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Micromachines
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Flows in Micro- and Nano-Systems
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Flows in Micro- and Nano-Systems

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

Deadline for manuscript submissions: 30 November 2025 | Viewed by 12449

Special Issue Editor

Dr. S. Kokou Dadzie

E-Mail Website
Guest Editor
School of Engineering and Physical Sciences, Heriot-Watt University, Edinburgh EH14 4AS, UK
Interests: kinetic theory; gas dynamics; non-continuum flows; micro/nano-fluidics; compressible fluid mechanics; multiphase and granular flows; swarm behaviour

Special Issue Information

Dear Colleagues,

Experimental data from pressure-driven liquid that flow through nanotubes have shown flow velocities over the past two decades that are four to five orders of magnitude higher than those predicted by the classical theory. Attempts to explain these enhanced mass flow rates at the nanoscale are still to be successful. In classical gas dynamics, the temperature field alone cannot induce a steady flow in gas without an external force such as gravity. However, in rarefied gas, the temperature field in gas under surface effects can induce a variety of flows. Interfacial phenomena at the solid–liquid interface at the micro- and nanoscale have various functions in micro- and nanofluidic device fabrications. Some of these devices operate on the physical mechanism of flows of electrolyte solutions. Viscous thickening due to electrostatic interactions is an example of a phenomenon to be understood in these flows. The influence of effects such as velocity slip and surface diffusion in the prediction of profiles of velocity, electrical potential, charge, and ion-transport characteristics is also to be understood. Multiphase flow in microfluidic systems generally shows complicated behaviour, but it has many practical uses. This Special Issue aims to discuss the current theoretical, numerical, and experimental progress in understanding the physical mechanisms of these flows.

We look forward to receiving your submissions.

Dr. S. Kokou Dadzie
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 2100 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

  • nanofluid heat transfer
  • nano-liquid flows
  • thermal-creeping flows/thermophoresis
  • diffusion in microfluidics
  • Newtonian/non-Newtonian electrolyte flows
  • electro-viscous flows
  • multiphase flows in micro- and nanochannels

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Published Papers (7 papers)

