A superposable double-gradient droplet array chip for preparation of PEGDA microspheres containing concentration-gradient drugs

This article describes a superposable double-concentration-gradient droplet array chip, which allows a variety of concentration combinations of two components to be formed simultaneously. The concentration gradients generated from two layers of the chip could be arbitrarily superimposed by adjusting the center angle between the two bonding layers. With the aqueous phase flow rate of 1.0 μL min^?1 and the oil phase flow rate of 30.0 μL min^?1, the droplets about 58 μm in diameter were produced, and the coefficients of variation were below 6.0% for single channel and 5.7% for all the channels. Using a dual-32-channel superposable gradient droplet array chip, poly(ethylene glycol) diacrylate (PEGDA) microspheres containing concentration-gradient combinations of rhodamine B and fluorescein were fabricated to demonstrate the capability of PEGDA for encapsulating hydrophilic and hydrophobic substances, as well as the proper concentration-gradient distribution. Furthermore, PEGDA microspheres loaded with two anticancer drugs, hydrophilic doxorubicin hydrochloride and hydrophobic paclitaxel, of 17 concentration combinations were simultaneously prepared. The drug-induced apoptosis of human uterine cervix cancer cells was investigated using the dual-drug-loaded PEGDA microspheres. The optimum synergistic concentration combination of the two drugs was 12.5 μg mL^?1 for doxorubicin hydrochloride and 43.75 μg mL^?1 for paclitaxel according to the preliminary screening. The superposable double-gradient droplet array generator was demonstrated to be a promising platform for screening multiple drug combination in microcarriers.     

An on-chip cell culturing and combinatorial drug screening system

A low-cost, convenient and precise drug combination screening microfluidic platform is developed, in which cell culture chambers designed with micropillars integrate with three laminar flow diffusion channels. This platform has several distinct features, including minimum shear stress on cells, biocompatibility, optimum concentration distribution and automatic combinatorial gradient generation, which can potentially speed up the discovery of an effective drug combination for cancer ablations. The presented device can generate two-drug combination gradients at the optimum flow rate of 90 μL/h and can be applied to identify the optimal combination of two clinically relevant chemotherapy drugs. For demonstration, paclitaxel at 0.77 × 10^?3 mg/mL and cisplatin at 0.23 × 10^?4 mg/mL were studied against lung cancer cells (A549). This microfluidic device has the potential to provide a precise and robust screening for anticancer combinational drugs practiced in clinics.     

An effective PDMS microfluidic chip for chemiluminescence detection of cobalt (II) in water

A microfluidic chip for the chemiluminescence detection of cobalt (II) in water samples, based on the measurement of light emitted from the cobalt (II) catalysed oxidation of luminol by hydrogen peroxide in basic aqueous solution, is presented. The microfluidic chip was designed and fabricated from polydimethylsiloxane using micro-molding method. Optimized reagents conditions were found to be 5.0 × 10^?4 mol/L luminol, 1.0 × 10^?2 mol/L hydrogen peroxide, and 8.0 × 10^?2 mol/L sodium hydroxide. The system can perform fully automated detection with a reagent consumption of only 2.4 μL each time. The linear range of the cobalt (II) ions concentration was 1.0 × 10^?10–1.0 × 10^?3 mol/L and the detection limit was 5.6 × 10^?11 mol/L with the S/N ratio of 3. The relative standard deviation was 4.6 % for 1.0 × 10^?5 mol/L cobalt (II) ions (n = 10).     

Impact of thermal contact resistances on micro-gap heat losses for microthermionic power generators

Thermionic power generation is a safe and clean energy source that allows for converting heat into electrical energy using thermionic electrons. The miniaturization is an advantage of this technology that led to the recent development of micro-gap thermionic power generators. In this work, thermal contact resistances between the micro-gap insulators and the emitter as well as between the micro-gap insulators and the collector are measured. A thermal resistance of 48.6 K/W is obtained by downsizing the insulators until 60 × 45 μm² of contact area with the emitter, demonstrating a high impact for decreasing the micro-gap conduction heat loss density from the emitter to the collector from 28 W/cm² (theoretical value obtained without considering contact resistances) to 5.6 W/cm². Downsizing the contact area between the insulators and the emitter from 320 × 300 to 60 × 45 μm² leads to an increase of the power conversion efficiency from 9.1 × 10^?5 until 1.5 × 10^?3.     

