Precise and effective control of droplet generation is critical for applications of droplet microfluidics ranging from materials synthesis to lab-on-a-chip systems. Methods for droplet generation can be either passive or active, where the former generates droplets without external actuation, and the latter makes use of additional energy input in promoting interfacial instabilities for droplet generation. A unified physical understanding of both passive and active droplet generation is beneficial for effectively developing new techniques meeting various demands arising from applications. Our review of passive approaches focuses on the characteristics and mechanisms of breakup modes of droplet generation occurring in microfluidic cross-flow, co-flow, flow-focusing, and step emulsification configurations. The review of active approaches covers the state-of-the-art techniques employing either external forces from electrical, magnetic and centrifugal fields or methods of modifying intrinsic properties of flows or fluids such as velocity, viscosity, interfacial tension, channel wettability, and fluid density, with a focus on their implementations and actuation mechanisms. Also included in this review is the contrast among different approaches of either passive or active nature.

Passive and active droplet generation with microfluidics: a review

Applications Yuanyuan Yang,† Eka Noviana,† Michael P. Nguyen,† Brian J. Geiss,‡ David S. Dandy, and Charles S. Henry*,†,§ †Department of Chemistry, Colorado State University, Fort Collins, Colorado 80523, United States ‡Department of Microbiology, Immunology, and Pathology, Colorado State University, Fort Collins, Colorado 80523, United States Department of Chemical and Biological Engineering, Colorado State University, Fort Collins, Colorado 80523, United States ■ CONTENTS Fabrication 71 Hydrophobic/Solvent Barrier 72 Deposition 73 Flow and Injection Control 74 Three-Dimensional Devices 75 Incorporating Nonsensing Electrodes 75 Colorimetric Detection 75 Detectors and Readout 75 Reflectance-Based Measurement 75 Transmittance-Based Measurement 77 Instrument-Free Measurement 77 Biomedical Applications 77 Enzymatic Methods 77 Immunoassays 78 Other 79 Environmental Applications 79 Other Applications 80 Electrochemical Detection 80 Electrodes 80 Carbon Electrodes 81 Metallic Electrodes 81 Biological Applications 82 Glucose Sensors 82 Immunosensors 84 Other Examples 84 Environmental Applications 84 Other Technologies 85 Chemiluminescence and Electrochemiluminescence 85 Fluorescence 85 Surface-Enhanced Raman Spectroscopy 85 Separation 86 Preconcentration 86 Conclusions and Future Directions 87 Author Information 87 Corresponding Author 87 ORCID 87 Notes 87 Biographies 87 Acknowledgments 88 References 88

Paper-Based Microfluidic Devices: Emerging Themes and Applications

We numerically analyze the mixing and pressure drop characteristics for flow through wavy micromixer of two geometrical configurations, namely raccoon and serpentine for different values of amplitude of the waviness of the mixer (α), wavelength of the waviness (λ), Reynolds number(Re) and Schmidt number(Sc). Three different flow regimes are identified depending on the parameters influencing the mixing index. The mixing index for both the raccoon and serpentine mixer is very close to unity in the first regime (0.1

Numerical study of mixing in wavy micromixers: comparison between raccoon and serpentine mixer

A colorimetric paper-based sensor is proposed for the rapid monitoring of six major organophosphate and carbamate pesticides. The assay was constructed by dropping gold and silver nanoparticles on the hydrophilic zones of a paper substrate. The nanoparticles were modified by L-arginine, quercetin, and polyglutamic acid. The mechanism of sensing is based on the interaction between the pesticide and the nanoparticles. The color of nanoparticles changed during the interactions. A digital camera recorded these changes. The assay provided a unique response for each studied pesticide. This method can determine six individual pesticides including carbaryl, paraoxon, parathion, malathion, diazinon, and chlorpyrifos. The limit of detection for these pesticides were 29.0, 22.0, 32.0, 17.0, 45.0, and 36.0 ng mL−1, respectively. The assay was applied to simultaneously determine the six studied pesticides in a mixture using the partial least square method (PLS). The root mean square errors of prediction were 11, 8.7, 9.2, 10, 12, and 11 for carbaryl, paraoxon, parathion, malathion, diazinon, and chlorpyrifos, respectively. The paper-based device can differentiate two types of studied pesticide (organophosphate and carbamate) as well as two types of organophosphate structures (oxon and thion). Furthermore, this sensor showed high selectivity to the pesticides in the presence of other potential species (e.g., metal ions, anions, amino acids, sugar, and vitamins). This assay is capable of determining the pesticide compounds in tap water, apple juice, and rice samples. Graphical abstract

A paper-based colorimetric sensor array for discrimination and simultaneous determination of organophosphate and carbamate pesticides in tap water, apple juice, and rice

We present a rapid (<10 s), cost-effective, unique single-step method for fabricating paper-based devices without necessitating any expensive instrumentation, simply by deploying correction pens that are otherwise commonly used for masking typos in printed or written matters. The marked regions formed by deposits from the correction pen demonstrate ubiquitous flow resistances to typical aqueous solutions and organic solvents in the transverse direction, resulting in a preferential bulk flow along the axial direction of the paper channels ‘fabricated’ in the process. Considering the simplicity and cost-effectiveness of this platform, it is deemed to be ideal for (bio) chemical sensing and point-of-care diagnostics in resource-limited settings.

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Fabricating Paper Based Devices Using Correction Pens.

We demonstrate here a frugal, printing-based fabrication methodology for paper channels, in an effort towards developing an inexpensive micromixing device. The proposed fabrication methodology utilizes the normal ink-jet cartridge ink to create the barriers for the paper channels, without involving any additional complex materials or intermediary ink modification steps. We show through experimental observations, and pertinent scaling analysis, that the electrokinetic effects, along with the capillary and viscous forces, play a significant role in enhancing the liquid transport rate through such a paper channel under an applied electrical potential, in comparison with that observed due to natural imbibition. Thereafter, we delineate the modality of active electrical control of mixing of two liquids in such a printed ‘zigzag’ ‘paper-and-pencil’ device, by exploiting the interplay between the electrohydrodynamic flows stemming from the electrokinetic phenomena and the specific channel geometry. The electrokinetically mediated flow of the liquid samples through the ‘zigzag’ paper channel can be judiciously controlled to either appreciably enhance the mixing characteristics or artificially maintain the segregation of the liquid streams by overriding the inherent wicking action-driven mixing within the paper matrix. Hence, the present endeavour will usher in a new generation of paper microfluidic platforms for micromixing, with enhanced production feasibility, controllability, functioning efficiency, and multiplexing capability.

Ultra-low-cost ‘paper-and-pencil’ device for electrically controlled micromixing of analytes

We generate stable monodisperse droplets of nano-liter volumes and long serpentine liquid threads in a single, simple “Y”-shaped microchannel mounted on a rotationally actuated lab-on-a-compact-disk platform. Exploitation of Coriolis force offers versatile modus operandi of the present setup, without involving any design complications. Based on the fundamental understanding and subsequent analysis, we present scaling theories consistent with the experimental observations. We also outline specific applications of this technique, in the biological as well as in the physical domain, including digital polymerase chain reaction(PCR), controlled release of medical components, digital counting of colony forming units, hydrogelengineering, optical sensors and scaffolds for living tissues, to name a few.

Sumit Joshi

Papers

Ultra-low-cost ‘paper-and-pencil’ device for electrically controlled micromixing of analytes

Generation of droplets to serpentine threads on a rotating compact-disk platform