Microfluidic systems have advanced beyond natural and life science applications and lab‐on‐a‐chip uses. A growing trend of employing microfluidic technologies for environmental detection has emerged thanks to the precision, time‐effectiveness, and cost‐effectiveness of advanced microfluidic systems. This paper reviews state‐of‐the‐art microfluidic technologies for environmental applications, such as on‐site environmental monitoring and detection. Microdevices are extensively used in collecting environmental samples as a means to facilitate detection and quantification of targeted components with minimal quantities of samples. Likewise, microfluidic‐inspired approaches for separation and treatment of contaminated water and air, such as the removal of heavy metals and waterborne pathogens from wastewater and carbon capture are also investigated.

Abstract:In this paper, a low-cost yet effective method of irreversible bonding between two elastomeric polydimethylsiloxane (PDMS) interfaces using Piranha solution is investigated. Piranha solutions at a weight ratio of 3:1 using different acids and hydrogen peroxide were attempted. The average tensile strengths of the device bonded with concentrated sulfuric acid-based piranha solution and nitric acid-based piranha solution were found to be 200 ± 20 kPa and 100 ± 15 kPa respectively. A PDMS surface treated with Piranha Solution demonstrated an increase in hydrophilicity. In addition, relatively straightforward swelling studies of PDMS using a weight loss method with common organic solvents were also investigated. Experimental results show that hexane, toluene, ethyl acetate, n-propyl alcohol and acetone swell PDMS significantly over a duration of up to 1 h and above; PDMS samples reached a steady state of swelling only after 5 min of immersion in other solvents. This will enable researchers to develop devices for the future according to the interaction between the material and the solvents in contact.

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PDMS–MWCNTs/TiO2 microparticles made by microfluidics can achieve 85% removal efficiency of RhB pollutant in wastewater via synergetic treatment.

Scale-up in droplet microfluidics achieved by increasing the number of devices running in parallel or increasing the droplet makers in the same device can compromise the narrow droplet-size distribution, or requires high fabrication cost, when glass-or polymer-based microdevices are used. This paper reports a novel way using parallelization of needle-based microfluidic systems to form highly monodispersed droplets with enhanced production rates yet in cost-effective way, even when forming higher order emulsions with complex inner structure. Parallelization of multiple needle-based devices could be realized by applying commercially available two-way connecters and 3D-printed four-way connectors. The production rates of droplets could be enhanced around fourfold (over 660 droplets/min) to eightfold (over 1300 droplets/min) by two-way connecters and four-way connectors, respectively, for the production of the same kind of droplets than a single droplet maker (160 droplets/min). Additionally, parallelization of four-needle sets with each needle specification ranging from 34G to 20G allows for simultaneous generation of four groups of PDMS microdroplets with each group having distinct size yet high monodispersity (CV < 3%). Up to six cores can be encapsulated in double emulsion using two parallelly connected devices via tuning the capillary number of middle phase in a range of 1.31 × 10 −4 to 4.64 × 10 −4 . This study leads to enhanced production yields of droplets and enables the formation of groups of droplets simultaneously to meet extensive needs of biomedical and environmental applications, such as microcapsules with variable dosages for drug delivery or drug screening, or microcapsules with wide range of absorbent loadings for water treatment.

This paper demonstrates a low cost fabrication approach for microscale droplet fission and encapsulation. Using a modified xurography method, rapid yet reliable microfluidic devices with flexible designs (single layer and double layer) are developed to enable spatial control of droplet manipulation. In this paper, two different designs are demonstrated, i.e., droplet fission (single layer) and droplet encapsulation (double layer). In addition, the current fabrication approach reduces the overall production interval with the introduction of a custom-made polydimethylsiloxane (PDMS) aligner. Apart from that, the fabricated device is able to generate daughter droplets with the coefficient of variance (CV) below 5% and double emulsions with CV maintained within 10% without involvement of complex surface wettability modification.

Breakup of droplets at bi-layer bifurcating junction in polydimethylsiloxane (PDMS) microchannel has been investigated by experiments and numerical simulation. The pressure drop in bi-layer bifurcating channel was investigated and compared with single-layer bifurcating channel. Daughter droplet size variation generated in bi-layer bifurcating microchannel was analyzed. The correlation was proposed to predict the transition between breakup and non-breakup conditions of droplets in bi-layer bifurcating channel using a phase diagram. In the non-breakup regime, droplets exiting port can be switched via tuning flow resistance by controlling radius of curvature, and or channel height ratio. Compared with single-layer bifurcating junction, 3-D cutting in diagonal direction from bi-layer bifurcating junction induces asymmetric fission to form daughter droplets with distinct sizes while each size has good monodispersity. Lower pressure drop is required in the new microsystem. The understanding of the droplet fission in the novel microstructure will enable more versatile control over the emulsion formation, fission and sorting. The model system can be developed to investigate the encapsulation and release kinetics of emulsion templated particles such as drug encapsulated microcapsules as they flow through complex porous media structures, such as blood capillaries or the porous tissue structures, which feature with bifurcating junctions.

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