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Complex Flow Dynamics at Microscale
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Complex Flow Dynamics at Microscale

A special issue of Materials (ISSN 1996-1944). This special issue belongs to the section "Advanced Composites".

Deadline for manuscript submissions: closed (30 June 2020) | Viewed by 13532

Special Issue Editors

Dr. Francisco Galindo Rosales

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Guest Editor
Department of Chemical Engineering, Transport Phenomena Research Center (CEFT), Faculty of Engineering of the University of Porto, Rua Dr. Roberto Frias s/n, CP 4200-465 Porto, Portugal
Interests: complex fluids; rheology; printing techniques
Special Issues, Collections and Topics in MDPI journals
Dr. Laura Campo-Deaño

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Guest Editor
Centro de Estudos de Fenómenos de Transporte (CEFT), Departamento de Engenharia Mecânica, Faculdade de Engenharia da Universidade do Porto, Rua Dr. Roberto Frias s/n, CP 4200-465 Porto, Portugal
Interests: fluid mechanics; complex fluid flows; rheology; hemodynamics; biofluids and biomaterials
Special Issues, Collections and Topics in MDPI journals

Special Issue Information

Dear Colleagues,

Microfluidics deals with fluid flows confined in channels with a characteristic length scale of the order of hundreds of microns at most. Therefore, at microscale, most of the flows are intrinsically laminar, with low numerical values of Reynolds. This may wrongly be assumed to be synonymous to simple and predictable flow dynamics. Nevertheless, under severe confinement conditions, a wide variety of scientific problems emerge, leading to challenging problems that make this topic worthy of being the focus of this Special Issue. Among a plethora of very interesting problems, we would like to highlight the following ones:

Micromixing: At low Reynolds numbers, two streams of fluids will flow parallel to each other and will not mix, simply because laminar diffusion dominates the flow. This has led to extensive studies with different approaches (active and passive micromixers) aiming at increasing mixing efficiency.

Microrheometry: At small length scales, the elasticity number, which represents the balance between elastic and inertial forces, is intrinsically large for viscoelastic fluids. Thus, the microscale opens a new door for rheometric purposes, providing an excellent platform for characterizing complex fluids with a low relaxation time and low viscosity unreachable at macroscale. Moreover, at microscale, new flowfield configurations could be proposed for the rheological characterization of magneto and electrorheological fluids.

Flow around obstacles: The analysis of complex flow dynamics around microdevices with potential use in biomedicine is essential for the development of optimal microbots and biosensors to minimize their impact on the patient body and to improve the binding efficiency of analyte–ligand through a sensitive membrane, respectively. 

All these problems and many more, such as those topics related to elastic instabilities, particle migration, hemodynamics, and fluid–structure Interactions, require extensive fundamental study from all approaches, i.e., theoretical, numerical, and experimental. This Special Issue is expected to gather contributions that describe recent results obtained in complex flow dynamics at microscale and to demonstrate how those results could be important in terms of real-world application.

Dr. Francisco J. Galindo-Rosales
Dr. Laura Campo-Deaño
Guest Editors

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. Materials is an international peer-reviewed open access semimonthly 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

  • microfluidics
  • complex fluids
  • microrheometry
  • micromixing
  • particle migration
  • FSI
  • elastic instabilities
  • hemodynamics

Published Papers (4 papers)

