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Micromachines
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Droplet-Based Microfluidics: Design, Fabrication and Applications
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Droplet-Based Microfluidics: Design, Fabrication and Applications

A special issue of Micromachines (ISSN 2072-666X). This special issue belongs to the section "E：Engineering and Technology".

Deadline for manuscript submissions: closed (28 February 2023) | Viewed by 33409

Printed Edition Available!
A printed edition of this Special Issue is available here.

Special Issue Editor

Dr. Pingan Zhu

E-Mail Website
Guest Editor
Department of Mechanical Engineering, City University of Hong Kong, Tat Chee Avenue, Kowloon Tong, Hong Kong, China
Interests: microfluidics; fluid mechanics; wettability; biomimetics; micro/nanorobots
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Droplet-based microfluidics has emerged as a versatile and powerful platform for precise processing and/or manipulation of small volumes of fluids, fostering multidisciplinary applications in physics, chemistry, biology, engineering, the environment, and medicine, to name a few. Since its advent in the 2000s, the content of droplet-based microfluidics has been continuously enriched by interdisciplinary innovations. For example, the advancement of surface engineering greatly boosts the development of droplet manipulation in an open space for larger ramifications; the thriving of 3D printing technology substantially benefits the fabrication of microfluidic devices, especially those with complex configurations; and the integration of machine learning and droplet-based microfluidics revolutionizes droplet generation and biomedical applications, such as droplet digital polymerase chain reaction (ddPCR). Having been burgeoning for two decades, droplet-based microfluidics is entering a third “golden” decade with new concepts, technologies, and commercial products continuing to be witnessed. This Special Issue aims to showcase research papers, short communications, and review articles that focus on recent advancements in the design, fabrication, and applications of droplet-based microfluidics, including but not limited to (1) fundamental understanding of microscale fluid flows, (2) design and fabrication of innovative microfluidic systems, and (3) diverse applications ranging from lab-on-a-Chip to materials synthesis.

Dr. Pingan Zhu
Guest Editor

Manuscript Submission Information

Manuscripts should be submitted online at www.mdpi.com by registering and logging in to this website. Once you are registered, click here to go to the submission form. Manuscripts can be submitted until the deadline. All submissions that pass pre-check are peer-reviewed. Accepted papers will be published continuously in the journal (as soon as accepted) and will be listed together on the special issue website. Research articles, review articles as well as short communications are invited. For planned papers, a title and short abstract (about 100 words) can be sent to the Editorial Office for announcement on this website.

Submitted manuscripts should not have been published previously, nor be under consideration for publication elsewhere (except conference proceedings papers). All manuscripts are thoroughly refereed through a single-blind peer-review process. A guide for authors and other relevant information for submission of manuscripts is available on the Instructions for Authors page. Micromachines is an international peer-reviewed open access monthly journal published by MDPI.

Please visit the Instructions for Authors page before submitting a manuscript. The Article Processing Charge (APC) for publication in this open access journal is 2600 CHF (Swiss Francs). Submitted papers should be well formatted and use good English. Authors may use MDPI's English editing service prior to publication or during author revisions.

Keywords

  • droplet-based microfluidics
  • microfluidic devices
  • microscale fluid flows
  • droplet generation
  • droplet manipulation
  • surface wettability
  • lab-on-a-Chip
  • materials synthesis

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Published Papers (18 papers)

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Editorial

Jump to: Research

4 pages, 176 KiB  
Editorial
Editorial for the Special Issue on Droplet-Based Microfluidics: Design, Fabrication, and Applications
by Pingan Zhu
Micromachines 2023, 14(3), 693; https://doi.org/10.3390/mi14030693 - 21 Mar 2023
Viewed by 1098
Abstract
Microfluidics is a rapidly growing field of research that involves the manipulation and analysis of fluids in small-scale channels, usually with dimensions ranging from sub-micrometer to sub-millimeter [...] Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)

