Drag reduction and terminal velocity measurements on superhydrophobic

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, 24 th July 2015

2 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 3

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 4

Wenzel state 5 cos αW = r cos α r A = γGL cos αW Luca Co - University of Genova

Cassie-Baxter State Lotus leaf cos αF = f 1 cos α − f 2 f 1 = 1 − w/b f 2 = w/b Luca Co - University of Genova 6

Tilt angle 7 Δϑ = θf – θr Δϑ <10 ° for superhydrophobic surfaces Luca Co - University of Genova

Slip lenght 8 Luca Co - University of Genova

Laboratory 9 Luca Co - University of Genova

Laboratory 10 Luca Co - University of Genova

11 Luca Co - University of Genova

12 Luca Co - University of Genova

Results 13 Initial tests with different spheres falling in water Luca Co - University of Genova

Results 14 Initial tests with different spheres falling in water Luca Co - University of Genova

Results 15 Overlap of SH and uncoated 10 mm sphere falling in water Terminal velocity [m/s] Re Uncoated 1, 261 14112 SH 1, 153 12903 Luca Co - University of Genova

Results 16 Overlap of SH and uncoated 10 mm 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 17