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Research

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19 pages, 1093 KiB  
Article
Rheological Property Changes in Polyacrylamide Aqueous Solution Flowed Through Microchannel Under Low Reynolds Number and High Shear Rate Conditions
by Yishuai Li, Yukihiro Yonemoto, Yuki Yamahata and Akimaro Kawahara
Micromachines 2025, 16(5), 545; https://doi.org/10.3390/mi16050545 - 30 Apr 2025
Viewed by 215
Abstract
As an important structure of microfluidic devices, microchannels have the advantages of precise flow control and high reaction efficiency. This study investigates experimentally changing the rheological properties of a polyacrylamide (PAM) aqueous solution after flowing through a square microchannel with a hydraulic diameter [...] Read more.
As an important structure of microfluidic devices, microchannels have the advantages of precise flow control and high reaction efficiency. This study investigates experimentally changing the rheological properties of a polyacrylamide (PAM) aqueous solution after flowing through a square microchannel with a hydraulic diameter of 0.5 mm under low Reynolds number and high shear rate conditions. To know the effect of the channel length on the change in viscosity and relaxation time, the length is changed to 100 mm and 200 mm. From the experiment, it is found that both the viscosity and relaxation time of the solution decrease with increasing the shear rate and the microchannel length. Based on the present experimental data, an empirical model is proposed to predict the change ratio of the relaxation time before and after passing through the microchannel, and the calculation with the model has an agreement with the experiment with root-mean-square absolute error of 0.007. Full article
(This article belongs to the Special Issue Flows in Micro- and Nano-Systems)
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15 pages, 1516 KiB  
Article
Directional Fluidity of Dense Emulsion Activated by Transverse Wedge-Shaped Microroughness
by Giacomo Guastella, Daniele Filippi, Davide Ferraro, Giampaolo Mistura and Matteo Pierno
Micromachines 2025, 16(3), 335; https://doi.org/10.3390/mi16030335 - 14 Mar 2025
Viewed by 456
Abstract
The handling and fluidization of amorphous soft solids, such as emulsions, foams, or gels, is crucial in many technological processes. This is generally achieved by applying mechanical stress that overcomes a critical threshold, known as yield stress, below which these systems behave as [...] Read more.
The handling and fluidization of amorphous soft solids, such as emulsions, foams, or gels, is crucial in many technological processes. This is generally achieved by applying mechanical stress that overcomes a critical threshold, known as yield stress, below which these systems behave as elastic solids. However, the interaction with the walls can facilitate the transition from solid to fluid by activating rearrangements of the fluid constituents close to the wall, resulting in increased fluidity of the system up to distances greater than the spatial scale of the rearrangements. We address the impact of wedge-shaped microroughness on activating the fluidization of emulsion droplets in pressure-driven flow through microfluidic channels. We realize the micro wedges by maskless photolithography to texture one wall of the channel and measure the velocity profiles for flow directed accordingly and against the increasing ramp of the wedge-shaped grooves. We report the enhancement of the emulsion flow in the direction of the climbing ramp of the wedge activated by increasing the magnitude of the pressure gradient. A gain for the volumetric flow rate is registered with respect to the opposite direction as being to 30%, depending on the pressure drop. Full article
(This article belongs to the Special Issue Flows in Micro- and Nano-Systems)
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18 pages, 16263 KiB  
Article
The Deformation of a Liquid Metal Droplet Under Continuous Acceleration in a Variable Cross-Section Groove
by Hanyang Xu, Haojie Dang, Wenchao Tian and Zhao Li
Micromachines 2024, 15(12), 1472; https://doi.org/10.3390/mi15121472 - 4 Dec 2024
Viewed by 3729
Abstract
This paper constructs a numerical simulation model for the deformation of droplets in a variable cross-section groove of a liquid droplet MEMS switch under different directions, amplitudes, frequencies, and waveforms of acceleration. The numerical simulation utilizes the level set method to monitor the [...] Read more.
This paper constructs a numerical simulation model for the deformation of droplets in a variable cross-section groove of a liquid droplet MEMS switch under different directions, amplitudes, frequencies, and waveforms of acceleration. The numerical simulation utilizes the level set method to monitor the deformation surface boundary of the metal droplets. The simulation outcomes manifest that when the negative impact acceleration on the X-axis is 12.9 m/s2, the negative impact acceleration on the Y-axis is 90 m/s2, the negative impact acceleration on the Z-axis is 34.5 m/s2, and the metal droplet interfaces with the metal electrode. The droplet deformation under the effect of a sine wave acceleration signal in the X and Y directions is lower than that under impact acceleration, while in the Z direction, the deformation is higher than that under impact acceleration. The deformation of metal droplets under square wave acceleration is more pronounced than that under sinusoidal wave acceleration. The deformation escalates with the augmentation in square wave amplitude and dwindles with the reduction in square wave acceleration frequency. Furthermore, there exists a phase difference between the deformation curve of the metal droplet and the continuous acceleration signal curve, and the phase difference is dependent of the material properties of the metal droplet. This work elucidates the deformation of the liquid-metal droplets under continuous acceleration and furnishes the foundation for the continuous operation design of MEMS droplet switches. Full article
(This article belongs to the Special Issue Flows in Micro- and Nano-Systems)
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12 pages, 4079 KiB  
Article
Engineering Wettability Transitions on Laser-Textured Shark Skin-Inspired Surfaces via Chemical Post-Processing Techniques
by Elham Lori Zoudani, Nam-Trung Nguyen and Navid Kashaninejad
Micromachines 2024, 15(12), 1442; https://doi.org/10.3390/mi15121442 - 28 Nov 2024
Viewed by 933
Abstract
Surface wettability, the interaction between a liquid droplet and the surface it contacts, plays a key role in influencing droplet behavior and flow dynamics. There is a growing interest in designing surfaces with tailored wetting properties across diverse applications. Advanced fabrication techniques that [...] Read more.
Surface wettability, the interaction between a liquid droplet and the surface it contacts, plays a key role in influencing droplet behavior and flow dynamics. There is a growing interest in designing surfaces with tailored wetting properties across diverse applications. Advanced fabrication techniques that create surfaces with unique wettability offer significant innovation potential. This study investigates the wettability transition of laser-textured anisotropic surfaces featuring shark skin-inspired microstructures using four post-processing methods: spray coating, isopropyl alcohol (IPA) treatment, silicone oil treatment, and silanization. The impact of each method on surface wettability was assessed through water contact angle measurements, scanning electron microscopy (SEM), and laser scanning microscopy. The results show a transition from superhydrophilic behavior on untreated laser-textured surfaces to various (super)hydrophobic states following surface treatment. Chemical treatments produced different levels of hydrophobicity and anisotropy, with silanization achieving the highest hydrophobicity and long-term stability, persisting for one year post-treatment. This enhancement is attributed to the low surface energy and chemical properties of silane compounds, which reduce surface tension and increase water repellence. In conclusion, this study demonstrates that post-processing techniques can effectively tailor surface wettability, enabling a wide range of wetting properties with significant implications for practical applications. Full article
(This article belongs to the Special Issue Flows in Micro- and Nano-Systems)
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11 pages, 4563 KiB  
Article
A Spectroscopy Solution for Contactless Conductivity Detection in Capillary Electrophoresis
by Tomas Drevinskas, Audrius Maruška, Hirotaka Ihara, Makoto Takafuji, Linas Jonušauskas, Domantas Armonavičius, Mantas Stankevičius, Kristina Bimbiraitė-Survilienė, Elzbieta Skrzydlewska, Ona Ragažinskienė, Yutaka Kuwahara, Shoji Nagaoka, Vilma Kaškonienė and Loreta Kubilienė
Micromachines 2024, 15(12), 1430; https://doi.org/10.3390/mi15121430 - 28 Nov 2024
Viewed by 1182
Abstract
This paper introduces a novel contactless single-chip detector that utilizes impedance-to-digital conversion technology to measure impedance in the microfluidic channel or capillary format analytical device. The detector is designed to operate similarly to capacitively coupled contactless conductivity detectors for capillary electrophoresis or chromatography [...] Read more.
This paper introduces a novel contactless single-chip detector that utilizes impedance-to-digital conversion technology to measure impedance in the microfluidic channel or capillary format analytical device. The detector is designed to operate similarly to capacitively coupled contactless conductivity detectors for capillary electrophoresis or chromatography but with the added capability of performing frequency sweeps up to 200 kHz. At each recorded data point, impedance and phase-shift data can be extracted, which can be used to generate impedance versus frequency plots, or phase-shift versus frequency plots. Real and imaginary parts can also be calculated from the data, allowing for the generation of Nyquist diagrams. This detector represents the first of its kind in the contactless conductivity class to provide spectrum-type data, as demonstrated in capillary electrophoresis experiments. Full article
(This article belongs to the Special Issue Flows in Micro- and Nano-Systems)
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24 pages, 10140 KiB  
Article
On the Complex Flow Dynamics of Shear Thickening Fluids Entry Flows
by Miguel Montenegro and Francisco J. Galindo-Rosales
Micromachines 2024, 15(11), 1281; https://doi.org/10.3390/mi15111281 - 22 Oct 2024
Viewed by 1364
Abstract
Due to their nature, using shear thickening fluids (STFs) in engineering applications has sparked an interest in developing energy-dissipating systems, such as damping devices or shock absorbers. The Rheinforce technology allows the design of customized energy dissipative composites by embedding microfluidic channels filled [...] Read more.
Due to their nature, using shear thickening fluids (STFs) in engineering applications has sparked an interest in developing energy-dissipating systems, such as damping devices or shock absorbers. The Rheinforce technology allows the design of customized energy dissipative composites by embedding microfluidic channels filled with STFs in a scaffold material. One of the reasons for using microfluidic channels is that their shape can be numerically optimized to control pressure drop (also known as rectifiers); thus, by controlling the pressure drop, it is possible to control the energy dissipated by the viscous effect. Upon impact, the fluid is forced to flow through the microchannel, experiencing the typical entry flow until it reaches the fully developed flow. It is well-known for Newtonian fluid that the entrance flow is responsible for a non-negligible percentage of the total pressure drop in the fluid; therefore, an analysis of the fluid flow at the entry region for STFs is of paramount importance for an accurate design of the Rheinforce composites. This analysis has been numerically performed before for shear-thickening fluids modeled by a power-law model; however, as this constitutive model represents a continuously growing viscosity between end-viscosity plateau values, it is not representative of the characteristic viscosity curve of shear-thickening fluids, which typically exhibit a three-region shape (thinning-thickening-thinning). For the first time, the influence of these three regions on the entry flow on an axisymmetric pipe is analyzed. Two-dimensional numerical simulations have been performed for four STFs consisting of four dispersions of fumed silica nanoparticles in polypropylene glycol varying concentrations (7.5–20 wt%). Full article
(This article belongs to the Special Issue Flows in Micro- and Nano-Systems)
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Review