Chemiluminescence detection for Cu (II) based on 1,10-Phenanthroline-hydrogen peroxide reaction on a PDMS microfluidic chip

A simple, rapid and effective method for the determination of copper (II) in water on a PDMS microfluidic chip with chemiluminescence (CL) detection is presented. The CL reaction was based on oxidation of 1,10-phenanthroline by hydrogen peroxide in basic aqueous solution. Polydimethylsiloxane (PDMS) was chosen as material for fabricating the microfluidic chip with two steps lithography method. Optimized reagents conditions were found to be 6.0 × 10^?5 mol/L 1,10-phenanthroline, 1.2 × 10^?3 mol/L hydrogen peroxide, 6.5 × 10^?2 mol/L sodium hydroxide and 2.0 × 10^?3 mol/L Hexadecyl trimethyl ammonium Bromide (CTMAB). In the continuous flow injection mode the system can perform fully automated detection with a reagent consumption of only 3.4 μL each time. The linear range of the Cu (II) ions concentration was 1.0 × 10^?8 mol/L to 1.0 × 10^?4 mol/L, and the detection limit was 9.2 × 10^?9 mol/L with the S/N ratio of 3. The relative standard deviation was 2.8 % for 1.0 × 10^?6 mol/L Cu (II) ions (n = 8). The most notable features of the detection method are simple operation, rapid detection and easy fabrication of the microfluidic chip.     

Measurement of dissolved oxygen diffusion coefficient in a microchannel using UV-LED induced fluorescence method

The diffusion coefficient of dissolved oxygen (DO) was measured in a microchannel using the UV-LED induced fluorescence method. Mass transfer between oxic and anoxic de-ionized (DI) water was quantitatively visualized in a Y-shaped microchannel. Oxygen-sensitive ruthenium (tris (2,2′-bipyridine) ruthenium (II) chloride hexahydrate] and a 450-nm UV-LED were used for the optical measurement of a DO concentration field. In situ pixel-by-pixel calibration was carried out to obtain Stern–Volmer equations to measure the DO concentration field with a spatial resolution of 0.625 μm/pixel. The diffusion layers are successfully acquired for different Reynolds numbers (Re = 0.14, 1.4, and 14). The DO diffusion coefficient is calculated by both the constant-assumed and the concentration-dependent diffusion coefficient methods. The measured DO diffusion coefficient, 2.32 × 10^?9 m²/s, is very close to that of the theoretical prediction of the oxygen gas diffusion coefficient, 2.16 × 10^?9 m²/s.     

Variation in aerosol black carbon concentration and its emission estimates at the mega-city Delhi

Simultaneous measurements of aerosol black carbon (BC) mass concentration using an Aethalometer Model AE-42 and mixing layer height (MLH) using a monostatic sonic detection and ranging (SODAR) system were carried out from January 2006 to January 2007 at the mega-city Delhi. The BC concentration generally had a typical diurnal variation with morning and late-afternoon/night peaks. The average BC concentration during the whole period of observation was fairly high at 14.75 μg m^?3. The BC concentration nearly doubled during cloudy-sky conditions compared to that during clear-sky conditions. The seasonal variation showed a maximum average concentration during the winter (25.5 μg m^?3) and a minimum during the monsoon season (7.7 μg m^?3), with post- and pre-monsoon values at 13.7 and 9.4 μg m^?3, respectively. The average BC concentrations were strongly affected by the ventilation coefficient, a product of average wind speed (WS) and average MLH, and were found to be strongly anticorrelated. A simple model of BC concentration along with the MLH and WS was applied to estimate the average BC emission, which was found to vary in the range 11?000–17?000 kg of BC per day. The maximum emission during the day averaged every hour for different months lay in the range 1000–2100 kg h^?1. The mean monthly emission varied in the range 0.35–0.52 Gg per month, giving rise to an annual estimated emission of 4.86 Gg in the year 2006 over Delhi.     

