Millisecond timescale reactions observed via X-ray spectroscopy in a 3D microfabricated fused silica mixer

Determination of electronic structures during chemical reactions remains challenging in studies which involve reactions in the millisecond timescale, toxic chemicals, and/or anaerobic conditions. In this study, a three-dimensionally (3D) microfabricated microfluidic mixer platform that is compatible with time-resolved X-ray absorption and emission spectroscopy (XAS and XES, respectively) is presented. This platform, to initiate reactions and study their progression, mixes a high flow rate (0.50–1.5 ml min−1) sheath stream with a low-flow-rate (5–90 µl min−1) sample stream within a monolithic fused silica chip. The chip geometry enables hydrodynamic focusing of the sample stream in 3D and sample widths as small as 5 µm. The chip is also connected to a polyimide capillary downstream to enable sample stream deceleration, expansion, and X-ray detection. In this capillary, sample widths of 50 µm are demonstrated. Further, convection–diffusion-reaction models of the mixer are presented. The models are experimentally validated using confocal epifluorescence microscopy and XAS/XES measurements of a ferricyanide and ascorbic acid reaction. The models additionally enable prediction of the residence time and residence time uncertainty of reactive species as well as mixing times. Residence times (from initiation of mixing to the point of X-ray detection) during sample stream expansion as small as 2.1 ± 0.3 ms are also demonstrated. Importantly, an exploration of the mixer operational space reveals a theoretical minimum mixing time of 0.91 ms. The proposed platform is applicable to the determination of the electronic structure of conventionally inaccessible reaction intermediates.

Tracing the Migration of Low‐level Radioactive Waste Using Rn‐222 as an Indicator of Hydrogeological and Geochemical Conditions

Port Hope, Ontario, located on the north shore of Lake Ontario, is home to some of largest amounts of low‐level radioactive waste contamination in the world. The waste is enriched in 238U and 226Ra, both of which decay into the invisible, radioactive gas radon‐222 (222Rn). This project is an attempt to determine the amounts of 222Rn that have migrated from scattered waste sites into selected streams and groundwater in the local watershed. This knowledge can be used to predict hydrogeological and geochemical conditions within the sample stream and surrounding area.

Investigation of Hydrodynamic Focusing in a Microfluidic Coulter Counter Device

The Coulter technique enables rapid analysis of particles or cells suspended in a fluid stream. In this technique, the cells are suspended in an electrically conductive solution, which is hydrodynamically focused by nonconducting sheath flows. The cells produce a characteristic voltage signal when they interrupt an electrical path. The population and size of the cells can be obtained through analyzing the voltage signal. In a microfluidic Coulter counter device, the hydrodynamic focusing technique is used to position the conducting sample stream and the cells and also to separate close cells to generate distinct signals for each cell and avoid signal jam. The performance of hydrodynamic focusing depends on the relative flow ratio between the sample stream and sheath stream. We use a numerical approach to study the hydrodynamic focusing in a microfluidic Coulter counter device. In this approach, the flow field is described by solving the incompressible Navier-Stokes equations. The sample stream concentration is modeled by an advection-diffusion equation. The motion of the cells is governed by the Newton-Euler equations of motion. Particle motion through the flow field is handled using an overlapping grid technique. A numerical model for studying a microfluidic Coulter counter has been validated. Using the model, the impact of relative flow rate on the performance of hydrodynamic focusing was studied. Our numerical results show that the position of the sample stream can be controlled by adjusting the relative flow rate. Our simulations also show that particles can be focused into the stream and initially close particles can be separated by the hydrodynamic focusing. From our study, we conclude that hydrodynamic focusing provides an effective way to control the position of the sample stream and cells and it also can be used to separate cells to avoid signal jam.

Microflow Cytometer: Hydrodynamic Focusing and Separation of Sample Stream

Using grooves in the walls of a microchannel and two sheath streams, we have passively focused a sample stream in the center of the microchannel for optical analysis. Even though the sample stream is completely surrounded by sheath fluid, reversing the orientation of the grooves in the channel walls returns the sample stream to its original position with respect to the sheath streams. The use of this sheathing technique has already been demonstrated in a sensitive microflow cytometer; the unsheathing capability now provides the opportunity to recover particles from the sensor with minimal dilution or to recycle the sheath fluid for long-term unattended operation. The ability to reverse focused laminar flows opens a variety of options for combining target transport, processing and analysis procedures.

Sample Flow Switching Technique Based on Combined Effect of Hydrodynamic and Electroosmosis

This paper presents theoretical and experimental investigations on valveless microfluidic switch using the coupled effect of hydrodynamics and electroosmosis. In the experiment, two sheath streams (aqueous NaCl and glycerol) and the sample stream (silicon oil) are introduced by syringe pumps to flow side by side in a straight rectangular microchannel. External electric fields are applied on the two sheath streams. Under the constant inlet volumetric flowrates, the sample stream is delivered to the desired outlet ports using electroosmotic effect. The liquid fractions of sheath streams are measured using fluorescence imaging technique. The results indicate that under suitable cooperation of electric fields, the sample stream can be delivered to the desired outlet ports.

Continuous Microfluidic Biosensing With Conjugated Paramagnetic Beads

A microfluidic biosensing assay has been designed and tested with streptavidin coated paramagnetic microbeads and fluorescently conjugated biotin (Biotin-FITC). The device is a three-inlet, three-outlet channel made by soft lithography of polydimethylsiloxane (PDMS). A novel magnetic actuation scheme is used to manipulate the beads within the channel. The device has proven capable of measuring the antigen concentration of a continuous sample stream. It is proposed that this technology could be applied as a real-time immunosensing assay.