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Formation of colloidal threads in geometrically varying flow-focusing channels
KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW.
KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Fibre- and Polymer Technology, Fibre Technology. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.ORCID iD: 0000-0003-3737-0091
KTH, School of Engineering Sciences (SCI), Engineering Mechanics. KTH, School of Engineering Sciences (SCI), Centres, Linné Flow Center, FLOW. KTH, School of Engineering Sciences in Chemistry, Biotechnology and Health (CBH), Centres, Wallenberg Wood Science Center.ORCID iD: 0000-0002-2504-3969
2021 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 6, no 11, article id 114001Article in journal (Refereed) Published
Abstract [en]

Threads of colloidal dispersions can be formed in microfluidic channel systems and are often used for analytical purposes or to assemble macroscopic structures from colloidal particles. Here, we report a combined experimental and numerical study of thread formation in channel systems with varying geometry. In the reference flow-focusing configuration, the sheath flows impinge the core flow orthogonally while in four other channel configurations, the sheath flows impinge the core flow at different confluence angles, which are both positive and negative with respect to the reference sheath direction. Tomographic measurements of the thread development are made using optical coherence tomography (OCT) and are compared to numerically simulated 3D data. The numerical simulations performed with an immiscible fluid solver show good agreement with the experiments in terms of 3D thread shapes, wetted region morphologies, and velocity fields provided an ultralow interfacial tension is applied between the low viscosity (solvent) sheath flows and the high viscosity (dispersion) core flow. Such an ultralow interfacial tension is motivated by the so-called Korteweg stresses induced as a result of the concentration gradient between two miscible fluids in nonequilibrium state. These stresses mimic the effect of interfacial tension, and are often modeled as an effective interfacial tension (EIT), an approach chosen in the present work as well. The value of interfacial tension applied in the simulations was determined through an optimization procedure and compares well with a value deduced from a scaling analysis utilizing the downstream development of experimentally determined thread shape. The experimental and numerical results show that for channel configurations with modest deviations from orthogonal sheath flows, the effect on the thread is similar regardless of whether the sheath flows are co- or counterflowing the core flow. In fact, for these cases, the effect of co- and counterflowing sheath flows can be reproduced with orthogonal sheath flows, if the sheath channel width is increased. However, for channel configurations with larger deviations from orthogonal sheath flows, the effects on the thread are direction dependent. The one-to-one comparison and analysis of numerical and experimental results bring useful insights to understand the behavior of miscible systems involving high-viscosity contrast fluids. These key results provide the foundation to tune the flow-focusing for specific applications, for example in tailoring the assembly of nanostruc-tured materials.

Place, publisher, year, edition, pages
American Physical Society (APS) , 2021. Vol. 6, no 11, article id 114001
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-305366DOI: 10.1103/PhysRevFluids.6.114001ISI: 000717535300002Scopus ID: 2-s2.0-85119171982OAI: oai:DiVA.org:kth-305366DiVA, id: diva2:1615808
Note

QC 20211201

Available from: 2021-12-01 Created: 2021-12-01 Last updated: 2024-03-18Bibliographically approved
In thesis
1. Experimental and numerical investigations of hydrodynamic focusing of colloidal dispersions
Open this publication in new window or tab >>Experimental and numerical investigations of hydrodynamic focusing of colloidal dispersions
2023 (English)Doctoral thesis, comprehensive summary (Other academic)
Alternative title[sv]
Experimentell och numerisk undersökningar av hydrodynamisk fokusering med kolloidala dispersioner
Abstract [en]

Dispersed non-spherical particles are the fundamental constituent of many complex fluids. Such fluids are studied both for their industrial and scientific importance, and for their peculiar functional properties (mechanical, optical, thermal, fluidic). One exemplar is cellulose nanofibrils (CNF), a biopolymer made of nanoscale particles with remarkable mechanical properties that has been found to be the potential candidate for the fabrication of sustainable and bio-compatible materials. To synthesize and characterise the behaviour of such non-spherical particles in flowing dispersions, microfluidic platforms have emerged as powerful tools. However, the scientific understanding of the fundamental role of the fluid dispersion properties and flow parameters on the microflow dynamics is still inadequate.  