Droplets coalescence and mixing with identical and distinct surface tension on a wettability gradient surface

The influence of identical and distinct surface tensions on the coalescence and mixing of droplets after a direct collision on a wettability gradient surface (made from a self-assembled monolayer, SAM technique) was investigated. The results indicate that their mixing is driven sequentially by interior convection and diffusion; the convection endures less than 100 ms but dominates more than 60 % of the mixing. If the stationary droplet has a large surface tension (73.28 mN × m^?1), whether the moving droplet has a large surface tension (73.28 mN × m^?1) or a small surface tension (38.63 mN × m^?1), the mushroom-shaped mixing pattern is generated within the coalesced droplet that enhances the convective mixing and also significantly enlarges the interface for mass diffusion. The mixing index of these two cases was greater than 0.8 at 120 s after the collision. For the cases in which the stationary droplet with a small surface tension collided by the moving droplet with a large surface tension, a mixing pattern with a round-head shape developed, which was insufficient to benefit the mixing. When the stationary and moving droplets both had small surface tension, the moving droplet was unable to merge with stationary droplet and had poor mixing quality due to the small surface Gibbs energy of both stationary and moving droplets. For the collision of droplets of identical surface tension, the surface tension affects the coalescence behavior; for the collision of droplets with distinct surface tension, the coalescence behavior and mixing quality depend on the colliding arrangement of stationary and moving droplets.     

Variations of surface boundary layer parameters associated with Cyclone Gonu over the Arabian Sea using QuikSCAT data

This study attempted to quantify the variations of the surface marine atmospheric boundary layer (MABL) parameters associated with the tropical Cyclone Gonu formed over the Arabian Sea during 30 May–7 June 2007 (just after the monsoon onset). These characteristics were evaluated in terms of surface wind, drag coefficient, wind stress, horizontal divergence, and frictional velocity using 0.5° × 0.5° resolution Quick Scatterometer (QuikSCAT) wind products. The variation of these different surface boundary layer parameters was studied for three defined cyclone life stages: prior to the formation, during, and after the cyclone passage. Drastic variations of the MABL parameters during the passage of the cyclone were observed. The wind strength increased from 12 to 22 m s^?1 in association with different stages of Gonu. Frictional velocity increased from a value of 0.1–0.6 m s^?1 during the formative stage of the system to a high value of 0.3–1.4 m s^?1 during the mature stage. Drag coefficient varied from 1.5 × 10^?3 to 2.5 × 10^?3 during the occurrence of Gonu. Wind stress values varied from 0.4 to 1.1 N m^?2. Wind stress curl values varied from 10 × 10^?7 to 45 × 10^?7 N m^?3. Generally, convergent winds prevailed with the numerical value of divergence varying from 0 to –4 × 10^?5 s^?1. Maximum variations of the wind parameters were found in the wall cloud region of the cyclone. The parameters returned to normally observed values in 1–3 days after the cyclone passage.     

Selective and in-situ determination of carbonate and oxide particles in aqueous solution using laser-induced breakdown spectroscopy (LIBS) for wearable information equipment

