Drag reduction and terminal velocity
measurements on superhydrophobic spheres
University of Genova – Polytech Orleans
Student: Luca Cò
Supervisor: Prof. Alessandro Bottaro
Correlator: Prof. Nicolas Mazellier
Bachelor’s thesis
Genova, 24th July 2015
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Outline
 Ultra Ever Dry® painture
 Wettability and superhydrophobicity
 Wenzel and Cassie-Baxter state
 Laboratory
 Measurements
 Conclusions and future applications
Luca Co - University of Genova
Static contact angle
A = γSG − γLS = γGL cos α
Luca Cò - University of Genova
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Static contact angle
 hydrophilic , high wettability , 0° < α < 90°
 Hydrophobic, low wettability , α > 90°
 Superhydrophobic, very low wettability, α > 150°
Luca Co - University of Genova
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Wenzel state
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cos αW = r cos α
r A = γGL cos αW
Luca Co - University of Genova
Cassie-Baxter State
Lotus leaf
cos αF = f1 cos α − f2
f1 = 1 − w/b
f2 = w/b Luca Co - University of Genova
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Tilt angle
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Δϑ = θf – θr
Δϑ <10 ° for superhydrophobic surfaces
Luca Co - University of Genova
Slip lenght
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Laboratory
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Laboratory
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Luca Co - University of Genova
Results
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Initial tests with different spheres falling in water
Luca Co - University of Genova
Results
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Initial tests with different spheres falling in water
Luca Co - University of Genova
Results
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Overlap of SH and uncoated 10mm sphere falling in water
Terminal velocity [m/s]
Re
Uncoated
1,261
14112
SH
1,153
12903
Luca Co - University of Genova
Results
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Overlap of SH and uncoated 10mm sphere falling in glycerol
Terminal velocity [m/s]
Re
Uncoated
0,3057
3,675
SH
0,3074
3,696
Luca Co - University of Genova
Conclusions and future applications
Luca Co - University of Genova
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