Research

Jump to: Editorial

12 pages, 2906 KiB  
Article
Ultrasoft and Ultrastretchable Wearable Strain Sensors with Anisotropic Conductivity Enabled by Liquid Metal Fillers
by Minjae Choe, Dongho Sin, Priyanuj Bhuyan, Sangmin Lee, Hongchan Jeon and Sungjune Park
Micromachines 2023, 14(1), 17; https://doi.org/10.3390/mi14010017 - 21 Dec 2022
Cited by 7 | Viewed by 2058
Abstract
Herein, ultrasoft and ultrastretchable wearable strain sensors enabled by liquid metal fillers in an elastic polymer are described. The wearable strain sensors that can change the effective resistance upon strains are prepared by mixing silicone elastomer with liquid metal (EGaIn, Eutectic gallium-indium alloy) [...] Read more.
Herein, ultrasoft and ultrastretchable wearable strain sensors enabled by liquid metal fillers in an elastic polymer are described. The wearable strain sensors that can change the effective resistance upon strains are prepared by mixing silicone elastomer with liquid metal (EGaIn, Eutectic gallium-indium alloy) fillers. While the silicone is mixed with the liquid metal by shear mixing, the liquid metal is rendered into small droplets stabilized by an oxide, resulting in a non-conductive liquid metal elastomer. To attain electrical conductivity, localized mechanical pressure is applied using a stylus onto the thermally cured elastomer, resulting in the formation of a handwritten conductive trace by rupturing the oxide layer of the liquid metal droplets and subsequent percolation. Although this approach has been introduced previously, the liquid metal dispersed elastomers developed here are compelling because of their ultra-stretchable (elongation at break of 4000%) and ultrasoft (Young’s modulus of <0.1 MPa) mechanical properties. The handwritten conductive trace in the elastomers can maintain metallic conductivity when strained; however, remarkably, we observed that the electrical conductivity is anisotropic upon parallel and perpendicular strains to the conductive trace. This anisotropic conductivity of the liquid metal elastomer film can manipulate the locomotion of a robot by routing the power signals between the battery and the driving motor of a robot upon parallel and perpendicular strains to the hand-written circuit. In addition, the liquid metal dispersed elastomers have a high degree of deformation and adhesion; thus, they are suitable for use as a wearable sensor for monitoring various body motions. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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10 pages, 3498 KiB  
Article
Surface Modification of 3D Printed Microfluidic Devices for Controlled Wetting in Two-Phase Flow
by Chandler A. Warr, Nicole G. Crawford, Gregory P. Nordin and William G. Pitt
Micromachines 2023, 14(1), 6; https://doi.org/10.3390/mi14010006 - 20 Dec 2022
Cited by 4 | Viewed by 1280
Abstract
Microfluidic devices (MFDs) printed in 3-D geometry using digital light projection to polymerize monomers often have surfaces that are not as hydrophobic as MFDs made from polydimethylsiloxane. Droplet microfluidics in these types of devices are subject to droplet adhesion and aqueous spreading on [...] Read more.
Microfluidic devices (MFDs) printed in 3-D geometry using digital light projection to polymerize monomers often have surfaces that are not as hydrophobic as MFDs made from polydimethylsiloxane. Droplet microfluidics in these types of devices are subject to droplet adhesion and aqueous spreading on less hydrophobic MFD surfaces. We have developed a post-processing technique using hydrophobic monomers that renders the surfaces of these devices much more hydrophobic. The technique is fast and easy, and involves flowing monomer without initiator into the channels and then exposing the entire device to UV light that generates radicals from the initiator molecules remaining in the original 3-D polymerization. After treatment the channels can be cleared and the surface is more hydrophobic, as evidenced by higher contact angles with aqueous droplets. We hypothesize that radicals generated near the previously printed surfaces initiate polymerization of the hydrophobic monomers on the surfaces without bulk polymerization extending into the channels. The most hydrophobic surfaces were produced by treatment with an alkyl acrylate and a fluorinated acrylate. This technique could be used for surface treatment with other types of monomers to impart unique characteristics to channels in MFDs. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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17 pages, 6656 KiB  
Article
Prediction of Dispersion Rate of Airborne Nanoparticles in a Gas-Liquid Dual-Microchannel Separated by a Porous Membrane: A Numerical Study
by Zohreh Sheidaei, Pooria Akbarzadeh, Carlotta Guiducci and Navid Kashaninejad
Micromachines 2022, 13(12), 2220; https://doi.org/10.3390/mi13122220 - 14 Dec 2022
Cited by 1 | Viewed by 1488
Abstract
Recently, there has been increasing attention toward inhaled nanoparticles (NPs) to develop inhalation therapies for diseases associated with the pulmonary system and investigate the toxic effects of hazardous environmental particles on human lung health. Taking advantage of microfluidic technology for cell culture applications, [...] Read more.
