Homogeneous and fast mixing of samples at microscale is a critical requirement for successful applications of microfluidics in biochemical analysis, chemical synthesis, drug delivery and nanomaterial synthesis. This paper reports the optimisation of bubble-induced mixing in a microfluidic device in terms of voltage, driving frequency, piezo transducer position and PDMS thickness. The microfluidic device consists of a microwell (with the diameter of 1mm and volume of ~95 nL) with two rectangular bubble traps (400×400μm) on both sides of the well. After the injection of liquid, air bubbles were spontaneously trapped in two rectangular traps. When the frequency of a piezo was equal to the resonance frequency of air bubbles, strong liquid recirculation formed (so called acoustic microstreaming) in the vicinity of the interface of air bubbles and water. The acoustic induced flow of microbeads and mixing of water and fluorescence dye were imaged to study the mixing efficiency. For a given voltage and PDMS thickness, when the piezo was placed on top of the well, the mixing was most vigorous. For a given frequency, the mixing efficiency was directly proportional to the voltage (4-20V) and inversely proportional to the PDMS thickness (0.3-2mm). When the frequency driving the piezo was approaching the resonance frequency of air bubbles, the mixing efficiency was maximal, while when it was far away from the resonance frequency of air bubbles, the mixing efficiency was much lower. This work provides guidance to the design and the application of bubble-induced acoustic mixing in microfluidics.
Analysis of chemical warfare agents (CWAs) and their degradation products is an important verification
component in support of the Chemical Weapons Convention and urgently demanding rapid and reliable
analytical methods. A portable microchip electrophoresis (ME) device with contactless conductivity (CCD)
detection was developed for the in situ identification of CWA and their degradation products. A 10mM
MES/His, 0.4mM CTAB - based separation electrolyte accomplished the analysis of Sarin (GB), Tabun( GA)
and Soman (GD) in less than 1 min, which is the fastest screening of nerve agents achieved with portable ME
and CCD based detection methods to date. Reproducibility of detection was successfully demonstrated on
simultaneous detection of GB (200ppm) and GA (278ppm). Reasonable agreement for the four consecutive runs
was achieved with the mean peak time for Sarin of 29.15s, and the standard error of 0.58s or 2%. GD and GA
were simultaneously detected with their degradation products methylphosphonic acid (MPA), pinacolyl
methylphosphonic acid (PMPA) and O-Ethyl Phosphorocyanidate (GAHP and GAHP1) respectively. The
detection limit for Sarin was around 35ppb. To the best of our knowledge this is the best result achieved in
microchip electrophoresis and contactless conductivity based detection to date.
Capillary flow in microchannels has received substantial attention of investigation recently due to its potential
applications in microfluidics. This paper will report the new findings of capillary flow behavior in microfluidic Hele-
Shaw flow cells. Flow cells with a rectangular cross section of 50×50, 50×500, 20×200, and 50×1000μm were used. It
was observed that the shape of the meniscus varied with cell size and aspect ratio. The shape was also strongly affected
by the contact lines on both sidewalls and the gas phase flow in front of the meniscus. For cells with same height, the
meniscus was stretched more in the flow direction for wider cells. The measured averaged speed of the interface was 5.7,
8.3, 8.3 and ~8 mm/s for the flow cells with a cross section of 50×50, 50×500 and 50×1000μm, respectively. The speed
of interface movement was not affected significantly by the aspect ratio for the values used in the current study. The
average speed for the flow cells agree reasonably well with the value from the theoretical analysis.
This paper reports on the development of a hand-held device for on-site detection of
organophosphonate nerve agent degradation products. This field-deployable analyzer relies on
efficient microchip electrophoresis separation of alkyl methylphosphonic acids and their sensitive
contactless conductivity detection. Miniaturized, low-powered design is coupled with promising
analytical performance for separating the breakdown products of chemical warfare agents such as
Soman, Sarin and VX . The detector has a detection limit of about 10 μg/mL and has a good linear
response in the range 10-300 μg/mL concentration range. Applicability to environmental samples is
demonstrated .The new hand-held analyzer offers great promise for converting conventional ion
chromatography or capillary electrophoresis sophisticated systems into a portable forensic laboratory
for faster, simpler and more reliable on-site screening.
