Autonomous system for rapid field quantification of Escherichia coli in surface waters.

AIMS: The purpose of this work is to present and evaluate the performance of a novel Automatic Lab-in-vial Escherichia coli Remote Tracking technology based on an automated real-time defined substrate approach, implemented in both portable and in situ instruments. METHODS AND RESULTS: We present the fresh water calibration procedure, and assess performance using side-by-side comparison with most probable number (MPN) approaches in terms of accuracy, reproducibility and capability to correctly generate early-warning alerts. Long-term data from an operational in situ deployment at a potential bathing site is presented as well. CONCLUSIONS: Automatic Lab-in-vial Escherichia coli Remote Tracking technology is shown to be an accurate and rapid bacterial quantification technology, capable of autonomous in situ measurements with metrological capabilities comparable to those of an approved laboratory using MPN microplate techniques. SIGNIFICANCE AND IMPACT OF THE STUDY: Rapid quantification of bacterial pollution is a requirement in water quality applications ranging from recreational water use, agriculture and aquaculture to drinking and wastewater treatment. The method and instruments presented in this work should enable fast and accurate bacterial concentration measurements to be performed in a portable or in situ manner, thus simplifying operational logistics, reducing time-to-result delays, and eliminating sample transportation constraints associated with traditional techniques.

Bubble production mechanism in a microfluidic foam generator.

We present the design and characterization of a microfluidic bubble generator that has the potential of producing monodisperse bubbles in 256 production channels that can operate in parallel. For a single production channel we demonstrate a production rate of up to 4 kHz with a coefficient of variation of less than 1%. We observe a two-stage bubble production mechanism: initially the gas spreads onto a shallow terrace, and then overflows into a larger foam collection channel; pinning of the liquid-gas meniscus is observed at the terrace edge, the result being an asymmetric pinch-off. A semiempirical physical model predicts the scaling of bubble size with fluid viscosity and gas pressure from measurements of the pinned meniscus width.

Microfluidic capillary separation and real-time spectroscopic analysis of specific components from multiphase mixtures.

We present a technique of phase separation suitable for microfluidic systems and demonstrate its efficient integration with a microfluidic optical cell for performing real-time spectrometric measurements on one specific phase from a mixture. We demonstrate that efficient and robust phase separation based on capillarity is possible within a microfluidic chip using either microfabricated capillary channels in polydimethylsiloxane (PDMS) or oil-wet fluoropolymer membranes, allowing for extraction of either the continuous or of the dispersed phases from a multiphase mixture. We analyze the dependence of phase separation efficiency on the operating parameters of the device and observe the presence of a hysteresis cycle during pressure sweeps above a water breakthrough pressure (P(b)); we also observe and analyze the reversibility of the oil-wet state of the membrane upon pressure reduction below a reset pressure (P(r) < P(b)). We test the capillary separation method extensively with several types of organic/water mixtures and emulsions and derive criteria for design and operation of a robust microfluidic capillary separator. As an example of monitoring application we describe the design and manufacturing of a microfluidic spectrometer cell optimized for fast response time, which was used to analyze the oil extracted from an oil/water emulsion using a capillary separator. The complete separator-sensor system is characterized in terms of response and cleanup times to instantaneous changes in the dye concentration of the phase of interest.

Two-dimensional melting transition observed in a block copolymer.

We report the observation of two-dimensional melting in a monolayer film of a sphere-forming diblock copolymer. By annealing in a well-controlled temperature gradient we obtain a complete record of the transition from a low-temperature hexatic phase to a high-temperature liquid in a single experiment. We investigate the temperature dependence of the orientational and translational correlation lengths, as well as of the topological defect density. All evidence suggests that the melting transition is first-order, but correlations in the liquid phase indicate the existence of an underlying second-order transition preempted by the first-order freezing.