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Molecular Processes in Dynamic Wetting
KTH, School of Engineering Sciences (SCI), Applied Physics. (Molecular Biophysics)ORCID iD: 0000-0001-9160-2549
2020 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

The spreading of liquids onto and over surfaces is a fundamental process in nature. It is present in all forms and sizes: From rivers carving through landscapes, to our blood stream transporting nutrients to cells, and even single water molecules moving through channels into these cells. We now have a good understanding of how fluid movement works inside the fluid itself. However, we do not fully understand the processes close to the contact line, where the liquid is spreading onto the surface. We are forced to make assumptions about this behaviour and none of these assumptions have yet proven to be universally valid.

As everything in nature, liquid spreading is a fundamentally molecular process. This thesis summarises my work on applying this lens to the process. By studying molecules we begin at the smallest combined building blocks of nature and do not have to make any prior assumptions of the involved processes. Instead, we simply observe their behaviour. This is accomplished through the use of molecular dynamics simulation, which are an atomistic form of computer experiments. We use a realistic model of water molecules as our base liquid, since this captures realistic effects such as hydrogen bonding which are not present when using simpler models. Combined with large-scale systems which minimise the influence of finite-size effects, we have a realistic treatment of complex liquid systems.

We find that the molecular processes of wetting have an important influence on large-scale wetting. Most importantly, the hydrogen bonding nature of water to realistic substrates yields the no-slip condition often used as a boundary condition for models of wetting. Furthermore, since molecular processes are thermal in nature they create energy barriers which impede contact line advancement. We show how these barriers are created and how they can be diminished, for example in the case of electrowetting. This highlights that understanding the molecular behaviour of fluids remains an important field of study.

Abstract [sv]

Hur vätskor breder ut sig över ytor är en grundläggande process i naturen. Den dyker upp i alla former och storleksgrader: från floder som skär genom berg, till vår blodström som levererar näring till våra celler, och till och med enstaka vattenmolekyler som rör sig genom de kanaler som celler tar in näringen från. Hur vätskor beter sig i stora flöden är sedan länge känt, men vi vet ännu inte hur de beter sig nära ytor. Istället gör vi antaganden, varav inga ännu är korrekta för alla tillämpningar.

Fundamentalt sett är en vätska som breder ut sig en molekylär process. Denna avhandling sammanfattar mitt arbete med att förstå den ur denna synvinkel. Genom att studera molekyler använder vi naturens minsta sammansatta byggstenar. Vi behöver inte göra antaganden om hur de beter sig, vi behöver bara titta. Det fönster som vi tittar igenom är molekylär dynamik-simuleringar, en atomistisk typ av datorexperiment. För att fånga verkliga effekter som vätebindningar, använder vi realistiska modeller av vattenmolekyler och ytor. Vi använder tillräckligt stora system för att se hur molekylära effekter påverkar större processer.

Vi visar med dessa metoder att molekylära processer har stor påverkan på hur vätskor flödar över ytor. En stor effekt är att vätebindningarna mellan vatten och realistiska ytor förhindrar vätskan från att glida över den, vilket är ett vanligt antagande i modeller. Vi visar också hur molekyler vid gränsen där vätskor sprider på ytor ger upphov till en energibarriär som förhindrar att vätskan enkelt sprider sig framåt. Denna barriär beskrivs i detalj och vi visar vilka effekter som kan förminska den. Detta genomlyser hur molekylära processer i vätning är en viktig ingrediens för ökad förståelse av vätskespridning i system.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2020. , p. 58
Series
TRITA-SCI-FOU ; 2020:05
Keywords [en]
contact lines, nanodroplets, computational physics, molecular dynamics, fluid dynamics, multi-phase flows, electrowetting
National Category
Other Physics Topics
Research subject
Biological Physics
Identifiers
URN: urn:nbn:se:kth:diva-268935ISBN: 978-91-7873-480-1 (print)OAI: oai:DiVA.org:kth-268935DiVA, id: diva2:1415588
Public defence
2020-04-16, F3, Lindstedtsvägen 26, Stockholm, 09:00 (English)
Opponent
Supervisors
Available from: 2020-03-23 Created: 2020-03-19 Last updated: 2020-03-23Bibliographically approved
List of papers
1. Water-substrate physico-chemistry in wetting dynamics
Open this publication in new window or tab >>Water-substrate physico-chemistry in wetting dynamics
2015 (English)In: Journal of Fluid Mechanics, ISSN 0022-1120, E-ISSN 1469-7645, Vol. 781, p. 695-711Article in journal (Refereed) Published
Abstract [en]

We consider the wetting of water droplets on substrates with different chemical composition and molecular spacing, but with an identical equilibrium contact angle. A combined approach of large-scale molecular dynamics simulations and a continuum phase field model allows us to identify and quantify the influence of the microscopic physics at the contact line on the macroscopic droplet dynamics. We show that the substrate physico-chemistry, in particular hydrogen bonding, can significantly alter the flow. Since the material parameters are systematically derived from the atomistic simulations, our continuum model has only one adjustable parameter, which appears as a friction factor at the contact line. The continuum model approaches the atomistic wetting rate only when we adjust this contact line friction factor. However, the flow appears to he qualitatively different when comparing the atomistic and continuum models, highlighting that non-trivial continuum effects can come into play near the interface of the wetting front.