Laser-induced breakdown spectroscopy (LIBS) has been applied for quantitative analysis of Al₂O₃, CaCO₃ and MgO particles suspended in water. In the single elemental system, the plasma emission intensities of Al, Ca and Mg were linearly increased with concentration of elements in the range of 1.0×10^?5–1.0×10^?3  M, 1.0×10^?4–2.0×10^?3 M and 8.0×10^?5 –4.0×10^?3 M, respectively. We also investigated the concentration dependence of breakdown spectra for suspended mixtures of Al₂O₃, CaCO₃ and MgO particles. The emission lines, such as Al I, Ca I, Ca II and Mg I, were appeared in the LIBS spectrum simultaneously, and each emission peak could be deconvoluted. The plasma emission intensities of Al, Ca and Mg in the multielemental system were also linearly increased with their concentrations in the range of 1.0×10^?5–1.0×10^?3 M, 1.0×10^?4–2.0×10^?3 M and 4.0×10^?4–2.0×10^?3 M, respectively. LIBS was found to be available for quantitative and qualitative measurement of the concentrations of Al₂O₃, CaCO₃ and MgO particles suspended in water. The present results suggest that LIBS is a potentially useful tool for in-situ analysis on particles composition and concentrations for environmental monitoring by the wearable information equipments.     

Particulate organic carbon in the surface waters of the North Atlantic: spatial and temporal variability based on satellite ocean colour

We have examined the 16-year time series of particulate organic carbon (POC) concentration in the surface waters of the North Atlantic derived from SeaWiFS and MODIS-Aqua data. The annual mean POC concentrations are the highest in the northern North Atlantic, reaching 120 mg m^?3. Moving south, the mean annual POC concentrations decrease to minimum values of about 30 mg m^?3 at around 30° N and increase in the equatorial region to about 70 mg m^?3. The seasonal amplitude of POC concentration in the northern North Atlantic region is larger when compared to other regions. The annual mean surface POC concentrations in the entire North Atlantic basin show a statistically significant trend with an average decrease of 0.79 mg m^?3 year^?1. Regionally averaged 16-year mean POC biomass integrated over the optical depth, euphotic depth, and mixed-layer depth is estimated at about 1.27, 4.34, and 4.59 g m^?2, respectively. Even larger biomass of 6.26 g m^?2 is estimated if one chooses to use in the calculations the greatest from the daily values of the estimates listed above at each pixel of the satellite data. Comparisons of POC biomass with primary productivity allowed us to assess temporal and spatial patterns of POC losses.     

Hygrometer-based calibration method for syringe pumps providing ultra-low liquid flows

Traceable and accurate measurements are critical in many areas where syringe pumps are used to provide microscale liquid flow. Especially, drug delivery demands for low flow rate calibrations have been increasing, but currently available calibration methods are very limited in range and accuracy at flow rates below 100 μl min^?1. This paper presents a new hygrometer-based method for the calibration of syringe pumps providing traceability down to the 0.1 μl min^?1 level. In this method, liquid water from the syringe pump under calibration is injected into a porous cloth from which it evaporates and mixes with a dry gas flow. By measuring the humidity, temperature, pressure and flow rate of the gas, the injected water flow rate can be calculated. To test the method, a calibration set-up was constructed and a syringe pump was calibrated at flow rates from 0.1 to 10 μl min^?1. A thorough uncertainty analysis carried out as a part of the work shows that a relative uncertainty of 0.4 % (at 95 % confidence level) is achieved at the largest calibration flow rate. The uncertainty increases to 3.3 % towards low end of the flow rate range. The achieved uncertainty level is significantly better than achieved with gravimetric methods so far.     

Development and characterization of a microthermoelectric generator with plated copper/constantan thermocouples

This work reports the development and the characterization of a microthermoelectric generator (μTEG) based on planar technology using electrochemically deposited constantan and copper thermocouples on a micro machined silicon substrate with a SiO₂/Si₃N₄/SiO₂ thermally insulating membrane to create a thermal gradient. The μTEG has been designed and optimized by finite element simulation in order to exploit the different thermal conductivity of silicon and membrane in order to obtain the maximum temperature difference on the planar surface between the hot and cold junctions of the thermocouples. The temperature difference was dependent on the nitrogen (N₂) flow velocity applied to the upper part of the device. The fabricated thermoelectric generator presented maximum output voltage and power of 118 mV/cm² and of 1.1 μW/cm², respectively, for a device with 180 thermocouples, 3 kΩ of internal resistance, and under a N₂ flow velocity of 6 m/s. The maximum efficiency (performance) was 2 × 10^?3 μW/cm² K².     

