Definition and measurement of the normalized electrical impedance of lossy piezoelectric resonators for ultrasonic transducers

Relevant equivalent circuit parameters and values of material constants of a piezoelectric resonator can be determined from measurements of its electrical input impedance as a function of frequency. The complex electrical impedance curves and the associated critical frequencies are the basis of this characterization by the piezoelectric resonance method. In this paper, the previously introduced concept of normalized electrical impedance of the lossy resonator, extended to include piezoelectric losses, is applied to the analysis of the effects of different types of intrinsic losses on peak values, bandwidths and characteristic frequencies. The resulting impedance patterns depend solely on the electromechanical coupling coefficient and the loss tangents, providing a useful tool for the analysis of low-Q resonators. The normalized impedance is experimentally evaluated from the basic data provided by an HP 4194A impedance analyser by means of specifically developed ASP programs.