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42 pages, 10634 KiB  
Review
Computational Fluid–Structure Interaction in Microfluidics
by Hafiz Muhammad Musharaf, Uditha Roshan, Amith Mudugamuwa, Quang Thang Trinh, Jun Zhang and Nam-Trung Nguyen
Micromachines 2024, 15(7), 897; https://doi.org/10.3390/mi15070897 - 9 Jul 2024
Cited by 6 | Viewed by 3578
Abstract
Micro elastofluidics is a transformative branch of microfluidics, leveraging the fluid–structure interaction (FSI) at the microscale to enhance the functionality and efficiency of various microdevices. This review paper elucidates the critical role of advanced computational FSI methods in the field of micro elastofluidics. [...] Read more.
Micro elastofluidics is a transformative branch of microfluidics, leveraging the fluid–structure interaction (FSI) at the microscale to enhance the functionality and efficiency of various microdevices. This review paper elucidates the critical role of advanced computational FSI methods in the field of micro elastofluidics. By focusing on the interplay between fluid mechanics and structural responses, these computational methods facilitate the intricate design and optimisation of microdevices such as microvalves, micropumps, and micromixers, which rely on the precise control of fluidic and structural dynamics. In addition, these computational tools extend to the development of biomedical devices, enabling precise particle manipulation and enhancing therapeutic outcomes in cardiovascular applications. Furthermore, this paper addresses the current challenges in computational FSI and highlights the necessity for further development of tools to tackle complex, time-dependent models under microfluidic environments and varying conditions. Our review highlights the expanding potential of FSI in micro elastofluidics, offering a roadmap for future research and development in this promising area. Full article
(This article belongs to the Special Issue Flows in Micro- and Nano-Systems)
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