Place, publisher, year, edition, pages
Cambridge University Press, 2015
Keywords
contact lines, micro-/nano-fluid dynamics, molecular dynamics
National Category
Physical Sciences
Identifiers
urn:nbn:se:kth:diva-182184 (URN)10.1017/jfm.2015.517 (DOI)000368737000011 ()2-s2.0-84958981400 (Scopus ID)
Funder
Swedish e‐Science Research CenterEU, European Research Council, 258980
Note

QC 20160216

Available from: 2016-02-16 Created: 2016-02-16 Last updated: 2020-03-23Bibliographically approved
2. Molecular origin of contact line friction in dynamic wetting
Open this publication in new window or tab >>Molecular origin of contact line friction in dynamic wetting
2018 (English)In: Physical Review Fluids, E-ISSN 2469-990X, Vol. 3, no 7, article id 074201Article in journal (Refereed) Published
Abstract [en]

A hydrophilic liquid, such as water, forms hydrogen bonds with a hydrophilic substrate. The strength and locality of the hydrogen bonding interactions prohibit slip of the liquid over the substrate. The question then arises how the contact line can advance during wetting. Using large-scale molecular dynamics simulations we show that the contact line advances by single molecules moving ahead of the contact line through two distinct processes: either moving over or displacing other liquid molecules. In both processes friction occurs at the molecular scale. We measure the energy dissipation at the contact line and show that it is of the same magnitude as the dissipation in the bulk of a droplet. The friction increases significantly as the contact angle decreases, which suggests suggests thermal activation plays a role. We provide a simple model that is consistent with the observations.

Place, publisher, year, edition, pages
AMER PHYSICAL SOC, 2018
National Category
Physical Chemistry
Identifiers
urn:nbn:se:kth:diva-232392 (URN)10.1103/PhysRevFluids.3.074201 (DOI)000437675700001 ()2-s2.0-85051108122 (Scopus ID)
Funder
EU, European Research Council, 258980Swedish Research Council, 2014-04505
Note

QC 20180726

Available from: 2018-07-26 Created: 2018-07-26 Last updated: 2020-03-23Bibliographically approved
3. Steady moving contact line of water over a no-slip substrate
Open this publication in new window or tab >>Steady moving contact line of water over a no-slip substrate
Show others...
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The movement of the triple contact line plays a crucial role inmany applications such as ink-jet printing, liquid coating and drainage(imbibition) in porous media. To design accurate computational toolsfor these applications, predictive models of the moving contact line areneeded. However, the basic mechanisms responsible for movement ofthe triple contact line are not well understood but still debated. We investigatethe movement of the contact line between water, vapour anda silica-like solid surface under steady conditions in low capillary numberregime. We use molecular dynamics (MD) with an atomistic watermodel to simulate a nanoscopic drop between two moving plates. Weinclude hydrogen bonding between the water molecules and the solidsubstrate, which leads to a sub-molecular slip length. We benchmarktwo continuum methods, the Cahn{Hilliard phase-eld (PF) model anda volume-of-uid (VOF) model, against MD results.We show that bothcontinuum models reproduce the statistical measures obtained fromMD reasonably well, with a trade-o in accuracy. We demonstrate theimportance of the phase-eld mobility parameter and the local sliplength in accurately modelling the moving contact line.

Keywords
contact lines, forced wetting, couette flow, multi-phase flows
National Category
Other Physics Topics
Research subject
Physics, Material and Nano Physics; Biological Physics
Identifiers
urn:nbn:se:kth:diva-271219 (URN)
Projects
INTERFACE
Funder
Swedish Research Council, 2014-5680Knut and Alice Wallenberg Foundation, 2016.0255
Note

QC 20200324

Available from: 2020-03-23 Created: 2020-03-23 Last updated: 2020-03-24Bibliographically approved
4. Electrowetting diminishes contact line friction in dynamic wetting
Open this publication in new window or tab >>Electrowetting diminishes contact line friction in dynamic wetting
(English)Manuscript (preprint) (Other academic)
Abstract [en]

We use large-scale molecular dynamics to study dynamics at the three-phase contact line in electrowetting of water and electrolytes on no-slip substrates. Under the applied electrostatic potential the line friction at the contact line is diminished. The effect is consistent for droplets of different sizes as well as for both pure water and electrolyte solution droplets. We analyze the electric field at the contact line to show how it assists ions and dipolar molecules to advance the contact line. Without an electric field, the interaction between a substrate and a liquid has a very short range, mostly affecting the bottom, immobilized layer of liquid molecules which leads to high friction since mobile molecules are not pulled towards the surface. In electrowetting, the electric field attractscharged and polar molecules over a longer range which diminishes the friction.

Keywords
electrowetting, contact lines, computational physics, molecular dynamics
National Category
Other Physics Topics
Research subject
Biological Physics
Identifiers
urn:nbn:se:kth:diva-271218 (URN)
Funder
Swedish Research Council, 2014-4505Swedish National Infrastructure for Computing (SNIC), 2018/1-22Swedish National Infrastructure for Computing (SNIC), 2019/1-22
Note

QC 20200324

Available from: 2020-03-20 Created: 2020-03-20 Last updated: 2020-03-24Bibliographically approved

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