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Case Study Tutorial Wetting and Non-Wetting. Basics of Wetting. surface. G. L. bulk. S. contact line. Three phase contact (TPC) zone. Three phase contact (TPC) line. droplet. steel surface. Three phase contact (TPC) line. droplet. steel surface. P e. P i. Capillary pressure. - PowerPoint PPT Presentation
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Case Study TutorialWetting and Non-Wetting
Basics of Wetting
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G
L
S
surface
contact line
bulk
Three phase contact (TPC) zone
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Three phase contact (TPC) line
steel surface
droplet
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Three phase contact (TPC) line
steel surface
droplet
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Capillary pressure
Pe
Pi
PPP ei
21 R1
R1P
is the interfacial tension, R1 and R2 are the two principal radii of curvature
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Young equation
SGYLGSL cos
Y
LG
SL
SG
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Hysteresis
Viscous flow:Hindered TPC (pinned)Non-slip
Ideal flow: Barriereless TPCFree slippage
r < Y < a
r
a
Y
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The TPC line resistance (hysteresis) is due to solid surface heterogeneities:
morphologic and/or energetic
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Morphologic heterogeneity
The intrinsic contact angle at a rough surface is different from measured one:
Wenzel, Cassie-Baxter, wicking models
"God created the solids, the devil their surfaces"
Wolfgang Pauli (1900-1958)
REAL SURFACES ARE ROUGH
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Form
WaveGroove
1 order
2 order3 order4 order
Topometric characterisation parameters
according to DIN EN ISOflatness, waveness, roughness
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Morphologic heterogeneity
Cassie-Baxter
1fcosfrcos sessrough
-1
1
C
tg = rs
1 - f
cos rough
cos flat0-1 1
Johnson & Dettre in “Wettability”, Ed. by John C. Berg, 1993
Wenzel
esrough cosrcos
Bico et al. wicking
)cos1(f1cos esrough
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Adhesion, viscous friction and contact line barriers have the same nature: van der Waals interactions
In the case of: - non-slip boundary conditionsviscous fluids - barrier contact line motion
- TPC angle hysteresis
In the case of: - free boundary slippageideal fluids - barriereless contact line motion
- no TPC hysteresis (Young Model)
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30 mm
30 mm
hydrophobic hydrophilic superhydrophobic
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Super-hydrophobicity
We learn from nature ...
... and want to mimic
- adhesives- coatings- în microelectronics
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Super-hydrophobicity
Wettability can be manipulated through
- changes in surface energy- changes in surface morphology/topography
(roughness, geometry)
CA = 90 - 120°CA 150°
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Super-hydrophobicity
Structure of rough surfaces can be:
RegularIrregular (Random)Hierarchical (Fractal): flat
2Dfractal cos)l/L(cos
L and l are the upper (of several micrometers) and lower limit (particle diameter) scales of the fractal behaviour on the surfaceD is the fractal dimension
Surface modified by particles: Regular Structure
10 mm
R = 200 nm R = 1 mm R = 2.4 mm R = 5 mm
9069.13R
2R3RR
SSS
SS
r2
222
triangle
poresegmentsphere
geometric
actualS
Regular particle structure: no superhydrophobicity
The height roughness (not the roughness factor) influences wetting
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ah
a
1 Under what condition is the Wenzel regime more stable than the Cassie-Baxter regime?
Wenzel, 1936 Cassie-Baxter, 1944
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ah
a
1 Under what condition is the Wenzel regime more stable than the Cassie-Baxter regime?
ah1
a2ah2
areaprojectedarearealrs
Wenzel roughness factor
Wenzel CA YYsW cosah1cosrcos
Cassie-Baxter CA 1fcosfrcos YfCB
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ah
a
1 Under what condition is the Wenzel regime more stable than the Cassie-Baxter regime?
If the liquid touch only the top of the surface, then f = ½ and rf = 1
21cos
21cos YCB
Wenzel regime more stable if W CB
Ycos 1
ah21
Wenzel regime is always more stable if Y 90°
21
ah
a
2 Under what condition can this surface become non-wettable, i.e. superhydrophobic with a ? CA 150°
CBcos 866.0150cos
21cos
21cos YCB 866.0
Ycos 732.0
Y 137 but
120Y
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