Recently, there has been increasing attention toward inhaled nanoparticles (NPs) to develop inhalation therapies for diseases associated with the pulmonary system and investigate the toxic effects of hazardous environmental particles on human lung health. Taking advantage of microfluidic technology for cell culture applications, lung-on-a-chip devices with great potential in replicating the lung air–blood barrier (ABB) have opened new research insights in preclinical pathology and therapeutic studies associated with aerosol NPs. However, the air interface in such devices has been largely disregarded, leaving a gap in understanding the NPs’ dynamics in lung-on-a-chip devices. Here, we develop a numerical parametric study to provide insights into the dynamic behavior of the airborne NPs in a gas–liquid dual-channel lung-on-a-chip device with a porous membrane separating the channels. We develop a finite element multi-physics model to investigate particle tracing in both air and medium phases to replicate the in vivo conditions. Our model considers the impact of fluid flow and geometrical properties on the distribution, deposition, and translocation of NPs with diameters ranging from 10 nm to 900 nm. Our findings suggest that, compared to the aqueous solution of NPs, the aerosol injection of NPs offers more efficient deposition on the substrate of the air channel and higher translocation to the media channel. Comparative studies against accessible data, as well as an experimental study, verify the accuracy of the present numerical analysis. We propose a strategy to optimize the affecting parameters to control the injection and delivery of aerosol particles into the lung-on-chip device depending on the objectives of biomedical investigations and provide optimized values for some specific cases. Therefore, our study can assist scientists and researchers in complementing their experimental investigation in future preclinical studies on pulmonary pathology associated with inhaled hazardous and toxic environmental particles, as well as therapeutic studies for developing inhalation drug delivery. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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16 pages, 4503 KiB  
Article
Kinetic Model and Experiment for Self-Ignition of Triethylaluminum and Triethylborane Droplets in Air
by Sergey M. Frolov, Valentin Y. Basevich, Andrey A. Belyaev, Igor O. Shamshin, Viktor S. Aksenov, Fedor S. Frolov, Pavel A. Storozhenko and Shirin L. Guseinov
Micromachines 2022, 13(11), 2033; https://doi.org/10.3390/mi13112033 - 21 Nov 2022
Cited by 2 | Viewed by 1775
Abstract
Triethylaluminum Al(C2H5)3, TEA, and triethylborane, B(C2H5)3, TEB, are transparent, colorless, pyrophoric liquids with boiling points of approximately 190 °C and 95 °C, respectively. Upon contact with ambient air, TEA, TEB, as [...] Read more.
Triethylaluminum Al(C2H5)3, TEA, and triethylborane, B(C2H5)3, TEB, are transparent, colorless, pyrophoric liquids with boiling points of approximately 190 °C and 95 °C, respectively. Upon contact with ambient air, TEA, TEB, as well as their mixtures and solutions, in hydrocarbon solvents, ignite. They can also violently react with water. TEA and TEB can be used as hypergolic rocket propellants and incendiary compositions. In this manuscript, a novel scheme of the heterogeneous interaction of gaseous oxygen with liquid TEA/TEB microdroplets accompanied by the release of light hydrocarbon radicals into the gas phase is used for calculating the self-ignition of a spatially homogeneous mixture of fuel microdroplets in ambient air at normal pressure and temperature (NPT) conditions. In the primary initiation step, TEA and TEB react with oxygen, producing an ethyl radical, which can initiate an autoxidation chain. The ignition delay is shown to decrease with the decrease in the droplet size. Preliminary experiments on the self-ignition of pulsed and continuous TEA–TEB sprays in ambient air at NPT conditions are used for estimating the Arrhenius parameters of the rate-limiting reaction. Experiments confirm that the self-ignition delay of TEA–TEB sprays decreases with the injection pressure and provide the data for estimating the activation energy of the rate-limiting reaction, which appears to be close to 2 kcal/mol. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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12 pages, 1713 KiB  
Article
Picoliter Droplet Generation and Dense Bead-in-Droplet Encapsulation via Microfluidic Devices Fabricated via 3D Printed Molds
by Tochukwu D. Anyaduba, Jonas A. Otoo and Travis S. Schlappi
Micromachines 2022, 13(11), 1946; https://doi.org/10.3390/mi13111946 - 10 Nov 2022
Cited by 5 | Viewed by 2845
Abstract
Picoliter-scale droplets have many applications in chemistry and biology, such as biomolecule synthesis, drug discovery, nucleic acid quantification, and single cell analysis. However, due to the complicated processes used to fabricate microfluidic channels, most picoliter (pL) droplet generation methods are limited to research [...] Read more.