In this thesis work, a combined numerical and experimental investigation with diverse set of microfluidic flow focusing devices are adopted to measure, analyse, and understand the micro-  and macro-scale morphologies of flowing dispersions. A high-viscosity colloidal dispersion liquid made of cellulose nanofibrils suspended in water (the solvent) is hydrodynamically focused with the low-viscosity solvent liquid. A 3D colloidal viscous thread structure is formed, which is characterized using optical coherence tomography (OCT) measurements and computational fluid dynamics (CFD) simulations. The studies show that if the Péclet number is large (diffusion of the particles is slower than the convective time scale of the flow), the concentration gradient between two in-homogeneous miscible fluids (colloidal dispersion and its own solvent) gives rise to Korteweg stresses, emulating the effect of interfacial tension in the form of effective interfacial tension (EIT). In addition, scaling laws describing the complex interplay between viscous, inertial and capillary effects in microchannels have been identified, and are in turn used to estimate the fluid properties.

Further, the collective behaviour of nanofibrils in the studied flow fields is investigated. Numerically modelled orientation distribution functions (ODF)  are compared with in-situ small angle X-ray scattering (SAXS) measurements. The calibrated SAXS-based digital twin model unveils complete 3D nanoparticle orientation both along the streamwise and cross-sectional planes of the channels. Overall, the key findings of this work open up possibilities in controlling the hydrodynamic assembly of nanoparticles in microchannels.

Abstract [sv]

Icke-sfäriska nanopartiklar är den grundläggande byggstenen i många komplexa vätskor. Sådana vätskor studeras både på grund av deras industriella och vetenskapliga betydelse och på grund av deras intressanta egenskaper (mekaniska, optiska, termiska och fluidiska). Ett exempel på sådana partiklar är cellulosanofibriller (CNF), en biopolymer med anmärkningsvärda mekaniska egenskaper som har stor potential för tillverkning av hållbara och biokompatibla material. Ett kraftfullt verktyg för syntes och karakterisering av sådana icke-sfäriska partiklar i strömmande dispersioner är mikrofluidik, men den vetenskapliga förståelsen av partiklarnas och dispersionernas beteende i mikrofluidiksystem är fortfarande otillräcklig.  

I denna avhandling kombineras numeriska och experimentella metoder för att mäta, analysera och förstå flödande dispersioners makroskopiska och de ingående partiklarnas mikroskopiska beteende i olika strömningssituationer. Det specifika strömningsfall som studeras är strömningsfokusering: en högviskös kolloidal dispersion bestående av cellulosanofibriller i vatten fokuseras hydrodynamiskt av ett yttre flöde med rent vatten med låg viskositet i en kanal. Det kan då skapas högviskös "tråd" i kanalen. Detta flöde karakteriseras med hjälp av optisk koherenstomografi (OCT) och CFD-simuleringar (Computational Fluid Dynamics). Om Péclet-talet här stort (vilket betyder att partiklarnas diffusionshastighet är lägre än strömningshastigheten) ger koncentrationsgradienten mellan två homogena, blandbara  vätskor (kolloidal dispersion och dess eget lösningsmedel) upphov till Korteweg-spänningar, vilka kan modelleras med en effektiv ytspänning (EIT). Skalningslagar som beskriver de komplexa kopplingarna mellan effekter av viskositet, tröghet och ytspänning i mikrokanalen system har tagits fram, och skalningslagarna används i sin tur för att uppskatta vätskeegenskaperna.        

Även det kollektiva beteendet hos nanofibrillerna själva har studerats. Numeriskt modellerade orienteringfördelningar jämförs med in-situ röntgenspridningsmätningar (SAXS). Resultatet blir en experimentellt kalibrerad digital modell som avslöjar nanopartiklarnas 3D-orientering i hela systemet. Sammansatta gör resultaten i denna avhandling det möjligt att prediktera, optimera och kontrollera hydrodynamisk syntes av icke-sfäriska partiklar i olika kanalsystem.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2023
Series
TRITA-SCI-FOU ; 2023:40
Keywords
Microfluidics, flow focusing, colloidal fiber dispersion, effective interfacial tension, microflow morphology, nanoparticle orientation, Mikrofluidik, flödesfokusering, kolloidala dispersioner, effektiv ytspänning, mikroflödesmorfologi, orientering av nanopartiklar
National Category
Fluid Mechanics and Acoustics
Research subject
Engineering Mechanics
Identifiers
urn:nbn:se:kth:diva-336681 (URN)978-91-8040-695-6 (ISBN)
Public defence
2023-10-06, https://kth-se.zoom.us/j/61949181313, D2, Lindstedtsvägen 9, Kungliga Tekniska Högskolan, Stockholm, 10:15 (English)
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Note

QC 230918

Available from: 2023-09-18 Created: 2023-09-17 Last updated: 2023-10-02Bibliographically approved

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Gowda, V. KrishneRydefalk, CeciliaSöderberg, DanielLundell, Fredrik

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