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Research

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15 pages, 5901 KiB  
Article
Experimental Analysis of the Extensional Flow of Very Weakly Viscoelastic Polymer Solutions
by Manuel Rubio, Alberto Ponce-Torres, Emilio José Vega and José María Montanero
Materials 2020, 13(1), 192; https://doi.org/10.3390/ma13010192 - 02 Jan 2020
Cited by 7 | Viewed by 1929
Abstract
We study with ultra-high-speed imaging the thinning of the filament formed during the breakup of a pendant droplet of very weakly viscoelastic polymer solutions of polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO). In the latter case, we consider two molecular weights: 10 5 g/mol [...] Read more.
We study with ultra-high-speed imaging the thinning of the filament formed during the breakup of a pendant droplet of very weakly viscoelastic polymer solutions of polyvinylpyrrolidone (PVP) and polyethylene oxide (PEO). In the latter case, we consider two molecular weights: 10 5 g/mol (PEO100K) and 2 × 10 6 g/mol (PEO2M). The results allow us to measure with high reproducibility extensional relaxation times of the order of 10 μ s. Despite the noticeable differences between PVP and PEO100K, very similar values are obtained for the range of concentrations where the linear elasto-capillary is established. For PEO2M, the extensional relaxation time depends on the concentration even for values significantly smaller than the overlap one. The prediction c low for the concentration below which the linear elasto-capillary regime cannot be reached qualitatively agrees with the results for PVP and PEO2M, while it underestimates the critical concentration for PEO100K. The results for PEO2M are consistent with those reported in the literature for higher concentrations. Full article
(This article belongs to the Special Issue Complex Flow Dynamics at Microscale)
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20 pages, 16260 KiB  
Article
Microfluidics as a Platform for the Analysis of 3D Printing Problems
by Rui Mendes, Paola Fanzio, Laura Campo-Deaño and Francisco J. Galindo-Rosales
Materials 2019, 12(17), 2839; https://doi.org/10.3390/ma12172839 - 03 Sep 2019
Cited by 10 | Viewed by 4428
Abstract
Fused Filament Fabrication is an extrusion deposition technique in which a thermoplastic filament is melted, pushed through a nozzle and deposited to build, layer-by-layer, custom 3D geometries. Despite being one of the most widely used techniques in 3D printing, there are still some [...] Read more.
Fused Filament Fabrication is an extrusion deposition technique in which a thermoplastic filament is melted, pushed through a nozzle and deposited to build, layer-by-layer, custom 3D geometries. Despite being one of the most widely used techniques in 3D printing, there are still some challenges to be addressed. One of them is the accurate control of the extrusion flow. It has been shown that this is affected by a reflux upstream the nozzle. Numerical models have been proposed for the explanation of this back-flow. However, it is not possible to have optical access to the melting chamber in order to confirm the actual behavior of this annular meniscus. Thus, microfluidics seems to be an excellent platform to tackle this fluid flow problem. In this work, a microfluidic device mimicking the 3D printing nozzle was developed, to study the complex fluid-flow behavior inside it. The principal aim was to investigate the presence of the mentioned back-flow upstream the nozzle contraction. As the microfluidic chip was fabricated by means of soft-lithography, the use of polymer melts was restricted due to technical issues. Thus, the working fluids consisted of two aqueous polymer solutions that allowed replicating the printing flow conditions in terms of Elasticity number and to develop a D e – R e flow map. The results demonstrate that the presence of upstream vortices, due to the elasticity of the fluid, is responsible for the back-flow problem. Full article
(This article belongs to the Special Issue Complex Flow Dynamics at Microscale)
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19 pages, 10094 KiB  
Article
Towards an Optimal Pressure Tap Design for Fluid-Flow Characterisation at Microscales
by Tomás Rodrigues, Francisco J. Galindo-Rosales and Laura Campo-Deaño
Materials 2019, 12(7), 1086; https://doi.org/10.3390/ma12071086 - 02 Apr 2019
Cited by 9 | Viewed by 2702
Abstract
Measuring fluid pressure in microchannels is difficult and constitutes a challenge to even the most experienced of experimentalists. Currently, to the best of the authors’ knowledge, no optimal solution are being used for the design of pressure taps, nor guidelines concerning their shape [...] Read more.
Measuring fluid pressure in microchannels is difficult and constitutes a challenge to even the most experienced of experimentalists. Currently, to the best of the authors’ knowledge, no optimal solution are being used for the design of pressure taps, nor guidelines concerning their shape and its relation with the accuracy of the readings. In an attempt to address this issue, a parametric study was devised to evaluate the performance of different pressure tap designs, 18 in total. These were obtained by combining three shape parameters: sub-channel width (w) and sub-channel–tap radius (R) or angle (α), while having the sub-channel length kept constant. For each configuration, pressure drop measurements were carried out along several lengths of a straight microfluidic rectangular channel and later compared to an analytical solution. The microchannels were fabricated out of PDMS using standard soft-lithography techniques, pressure drop was measured with differential pressure sensors, the test fluid was DI water and the flow conditions varied from creeping flow up to R e c ∼100. Pressure taps, having smooth contours (characterised by the radius R) and a sub-channel width (w) of 108 μ m , performed the best with results from that of radius R = 50 μ m only falling short of the theory by a mere ∼ 5 % . Full article
(This article belongs to the Special Issue Complex Flow Dynamics at Microscale)
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Other

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13 pages, 5543 KiB  
Concept Paper
Direct Ink Writing Glass: A Preliminary Step for Optical Application
by Bo Nan, Przemysław Gołębiewski, Ryszard Buczyński, Francisco J. Galindo-Rosales and José M. F. Ferreira
Materials 2020, 13(7), 1636; https://doi.org/10.3390/ma13071636 - 01 Apr 2020
Cited by 18 | Viewed by 3857
Abstract
In this paper, we present a preliminary study and conceptual idea concerning 3D printing water-sensitive glass, using a borosilicate glass with high alkali and alkaline oxide contents as an example in direct ink writing. The investigated material was prepared in the form of [...] Read more.
In this paper, we present a preliminary study and conceptual idea concerning 3D printing water-sensitive glass, using a borosilicate glass with high alkali and alkaline oxide contents as an example in direct ink writing. The investigated material was prepared in the form of a glass frit, which was further ground in order to obtain a fine powder of desired particle size distribution. In a following step, inks were prepared by mixing the fine glass powder with Pluoronic F-127 hydrogel. The acquired pastes were rheologically characterized and printed using a Robocasting device. Differential scanning calorimetry (DSC) experiments were performed for base materials and the obtained green bodies. After sintering, scanning electron microscope (SEM) and X-ray diffraction (XRD) analyses were carried out in order to examine microstructure and the eventual presence of crystalline phase inclusions. The results confirmed that the as obtained inks exhibit stable rheological properties despite the propensity of glass to undergo hydrolysis and could be adjusted to desirable values for 3D printing. No additional phase was observed, supporting the suitability of the designed technology for the production of water sensitive glass inks. SEM micrographs of the sintered samples revealed the presence of closed porosity, which may be the main reason of light scattering. Full article
(This article belongs to the Special Issue Complex Flow Dynamics at Microscale)
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