Spatio-temporal distribution of chlorophyll-a concentration derived from MODIS satellite remote-sensing and in situ observations in Sanya Bay,China

This study examined satellite chlorophyll-a (chl-a) concentration and in situ observations in Sanya Bay (SYB). In situ observation of chl-a was conducted four times per year at 12 sampling stations in SYB from January 2004 to October 2008. Monthly satellite chl-a was derived from the Moderate Resolution Imaging Spectroradiometer (MODIS) during 2000–2012. This study compared satellite chl-a values to in situ measurements in SYB. The two data sets match well in the whole region except for two estuaries. Results show that the average in situ chl-a was 1.49 mg m^?3 in SYB. Chl-a was relatively higher (>2 mg m^?3) and more variable in coastal areas, with a tendency to decrease offshore (<0.4 mg m^?3). The chl-a level in summer displayed obviously vertical stratification, with higher values at the bottom and lower values at the surface. Analysis of monthly mean chl-a showed that the highest level (>2 mg m^?3) appeared in December, with the lowest in March (<1 mg m^?3). The gradients are ranked winter, autumn, summer and spring. There was higher chl-a in autumn and winter, which may be associated with the stronger wind monsoon then. Annual mean chl-a from 2000 to 2012 varied from 1.17 to 2.05 mg m^?3, with the minimum in 2001 and the maximum in 2005. The chl-a level presented a roughly increasing tendency from 2000 to 2012, which may be related to the increasing nutrients associated with the development of tourism and fishery.     

Mapping growing stock volume and biomass carbon storage of larch plantations in Northeast China with L-band ALOS PALSAR backscatter mosaics

ABSTRACT

Reliable spatial information on growing stock volume (GSV) and biomass is critical for creating management strategies for plantation forests. This study developed empirical models to map the GSV and biomass of larch plantations (LPs) in Northeast China (1.25 million km² total area) by integrating L-band synthetic aperture radar (SAR) data with ground-based survey data. The best correlation model was used to map the GSVs and biomasses of LPs. The total GSV and biomass carbon storage were estimated at 224.3 ± 59.0 million m³ and 113.0 ± 29.7 × 10¹² g C with average densities of 85.1 m³ ha^?1 and 42.9 10⁶ g × C ha^?1, respectively, over a total area of 2.64 million ha. The saturation effect of SAR was determined beyond 260 m³ ha^?1, which was expected to influence the estimations for a small proportion of the study area. The accuracy of the estimations has limitations mainly due to the uncertainties in the GSV inventories, discrimination of natural larch and the SAR dataset. Based on the mapping results of the GSVs of LPs, a planning strategy for multipurpose management was tentatively proposed. This study can inform policies and management practices to assure broader and sustainable benefits from plantation forests in the future.     

Time-series analysis of chlorophyll-a,sea surface temperature,and sea surface height anomalies during 2003–2014 with special reference to El Niño,La Niña,and Indian Ocean Dipole (IOD) Years