Picoliter-scale droplets have many applications in chemistry and biology, such as biomolecule synthesis, drug discovery, nucleic acid quantification, and single cell analysis. However, due to the complicated processes used to fabricate microfluidic channels, most picoliter (pL) droplet generation methods are limited to research in laboratories with cleanroom facilities and complex instrumentation. The purpose of this work is to investigate a method that uses 3D printing to fabricate microfluidic devices that can generate droplets with sizes <100 pL and encapsulate single dense beads mechanistically. Our device generated monodisperse droplets as small as ~48 pL and we demonstrated the usefulness of this droplet generation technique in biomolecule analysis by detecting Lactobacillus acidophillus 16s rRNA via digital loop-mediated isothermal amplification (dLAMP). We also designed a mixer that can be integrated into a syringe to overcome dense bead sedimentation and found that the bead-in-droplet (BiD) emulsions created from our device had <2% of the droplets populated with more than 1 bead. This study will enable researchers to create devices that generate pL-scale droplets and encapsulate dense beads with inexpensive and simple instrumentation (3D printer and syringe pump). The rapid prototyping and integration ability of this module with other components or processes can accelerate the development of point-of-care microfluidic devices that use droplet-bead emulsions to analyze biological or chemical samples with high throughput and precision. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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14 pages, 6465 KiB  
Article
A Microfluidic Approach for Probing Heterogeneity in Cytotoxic T-Cells by Cell Pairing in Hydrogel Droplets
by Bart M. Tiemeijer, Lucie Descamps, Jesse Hulleman, Jelle J. F. Sleeboom and Jurjen Tel
Micromachines 2022, 13(11), 1910; https://doi.org/10.3390/mi13111910 - 04 Nov 2022
Cited by 5 | Viewed by 2173
Abstract
Cytotoxic T-cells (CTLs) exhibit strong effector functions to leverage antigen-specific anti-tumoral and anti-viral immunity. When naïve CTLs are activated by antigen-presenting cells (APCs) they display various levels of functional heterogeneity. To investigate this, we developed a single-cell droplet microfluidics platform that allows for [...] Read more.
Cytotoxic T-cells (CTLs) exhibit strong effector functions to leverage antigen-specific anti-tumoral and anti-viral immunity. When naïve CTLs are activated by antigen-presenting cells (APCs) they display various levels of functional heterogeneity. To investigate this, we developed a single-cell droplet microfluidics platform that allows for deciphering single CTL activation profiles by multi-parameter analysis. We identified and correlated functional heterogeneity based on secretion profiles of IFNγ, TNFα, IL-2, and CD69 and CD25 surface marker expression levels. Furthermore, we strengthened our approach by incorporating low-melting agarose to encapsulate pairs of single CTLs and artificial APCs in hydrogel droplets, thereby preserving spatial information over cell pairs. This approach provides a robust tool for high-throughput and single-cell analysis of CTLs compatible with flow cytometry for subsequent analysis and sorting. The ability to score CTL quality, combined with various potential downstream analyses, could pave the way for the selection of potent CTLs for cell-based therapeutic strategies. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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14 pages, 3395 KiB  
Article
Dynamic Behaviours of Monodisperse Double Emulsion Formation in a Tri-Axial Capillary Device
by Yuchen Dai, Haotian Cha, Nhat-Khuong Nguyen, Lingxi Ouyang, Fariba Galogahi, Ajeet Singh Yadav, Hongjie An, Jun Zhang, Chin Hong Ooi and Nam-Trung Nguyen
Micromachines 2022, 13(11), 1877; https://doi.org/10.3390/mi13111877 - 31 Oct 2022
Cited by 2 | Viewed by 1529
Abstract
We investigated experimentally, analytically, and numerically the formation process of double emulsion formations under a dripping regime in a tri-axial co-flow capillary device. The results show that mismatches of core and shell droplets under a given flow condition can be captured both experimentally [...] Read more.
We investigated experimentally, analytically, and numerically the formation process of double emulsion formations under a dripping regime in a tri-axial co-flow capillary device. The results show that mismatches of core and shell droplets under a given flow condition can be captured both experimentally and numerically. We propose a semi-analytical model using the match ratio between the pinch-off length of the shell droplet and the product of the core growth rate and its pinch-off time. The mismatch issue can be avoided if the match ratio is lower than unity. We considered a model with the wall effect to predict the size of the matched double emulsion. The model shows slight deviations with experimental data if the Reynolds number of the continuous phase is lower than 0.06 but asymptotically approaches good agreement if the Reynolds number increases from 0.06 to 0.14. The numerical simulation generally agrees with the experiments under various flow conditions. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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14 pages, 4838 KiB  
Article
Influence of Filler Materials on Wettability and Mechanical Properties of Basalt/E-Glass Woven Fabric–Reinforced Composites for Microfluidics
by Ayyappa Atmakuri, Lalitnarayan Kolli, Arvydas Palevicius, Sigita Urbaite and Giedrius Janusas
Micromachines 2022, 13(11), 1875; https://doi.org/10.3390/mi13111875 - 31 Oct 2022
Cited by 3 | Viewed by 1500
Abstract
This paper presents the development of novel hybrid composites in the presence of filler particles and manufactured using a proposed new fabrication technique. The hybrid composites were fabricated using a basalt and E-glass woven fabric–reinforced epoxy resin matrix combined with graphite powder nanoparticles. [...] Read more.