Microchip-based electrophoretic separation systems are essential components in the development of fully integrated
micro total analysis systems. In this paper, a miniaturized analytical system for separating and detecting inorganic ions
is described. The system was based on a polycarbonate (PC) capillary electrophoresis (CE) chip and a contactless
conductivity detector, both being developed at CSIRO Microfluidics and Microfabrication Laboratories, Melbourne,
Australia. The PC chip was fabricated using the soft lithography technique in conjunction with nickel plating and hot
embossing. The detector electrodes were fabricated from a PCB board and attached on the separation chip bottom
surface. The thin capping layer (20 micron) of the chip allowed for sensitive detection of conductivity change. The
system was demonstrated to separate reliably the potassium, sodium and lithium ions in a 20mM MES/His buffer within
a minute at an electrical field of 28.5kV/m. The detection limit for the current design is around 100μM. Such a system
offers great promise to be integrated into robust hand-held devices for in-situ monitoring of chemical and biological
samples with high speed, reliability and low costs.
Disposable polymer microfluidic chips have been used more and more in miniaturized analytical devices. The surface of
the polymers often needs to be treated to acquire specific properties. This study investigates the characteristics of
capillary flow in three microfluidic chips under different surface conditions and the aim is to understand how the surface
property could affect the capillary flow over the shelf life of the chips. The channel surfaces of polymer chips were
treated using air plasma. The interface pattern and velocity were measured by a photographic technique and a micron
Particle Imaging Velocimetry (MicroPIV) method. The glass chip could maintain a capillary flow velocity of around 3.0
mm/s and showed little reduction with time. The velocity agreed well with theory by Washburn. The PDMS chip
surfaces could be easily modified and the capillary flow rate could reach 4 mm/s. However, the hydrophilicity decreased
rapidly over time and was lost completely within a few hours. The polycarbonate chips need more powerful surface
treatment. Once modified, the surface could sustain for much longer time. It took one month for the capillary flow
velocity to decrease by 50%.
Joule heating is a significant problem for microfluidic chips with electrokinetically driven flows. In this paper, we will present the modeling results of the Joule heating of a Polymethylmethacrylate (PMMA) polymer separation chip using both experimental and computational methods. The temperature distributions on the surface of the chip were measured by an advanced thermograph system. The numerical study was carried out using the multiphysics computational fluid dynamics (CFD) package CFD-Ace+. Different solutions and operating conditions were studied. Both the measurements and CFD data revealed that the heat generation was approximately uniform and the subsequent temperature increase was also uniform along the channel except for regions near the liquid ports. The highest temperature increase was observed along the centerline of the channel and the temperature reduced significantly away from the channel. At an electrical field of 45kV/m, the Joule heating effect was negligible for the solution used, even though at such a high electric field significant heating effect has been observed for micro capillary flows in literature. At a higher electrical field (68-120kV/m), the Joule heating could cause an increase of temperature of up to 40°C.
Experiments are presented in which acoustic microstreaming is investigated and applied to a batch micromixing case appropriate to a point-of-care pathology screening test. The flows presented can be created without complex engineering of contacts or surfaces in the microdevice, which could thus be made disposable. Fundamental flow patterns are measured with a micro-Particle-Image Velocimetry (micro-PIV) system, enabling a quantification of the fluiddynamical processes causing the flows. The design of micromixers based on this principle requires a quantification of the mixing. A simple technique based on digital image processing is presented that enables an assessment of the improvement in mixing due to acoustic microstreaming. The digital image processing technique developed was shown to be non-intrusive, convenient and able to generate useful quantitative data. Preliminary indications are that microstreaming can at least halve the time required to mix quantities of liquid typical of a point-of-care test, and significantly greater improvements seem feasible.