The present study was carried out to find the variability of chlorophyll-a (chl-a) concentration, sea surface temperature (SST), and sea surface height anomalies (SSHa) during 2003–2014, covering the Bay of Bengal (BoB) and Arabian Sea (AS) waters. These parameters were linked with El Niño, La Niña, and Indian Ocean Dipole (IOD) years. The observed results during 2003–2014 were evaluated and it was found that the monthly mean value for 12-year data ranged as follows: chl-a (0.11–0.46 mg m^?3), SST (27–31 °C), and SSHa (?0.2 to 20 cm). The annual mean range of chl-a for 12-year data was 0.1–0.23 mg m^?3, the SST range was 27–28 °C, and the SSHa range was 2.14–13.91 cm. It has been observed that with the SST range of 27–28 °C and the SSHa range of 7–9 cm, the chl-a concentration enhanced to 0.20–0.23 mg m^?3. With a higher SST range of 28–29 °C and with a positive SSHa range of 11–14 cm, the chl-a concentration appeared to be low (0.17–0.18 mgm^?3). During normal years, SSHa was positive with the >5 to <10 cm range during the months of April–June, which coincided with an increase in SST, >2 to <4 °C. During the normal years, SSHa (>?0.2 to a concentration (>0.3 to <0.5 mg m^?3) was noticed during December–February in the BoB and AS. Compared to the BoB chl-a range (<0.4 mg m^?3), a high chl-a concentration was observed in AS (>0.4 mg m^?3). However, during the phenomenon years, the study area had experienced low chl-a (<0.2 mg m^?3), high SST (>5 °C), and more positive SSHa (>10 to <20 cm) during January–March and October–December in AS and BoB. The present study infers that a positive IOD leads to low chl-a concentration (<2 mg m^?3) and low primary productivity in AS. El Niño caused the down-welling process, it results in a low chl-a concentration (<1 mg m^?3) in BoB and AS. La Niña caused the upwelling process, and it results in a high chl-a concentration (>2.0 mg m^?3) in BoB and AS. In the recent past years (2003–2014), the intensity and frequency of El Niño, La Niña, and IOD have been increasing, evidenced with few studies, and have impacts on the Indian Ocean climate. Therefore, the influences of the relative changes of these phenomena on the BoB and AS need to be understood for productivity assessment and ocean state monitoring.     

Lidar profiling of aerosol scavenging parameters at a tropical station,Pune, India

In this study, we deal with observations of aerosol column content (height integration of vertical distribution of aerosol number density) that have been carried out using an Ar⁺ lidar for three different measurement cycles (each cycle consisting of three experimental days associated with non-rain, rain, and non-rain, respectively) of weekly spaced observations for pre-monsoon (March/April 1994), monsoon (September 1991), and post-monsoon (October 1998). Based on these observed profiles of aerosol number concentration on rainy days with respect to those on non-rainy days, vertical distributions of scavenging collection efficiencies (SCEs) are computed and discussed in this article. The SCE is found to decrease from 0.3 to 0.01 between the heights, 100 and 800 m for thunderstorm rain in April 1994, and during monsoon, it increases from 0.1 to 0.7. In the October 1998 episode, SCE was found to increase initially from 0.35 to 0.75 for heights between 40 and 200 m and thereafter decrease to 0.35 in the height interval of 200–800 m. For the rainfall intensity increase from 1 to 10 mm hour^?1, the corresponding scavenging coefficient (SC) for atmospheric layer 50–100 m varies from 4 × 10^?6 to 4 × 10^?5 s^?1 for thunderstorm in April 1994 and between 5 × 10^?6 and 5 × 10^?5 s^?1 in October 1998, respectively. During monsoon, these values vary from 3 × 10^?5 to 5 × 10^?4 s^?1. They lie in the range of those observed in the earlier field studies. The results are found useful to establish links between aerosols and cloud properties, and the influence of such interactions on weather and climate.     

Identifying local anthropogenic CO2 emissions with satellite retrievals: a case study in South Korea

