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School of Mechanical EngineeringFACULTY OF ENGINEERING
School of Mechanical EngineeringFACULTY OF ENGINEERING
Thin Film Coating & Spreading Flows
Harvey Thompson, Nikil Kapur, Jon Summers,
Mark Wilson & Phil Gaskell School of Mechanical Engineering, University of Leeds, Leeds LS2 9JT, UK
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School of somethingFACULTY OF OTHER
School of Mechanical EngineeringFACULTY OF ENGINEERING
Overview:
1. Introduction
2. Brief Review of Industrial Coating & Drying Flows1. Self-metered processes
2. Pre-metered processes
3. Drying
3. Droplet formation and spreading
4. Modern simulation techniques
5. Conclusions
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School of somethingFACULTY OF OTHER
School of Mechanical EngineeringFACULTY OF ENGINEERING
School of Mechanical Engineering:
• 5** rated under the Government Research Exercise:
• 3 multidisciplinary research institutes:
1. Institute of Medical and Biomedical Engineering
2. Institute of Engineering Thermo-fluids, Surfaces and Interfaces
3. Institute of Engineering Systems and Design
Broad groupings designed to allow cross-fertilisation of research areas
• Excellent research staff and facilities
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School of somethingFACULTY OF OTHER
School of Mechanical EngineeringFACULTY OF ENGINEERING
•Combustion •Thin Films and Fluids •Tribology •Corrosion
•Engineering Optics/Metrology •Biomimetics •Microfluidics•Computational Simulations
Growth AreasCore Strengths
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School of somethingFACULTY OF OTHER
School of Mechanical EngineeringFACULTY OF ENGINEERING
Optics and Flow Diagnostics
Interferometric sensing:
Full field shape / colour / texture
Ultrahigh dynamic range distance metrology (to 1: 1011)
High bandwidth 10MHz
Flow Metrology:Multiphase / multi-constituent flows
Mixing
GDI sprays
Micro-reactors
4D flow metrology:
Time varying turbulent flow
In-cylinder automotive applications
Biophotonics:Signal processing
4D sensing fluorophor labelled cells
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School of somethingFACULTY OF OTHER
School of Mechanical EngineeringFACULTY OF ENGINEERING
Thin
Films
and
Fluid
Flow
Coating and PrintingNumerical and experimental investigations of industrial processes
Free Surface Flows and WettingDroplet motion on surfaces
Spreading films on heterogeneous surfaces
MicrofluidicsMicrofluidic device characterisation
Mixing
MicroPIV
Flow Modelling and SimulationMultiscale modelling: molecular dynamics / lattice Boltzmann
Computational Fluid Dynamics for Engineering Applications
Engineering Simulation Tools
Rheology and Fluid Characterisation
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Review of Industrial Coating Flows
Industrial coating flows have several key stages:
Fluids preparation, coating, drying and winding.
Key distinction in coating stage: self-metered, pre-meteredLiquid supply
Substrate
Coater
Dryer/CurerWinder
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Self-Metered Processes
Self-metered Coating Processes: where the wet film thickness is controlled by the process itself as opposed to controlled flow rate to the coater – hence self metering.
Simple example: roll coating Substrate
Bath
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Self-metered processes
Many different forms
Just a few basic principles
Fighting the same sets of problems
Substrate
Bath
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Self-metered processes
Pick up some liquid – Dip Coating
Split it between rolls
Get the split ratio right
Hope there’s no ribbing, barring or runback
Hope we have a wide coating window
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Self-metered processes
Liquid Pick-up:
Key parameter is Capillary number:
For vertical pick-out (Wilson 1982):
Can be modified to take account of angle of pick-up and effect of plate.
Simple tools aid coating design
Speed ViscosityCa
SurfaceTension
6/1
2/1
944.0 Cag
UH lift
lift
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s r o r
Coated Substrate
Wiper
Reservoir
Reservoir-fed Reverse Roll Coating:
Industrially-important variation of the robust reverse roll configuration used for manufacturing a variety of films and foils
Self-metered processes
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Reservoir-fed Reverse Roll Coating: Comparison with Experiment
S=0.1
S=0.5
Self-metered processes
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Controlling position of wetting line position is key.
Wetting line position vs speed ratio:
Hydrostatic head is important, as is the variation of dynamic wetting angle with metering speed.