This paper presents the development of novel hybrid composites in the presence of filler particles and manufactured using a proposed new fabrication technique. The hybrid composites were fabricated using a basalt and E-glass woven fabric–reinforced epoxy resin matrix combined with graphite powder nanoparticles. Six sets of samples were fabricated using the vacuum-assisted free lamination compression molding technique. After the fabrication, wettability, mechanical properties (tensile, flexural and impact properties) and moisture properties were evaluated. Surface morphology and chemical composition of the composite samples were examined using a scanning electron microscope (SEM) and spectroscopy. The obtained results showed that the use of filler materials in hybrid composites improves the properties of hybrid composites. Basalt/E-glass hybrid composites with 10% graphite material exhibited superior mechanical properties over the other composites, with high-quality, improved adhesion and surface morphology. Thus, novel composites with the combination of exceptional properties may be integrated in the design of flexible electronics and microfluidics devices as a structural layer of the system. High flexibility and good surface tension of the designed composites makes them attractive for using the thermal imprint technique for microfluidics channel design. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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16 pages, 16821 KiB  
Article
Relationship between Onset of Sliding Behavior and Size of Droplet on Inclined Solid Substrate
by Yukihiro Yonemoto, Yosuke Fujii, Yoshiki Sugino and Tomoaki Kunugi
Micromachines 2022, 13(11), 1849; https://doi.org/10.3390/mi13111849 - 28 Oct 2022
Cited by 4 | Viewed by 1112
Abstract
Whether a droplet slides or not on inclined solid surface is mainly influenced by a balance between the adhesion force at contact area and the gravitational force exerted on the droplet. Especially as the adhesion force is a key parameter for the determination [...] Read more.
Whether a droplet slides or not on inclined solid surface is mainly influenced by a balance between the adhesion force at contact area and the gravitational force exerted on the droplet. Especially as the adhesion force is a key parameter for the determination of the sliding behavior of droplets. The adhesion force is mainly estimated by experimental observation for the sliding motion of the droplet. However, at present it is unknown whether the adhesion force is a constant value regardless of the droplet size or not. In the present study, focused on the onset for sliding of water-ethanol binary mixture droplets on inclined solid surface, experimental investigation on the sliding droplets is performed by considering the droplet volumes ranging from 7 to 600 μL in order to understand the effect of the size of the droplet on the adhesive property. The results are discussed using the existing analytical models. From the results, it is found that the adhesion force increases in the case of large droplet volume, while the force reaches constant value in the case of small droplet volume. This difference is related to the degree of the droplet shape deformation, which leads to a change in the contact angle. Finally, a simple empirical model for the adhesion force including the size effect is proposed. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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15 pages, 4364 KiB  
Article
A Portable, Negative-Pressure Actuated, Dynamically Tunable Microfluidic Droplet Generator
by Martin Trossbach, Marta de Lucas Sanz, Brinton Seashore-Ludlow and Haakan N. Joensson
Micromachines 2022, 13(11), 1823; https://doi.org/10.3390/mi13111823 - 25 Oct 2022
Cited by 3 | Viewed by 2160
Abstract
Droplet microfluidics utilize a monodisperse water-in-oil emulsion, with an expanding toolbox offering a wide variety of operations on a range of droplet sizes at high throughput. However, translation of these capabilities into applications for non-expert laboratories to fully harness the inherent potential of [...] Read more.
Droplet microfluidics utilize a monodisperse water-in-oil emulsion, with an expanding toolbox offering a wide variety of operations on a range of droplet sizes at high throughput. However, translation of these capabilities into applications for non-expert laboratories to fully harness the inherent potential of microscale manipulations is woefully trailing behind. One major obstacle is that droplet microfluidic setups often rely on custom fabricated devices, costly liquid actuators, and are not easily set up and operated by non-specialists. This impedes wider adoption of droplet technologies in, e.g., the life sciences. Here, we demonstrate an easy-to-use minimal droplet production setup with a small footprint, built exclusively from inexpensive commercially sourced parts, powered and controlled by a laptop. We characterize the components of the system and demonstrate production of droplets ranging in volume from 3 to 21 nL in a single microfluidic device. Furthermore, we describe the dynamic tuning of droplet composition. Finally, we demonstrate the production of droplet-templated cell spheroids from primary cells, where the mobility and simplicity of the setup enables its use within a biosafety cabinet. Taken together, we believe this minimal droplet setup is ideal to drive broad adoption of droplet microfluidics technology. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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22 pages, 6085 KiB  