We used multiyear Greenhouse Gases Observing Satellite (GOSAT) dry air, column-integrated CO₂ (XCO₂) retrievals (2010–2013) to evaluate urban and local-scale CO₂ emissions over East Asia and examined whether GOSAT XCO₂ captures the impact of strong local CO₂ emissions over South Korea, an East Asian downwind region with high atmospheric aerosol loading and strong summer monsoons. We chose a region in western Mongolia (upwind region) as the XCO₂ background, and estimated XCO₂ enhancements in South Korea to gauge local and regional emissions. We found that the cold season (November–February) was better suited for estimating XCO₂ enhancements of local emissions due to the summer monsoon and stronger transboundary impacts in other seasons. In particular, we focused on three local GOSAT XCO₂ footprints (about 10.5 km in diameter) in South Korea: the Seoul Metropolitan Area (SMA), the Gwangyang Steelworks and Hadong power plants (GYG), and the Samcheonpo power plants (SCH). The range of XCO₂ enhancement was 7.3–10.7 ppm (14.1–21.3 mg m^?3 in standard temperature and pressure (STP)). By estimating other important contributions to XCO₂ enhancements such as the XCO₂ latitudinal gradients and Chinese fossil fuel combustions, we estimated the net enhancements caused mainly by local CO₂ emissions in the range of 4.2–7.6 ppm (8.1–14.7 mg m^?3 in STP) These high enhancements imply that large point source contributions are an important factor in determining these enhancements, even if contributions are also made by broader-scale emissions. Additionally, differences in net XCO₂ enhancements and trends between GYG (+ 4.2 ppm (+ 8.2 mg m^?3 in STP), – 0.2 ppm year^?1 (–0.4 mg m^?3 year^?1 in STP)) and SCH (+ 7.6 ppm (+ 14.9 mg m^?3 in STP), + 1.3 ppm year^?1 (+ 2.6 mg· m^?3 year^?1 in STP)) indicate that these closely located footprints (approximately 26 km apart) are separable. These results will be useful in evaluating and reducing uncertainties in regional and local anthropogenic greenhouse gas (GHG) emissions over East Asia.     

On the background design for microscale background-oriented schlieren measurements of microfluidic mixing

Using microfluidic mixing as the benchmark, we assess the influences of the background designs in the accuracy of the microscale background-oriented schlieren measurements in this study. Three parameters are considered, they are as follows: pattern configuration (random dot, random grid, and grid), dot diameter, and area fraction of dot coverage. A photomask covered with the defined pattern is placed on top of the microchannel to serve as the background. When miscible fluids with different refractive indices are mixed in a T-microchannel, light deflects and there are pattern shifts on the acquired image. After a calibration process is carried out to obtain the relationship between the pattern shift and gradient of mass fraction, we are able to evaluate the performance of each background design based on its corresponding uncertainty. Except for the grid configuration, we find that the lowest error level is achieved with a dot diameter of 6 μm, which corresponds to a dot-image diameter of 2.8 pixels. Because a sparse distribution leads to vacant interrogation windows, the optimal random-dot design has the highest area fraction of 0.178 (0.196 for the design value). In contrast, the random-grid design with too many dots becomes comparable to the grid design and has difficulties during the cross-correlation analysis. As a result, the best random-grid background has an area fraction of 0.098. For the grid design, on the other hand, accurate results can be obtained when there is only one dot in each interrogation window. Hence, a dot diameter of 16 μm leads to the lowest uncertainty for the grid design. Once these backgrounds are optimized, we prove that all three configurations are able to deliver satisfactory results for the reconstruction of a concentration field in a T-microchannel and an instantaneous profile of concentration gradient in a microfluidic oscillator.     

Flow-induced particle migration in microchannels for improved microfiltration processes

Microchannels can be used to induce migration phenomena of micron sized particles in a fluid. Separation processes, like microfiltration, could benefit from particle migration phenomena. Currently, microfiltration is designed around maximum flux, resulting in accumulation of particles in and on the membrane. In this paper it is shown that starting the design at the particle level will result in a new microfiltration process. The behaviour of suspensions between 9 and 38 volume% was studied by confocal scanning laser microscopy; migration as a result of shear-induced diffusion was observed in a rectangular microchannel with nonporous walls. Particles segregated on size within the first 10 cm of the channel. To illustrate this, at 20 volume% of small (1.53 μm) and large (2.65 μm) particles each, the larger particles migrated to the middle of the channel, while the small particles had high concentrations near the walls. The small particles could then be collected from their position close to the permeable walls, e.g. membranes, where the pore size of the membrane is no longer the determining factor for separation. Guidelines for using this phenomenon in a microfiltration process were derived and the selectivity of the process was experimentally evaluated. The small droplets could be removed from the mixtures with a membrane having pores 3.7 times larger than the droplets, thereby minimizing accumulation of droplets in and on the membrane. As long as the process conditions are chosen appropriately, no droplet deposition takes place and high fluxes (1.7 × 10³ L h^?1 m^?² bar^?1) can be maintained.