Self-metered processes
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Self-metered processes
Negative Gaps: Deformable roll coating
One roller rubber
Great way to make really thin coatings …
… if you’re not too bothered about coat quality
Depends critically on rubber, which can change with time or between batches
Set load or (-ve) gap
Back-up
Applicator
Metering
Elastomer Web
Three-roll pan reverse
Applicator Metering
Back-upWeb
Three-roll reservoir reverse
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Self-metered processes
A PRE-SET GAP is specified and the separation of the roll centres is set by the adjustment of mechanical stops.
A LOAD is specified and the separation of the roll centres is set by applying a force across the roll pair.
Deformable roll coating can be very complex – requires sophisticated Finite element analysis
BUT simple models can be developed which do a good enough job for practical design – take account of roll speeds, viscosity, load,…
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Self-metered processes
Gravure Coating is increasingly popular – small film thicknesses (a few microns) and good stability
Fluid properties
• viscosity, surface tension
Web & roll speeds
Doctor blade position
Gravure cell
Note - web tension and wrap don’t affect transfer!
Direct Gravure nip
Gravure rollDoctor blade
Reservoir
Web
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Self-metered processes
Gravure Cell Shape is Key
• Quadrangular
• Pyramidal
• Laser engraved ceramics
• various shapes
• also QCH
• trihelical
Volume (microns)
Density (lines per inch /lines per cm)
Screen angle
Photographs of gravure roll surfaces
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Self-metered processes
The film thickness & pickout is also sensitive to fluid properties, doctor blade pressure and roll speed.
Need for accurate predictive models!
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Pre-metered processes
You deliver just the right amount of liquid to the web through, for example, a slot.
Pre-metering is used to smooth the liquid surface without the need to throw away or recirculate any liquid.
You know your flow rate Q m3/s and the line speed U m/s – wet film thickness is then Q/U.
Pre-metered processes are often used in high precision applications.
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Pre-metered processes
Slide-bead Slot
Curtain
and others
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Slot coating
Slot coating is a versatile method for applying single layers to a web.
Examples include photosensitive materials, such as photo-resist, magnetic suspensions, waxes, inks, silicon, rubber and foams and hot melt adhesives, in addition to low viscosity melts of alloys, metals and organic materials
Slot Die
Coated Substrate
Roll
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Controlling slot coating
Slot coating is affected by:
Lip shape
Lip land length
Angle of slot
Type of backing (roll/web)
Radius of backing roller
Angle
RollerRadius
Web speed
Gap WetThickness
Land length
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web (wrapped around roll of radius, R)
dynamic contact line
downstreamm eniscus
Q = UH
G
H
L d
static contact line
upstream meniscus
D
S
SLOTDIE
U
Simple models useful – may need more detailed analyses of the velocity or pressure field – can use Finite Elements or other CFD methods.
Typical streamlines in a
slot coating system.
BUT simpler models are often effective…
shows eddies etc…
Slot coating
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A happy slot
A nice downstream meniscus
A healthy balance of pressure
Upstream meniscus under control …
… able to absorb fluctuations
Too much pressure and your pump has problems, too little and you’re out of control
Downstream meniscus
Upstream meniscus
Pressure curve
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Unhappy slots
Upstream overspill Unstable inflow
Ribbing
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Slide Coatingpopular in the photographic industry for producing multi-layer coatings
Substrate
Cascade
Slide coating
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Typical Finite Element Grid
Slide coating
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Streak-Line Formation
w eb
upper free surface
lower free surface
rec ircu la tion reg ion
(a ) = 25 S
o
(b ) = 45 S
o
in te rna lin te rface
Slide coating
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Curtain Coating
High impingement speed of
falling curtain enables high
coating speeds - up to ~600m/m
Very versatile due to large gap
Not so mechanically demanding
Highly robust against lines
Predictable performance
Can coat several layers at once
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Curtain Flow Zone
Simple and highly predictable
impingement velocity
But can be inherently unstable – minimum
flow rates to avoid break-up of curtain
X
X
V
V
0
0 0
20
2 2 XXgVV
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Curtain Impingement Zone
An unwanted heel can form
This can trap particles
and bubbles - causing lines
Can also entrain air
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Slot exits often used to supply pre-metered coatings such as slide and curtain
Defects Caused by Feed Flow
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Back-wetting of uppermost slot may occur during start-up – can lead to defect-causing solids deposits due to degradation in recirculation regions.