Article
Strategy for Fast Decision on Material System Suitability for Continuous Crystallization Inside a Slug Flow Crystallizer
by Anne Cathrine Kufner, Adrian Krummnow, Andreas Danzer and Kerstin Wohlgemuth
Micromachines 2022, 13(10), 1795; https://doi.org/10.3390/mi13101795 - 21 Oct 2022
Cited by 5 | Viewed by 1591
Abstract
There is an increasing focus on two-phase flow in micro- or mini-structured apparatuses for various manufacturing and measurement instrumentation applications, including the field of crystallization as a separation technique. The slug flow pattern offers salient features for producing high-quality products, since narrow residence [...] Read more.
There is an increasing focus on two-phase flow in micro- or mini-structured apparatuses for various manufacturing and measurement instrumentation applications, including the field of crystallization as a separation technique. The slug flow pattern offers salient features for producing high-quality products, since narrow residence time distribution of liquid and solid phases, intensified mixing and heat exchange, and an enhanced particle suspension are achieved despite laminar flow conditions. Due to its unique features, the slug flow crystallizer (SFC) represents a promising concept for small-scale continuous crystallization achieving high-quality active pharmaceutical ingredients (API). Therefore, a time-efficient strategy is presented in this study to enable crystallization of a desired solid product in the SFC as quickly as possible and without much experimental effort. This strategy includes pre-selection of the solvent/solvent mixture using heuristics, verifying the slug flow stability in the apparatus by considering the static contact angle and dynamic flow behavior, and modeling the temperature-dependent solubility in the supposed material system using perturbed-chain statistical associating fluid theory (PC-SAFT). This strategy was successfully verified for the amino acids l-alanine and l-arginine and the API paracetamol for binary and ternary systems and, thus, represents a general approach for using different material systems in the SFC. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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14 pages, 4030 KiB  
Article
Development of Finite Element Models of PP, PETG, PVC and SAN Polymers for Thermal Imprint Prediction of High-Aspect-Ratio Microfluidics
by Justas Ciganas, Paulius Griskevicius, Arvydas Palevicius, Sigita Urbaite and Giedrius Janusas
Micromachines 2022, 13(10), 1655; https://doi.org/10.3390/mi13101655 - 30 Sep 2022
Cited by 2 | Viewed by 1333
Abstract
Polymeric microstructures and microchannels are widely used in biomedical devices, optics, microfluidics and fiber optics. The quality, the shape, the spacing and the curvature of microstructure gratings are influenced by different mechanisms and fabrication techniques used. This paper demonstrates a cost-effective way for [...] Read more.
Polymeric microstructures and microchannels are widely used in biomedical devices, optics, microfluidics and fiber optics. The quality, the shape, the spacing and the curvature of microstructure gratings are influenced by different mechanisms and fabrication techniques used. This paper demonstrates a cost-effective way for patterning high-aspect-ratio thermoplastic microstructures using thermal imprint technology and finite element modeling. Polymeric materials polypropylene (PP), polyethylene terephthalate glycol (PETG), polyvinyl chloride (PVC) and styrene-acrylonitrile (SAN) were chosen for the experimental investigations. A finite element model was constructed to define the most suitable parameters (time, heating temperature, pressure, etc.) for the formation of microstructures using the thermal imprint procedure. To confirm the relevance of the finite element model, different types of PP, PETG, PVC and SAN microstructures were fabricated using theoretically defined parameters. Experimental investigations of imprinted microstructures’ morphological and optical properties were performed using scanning electron microscopy, atomic force microscopy and a diffractometer. Obtained results confirmed the relevance of the created finite element model which was applied in the formation of high-aspect-ratio microstructures. Application of this model in thermal imprint would not only reduce the fabrication time, but also would highly increase the surface quality and optical properties of the formed structures. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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21 pages, 6576 KiB  
Article
Interaction of Shock Waves with Water Saturated by Nonreacting or Reacting Gas Bubbles
by Sergey M. Frolov, Konstantin A. Avdeev, Viktor S. Aksenov, Illias A. Sadykov, Igor O. Shamshin and Fedor S. Frolov
Micromachines 2022, 13(9), 1553; https://doi.org/10.3390/mi13091553 - 19 Sep 2022
Cited by 2 | Viewed by 1596
Abstract
A compressible medium represented by pure water saturated by small nonreactive or reactive gas bubbles can be used for generating a propulsive force in large-, medium-, and small-scale thrusters referred to as a pulsed detonation hydroramjet (PDH), which is a novel device for [...] Read more.