Back wetting at the upper slot: (a) experimental, (b) CFD prediction
Defects Caused by Feed Flow
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Geometrical modifications to slots – effect of a curved diffusor (Schweizer (1988))
Diffusor can also remove downstream eddy
Defects Caused by Feed Flow
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CFD used to identify a more practical solution:Merging flow out of slot exits – effect of chamfering lower corner
Chamfer can remove eddies in both liquid layers simultaneously!
CFD Slot Optimisation
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Film drying is often the limiting factor in industrial coating systems:
In drying, two processes must occur simultaneously:
(a) The transfer of energy (heat) from the surrounding environment to the product in order to evaporate the surface moisture
(b) The transportation of solvent held within the
product to its surface where it can be removed
by process (a)
Film Drying
Liquid supply
Substrate
Coater
Dryer/CurerWinder
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For aqueous coatings, most of drying is in the Constant Rate Period.
For solvent-based coatings,
most drying is in the Falling Rate Period.
Film Drying
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For aqueous coatings, most of drying is in the Constant Rate Period.
For solvent-based coatings,
most drying is in the Falling Rate Period.
The Drying Curve
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Air Floatation Dryers
Double-sided floatation dryers usually have nozzles in a staggered arrangement on opposite sides with a stable, sinusoidal web profile.
Practical Dryer Design
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Understanding web stability is crucial
A typical nozzle design show below –
two angled jets separated by a Coanda
plate.
Practical Dryer Design
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Practical Dryer Design
Accurate predictions require sophisticated numerical models (Computational Fluid Dynamics):
But simpler models can also be extremely useful.
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Aqueous coating, 30% solids By weight
Single sided drying, Temp = 125oC, h = 100 W/m2K
Single zone – 4m in length
No recirculation
Water not completely evaporated by end of zone
Practical Dryer Design
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Effect of more volatile solvent
Methanol, 30% solids by weight
Single sided drying, Temp = 125oC, h=100W/m2K
Single zone – 4m in length
No recirculation
All methanol evaporated after 1.1m.
Practical Dryer Design
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Droplet Formation and Spreading
Droplet flows are increasingly important – coating of electronic circuits, spray deposition onto leaves, heat mitigation in circuits, ink-jet printing.
At Leeds, have developed range of experimental and numerical methods for analysing droplet flows: formation, coalescence and migration.
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Courtesy of S.L. Turner & T.P. Comyn, University of Leeds
Droplet Formation – Ink Jet Printing on Textiles
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Droplet Formation – Ink Jet Printing on Textiles
CFD predictions agree well with experiment – very useful design tool.
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Droplet coalescence is of fundamental importance for a variety of applications
Experiments Lubrication Theory
(a)
(c)
(e)
(b)
(d)
(f)
(a)
(c)
(e)
(g)
(b)
(d)
(f)
(h)
Droplet Coalescence
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Droplet motion on chemically- and topographically patterned surface
Through a small trench
Droplet Migration on Complex Surfaces
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Droplet motion on chemically- and topographically patterned surface
Through a larger trench
Droplet Migration on Complex Surfaces
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Droplet evaporation – formation of classical ‘coffee-ring’
Droplet Evaporation
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Lattice Boltzmann Method
The Lattice Boltzmann Method (Jon Summers, Mark Wilson)
•Capabilities to deal with complex physics are increasing rapidly.
• Can deal with flow in very complex geometry
• Can couple up to continuum and couple down to molecular
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Rayleigh-Taylor Instability
Fluid-fluid interface as heavier fluid on top plunges into lighter fluid below
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Droplet dynamics
Animations by Alan Davies
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Passive mixing with blocks.
Electric field
Species concentration
Electric field and fluid flow both solved using LBM. Very useful for microfluidic flows in micro-channels.
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Slug in a pore-throat
Lattice Boltzmann very powerful for porous media flows
Heterogeneous surface causes snap-off
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Conclusions
Coating and Drying phenomena can be very complex
Often best optimised by combination of targeted experiments, simple models and/or more complex numerical models
Simple models can often be produced which capture the essence of the process.