A compressible medium represented by pure water saturated by small nonreactive or reactive gas bubbles can be used for generating a propulsive force in large-, medium-, and small-scale thrusters referred to as a pulsed detonation hydroramjet (PDH), which is a novel device for underwater propulsion. The PDH thrust is produced due to the acceleration of bubbly water (BW) in a water guide by periodic shock waves (SWs) and product gas jets generated by pulsed detonations of a fuel–oxidizer mixture. Theoretically, the PDH thrust is proportional to the operation frequency, which depends on both the SW velocity in BW and pulsed detonation frequency. The studies reported in this manuscript were aimed at exploring two possible directions of the improvement of thruster performances, namely, (1) the replacement of chemically nonreacting gas bubbles by chemically reactive ones, and (2) the increase in the pulsed detonation frequency from tens of hertz to some kilohertz. To better understand the SW-to-BW momentum transfer, the interaction of a single SW and a high-frequency (≈7 kHz) sequence of three SWs with chemically inert or active BW containing bubbles of air or stoichiometric acetylene–oxygen mixture was studied experimentally. Single SWs and SW packages were generated by burning or detonating a gaseous stoichiometric acetylene–oxygen or propane–oxygen mixture and transmitting the arising SWs to BW. The initial volume fraction of gas in BW was varied from 2% to 16% with gas bubbles 1.5–4 mm in diameter. The propagation velocity of SWs in BW ranged from 40 to 580 m/s. In experiments with single SWs in chemically active BW, a detonation-like mode of reaction front propagation (“bubbly quasidetonation”) was realized. This mode consisted of a SW followed by the front of bubble explosions and was characterized by a considerably higher propagation velocity as compared to the chemically inert BW. The latter could allow increasing the PDH operation frequency and thrust. Experiments with high-frequency SW packages showed that on the one hand, the individual SWs quickly merged, feeding each other and increasing the BW velocity, but on the other hand, the initial gas content for each successive SW decreased and, accordingly, the SW-to-BW momentum transfer worsened. Estimates showed that for a small-scale water guide 0.5 m long, the optimal pulsed detonation frequency was about 50–60 Hz. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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13 pages, 5323 KiB  
Article
Asymmetric Jetting during the Impact of Liquid Drops on Superhydrophobic Concave Surfaces
by Chengmin Chen, Hongjun Zhong, Zhe Liu, Jianchun Wang, Jianmei Wang, Guangxia Liu, Yan Li and Pingan Zhu
Micromachines 2022, 13(9), 1521; https://doi.org/10.3390/mi13091521 - 14 Sep 2022
Cited by 2 | Viewed by 1674
Abstract
The impact of liquid drops on superhydrophobic solid surfaces is ubiquitous and of practical importance in many industrial processes. Here, we study the impingement of droplets on superhydrophobic surfaces with a macroscopic dimple structure, during which the droplet exhibits asymmetric jetting. Systematic experimental [...] Read more.
The impact of liquid drops on superhydrophobic solid surfaces is ubiquitous and of practical importance in many industrial processes. Here, we study the impingement of droplets on superhydrophobic surfaces with a macroscopic dimple structure, during which the droplet exhibits asymmetric jetting. Systematic experimental investigations and numerical simulations provide insight into the dynamics and underlying mechanisms of the observed phenomenon. The observation is a result of the interaction between the spreading droplet and the dimple. An upward internal flow is induced by the dimple, which is then superimposed on the horizontal flow inside the spreading droplet. As such, an inclined jet is issued asymmetrically into the air. This work would be conducive to the development of an open-space microfluidic platform for droplet manipulation and generation. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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11 pages, 2123 KiB  
Article
Variable Stiffness Conductive Composites by 4D Printing Dual Materials Alternately
by Fei Long, Gaojie Xu, Jing Wang, Yong Ren and Yuchuan Cheng
Micromachines 2022, 13(8), 1343; https://doi.org/10.3390/mi13081343 - 19 Aug 2022
Cited by 2 | Viewed by 2017
Abstract
Materials that can be designed with programmable properties and which change in response to external stimuli are of great importance in numerous fields of soft actuators, involving robotics, drug delivery and aerospace applications. In order to improve the interaction of human and robots, [...] Read more.
Materials that can be designed with programmable properties and which change in response to external stimuli are of great importance in numerous fields of soft actuators, involving robotics, drug delivery and aerospace applications. In order to improve the interaction of human and robots, materials with variable stiffness are introduced to develop their compliance. A variable stiffness composite has been investigated in this paper, which is composed of liquid metals (LMs) and silicone elastomers. The phase changing materials (LMs) have been encapsulated into silicone elastomer by printing the dual materials alternately with three-dimensional direct ink writing. Such composites enable the control over their own stiffness between soft and rigid states through LM effective phase transition. The tested splines demonstrated that the stiffness changes approximately exceeded 1900%, and the storage modulus is 4.75 MPa and 0.2 MPa when LM is rigid and soft, respectively. In the process of heating up, the stretching strain can be enlarged by at least three times, but the load capacity is weakened. At a high temperature, the resistance of the conductive composites changes with the deformation degree, which is expected to be applied in the field of soft sensing actuators. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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20 pages, 7701 KiB  
Article
An Ultra-Micro-Volume Adhesive Transfer Method and Its Application in fL–pL-Level Adhesive Distribution
by Huifang Liu, Xi Chen, Shuqing Wang, Shenhui Jiang, Ying Chen and Fuxuan Li
Micromachines 2022, 13(5), 664; https://doi.org/10.3390/mi13050664 - 23 Apr 2022
Cited by 3 | Viewed by 1701
Abstract
This study is aimed at addressing the urgent demand for ultra-micro-precision dispensing technology in high-performance micro- and nanometer encapsulation, connection, and assembly manufacturing, considering the great influence of colloid viscosity and surface tension on the dispensing process in micro- and nanometer scale. According [...] Read more.
This study is aimed at addressing the urgent demand for ultra-micro-precision dispensing technology in high-performance micro- and nanometer encapsulation, connection, and assembly manufacturing, considering the great influence of colloid viscosity and surface tension on the dispensing process in micro- and nanometer scale. According to the principle of liquid transfer, a method of adhesive transfer that can realize fL–pL levels is studied in this paper. A mathematical model describing the initial droplet volume and the transfer droplet volume was established, and the factors affecting the transfer process of adhesive were analyzed by the model. The theoretical model of the transfer droplet volume was verified by a 3D scanning method. The relationships between the transfer droplet volume and the initial droplet volume, stay time, initial distance, and stretching speed were systematically analyzed by a single-factor experiment, and the adhesive transfer rate was calculated. Combined with trajectory planning, continuous automatic dispensing experiments with different patterns were developed, and the problems of the transfer droplet size, appearance quality, and position accuracy were analyzed comprehensively. The results show that the average relative deviation of the transfer droplet lattice position obtained by the dispensing method in this paper was 6.2%. The minimum radius of the transfer droplet was 11.7 μm, and the minimum volume of the transfer droplet was 573.3 fL. Furthermore, microporous encapsulation was realized using the method of ultra-micro-dispensing. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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9 pages, 3353 KiB  
Communication
Fabrication of Transparent and Flexible Digital Microfluidics Devices
by Jianchen Cai, Jiaxi Jiang, Jinyun Jiang, Yin Tao, Xiang Gao, Meiya Ding and Yiqiang Fan
Micromachines 2022, 13(4), 498; https://doi.org/10.3390/mi13040498 - 23 Mar 2022
Cited by 4 | Viewed by 2690
Abstract
This study proposed a fabrication method for thin, film-based, transparent, and flexible digital microfluidic devices. A series of characterizations were also conducted with the fabricated digital microfluidic devices. For the device fabrication, the electrodes were patterned by laser ablation of 220 nm-thick indium [...] Read more.
This study proposed a fabrication method for thin, film-based, transparent, and flexible digital microfluidic devices. A series of characterizations were also conducted with the fabricated digital microfluidic devices. For the device fabrication, the electrodes were patterned by laser ablation of 220 nm-thick indium tin oxide (ITO) layer on a 175 μm-thick polyethylene terephthalate (PET) substrate. The electrodes were insulated with a layer of 12 μm-thick polyethylene (PE) film as the dielectric layer, and finally, a surface treatment was conducted on PE film in order to enhance the hydrophobicity. The whole digital microfluidic device has a total thickness of less than 200 μm and is nearly transparent in the visible range. The droplet manipulation with the proposed digital microfluidic device was also achieved. In addition, a series of characterization studies were conducted as follows: the contact angles under different driving voltages, the leakage current density across the patterned electrodes, and the minimum driving voltage with different control algorithms and droplet volume were measured and discussed. The UV–VIS spectrum of the proposed digital microfluidic devices was also provided in order to verify the transparency of the fabricated device. Compared with conventional methods for the fabrication of digital microfluidic devices, which usually have opaque metal/carbon electrodes, the proposed transparent and flexible digital microfluidics could have significant advantages for the observation of the droplets on the digital microfluidic device, especially for colorimetric analysis using the digital microfluidic approach. Full article
(This article belongs to the Special Issue Droplet-Based Microfluidics: Design, Fabrication and Applications)
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