Student: Luca Cò

Supervisor: Prof. Alessandro Bottaro

Correlator: Prof. Nicolas Mazellier

Bachelor’s thesis

Genova, 24th July 2015

Drag reduction and terminal velocitymeasurements on superhydrophobic

spheresUniversity of Genova – Polytech Orleans

Luca Co - University of Genova

2

Outline

Ultra Ever Dry® painture

Wettability and superhydrophobicity

Wenzel and Cassie-Baxter state

Laboratory

Measurements

Conclusions and future applications

Luca Cò - University of Genova

3Static contact angle

A = γSG − γLS = γGL cos α

Luca Co - University of Genova

4

hydrophilic , high wettability , 0° < α < 90°

Hydrophobic, low wettability , α > 90°

Superhydrophobic, very low wettability, α > 150°

Static contact angle

Luca Co - University of Genova

5Wenzel state

cos αW = r cos α

r A = γGL cos αW

Luca Co - University of Genova

6Cassie-Baxter State

cos αF = f1 cos α − f2

f1 = 1 − w/b f2 = w/b

Lotus leaf

Luca Co - University of Genova

7Tilt angle

Δϑ = θf – θr

Δϑ <10 ° for superhydrophobic surfaces

Luca Co - University of Genova

8Slip lenght

Luca Co - University of Genova

9

Laboratory

Luca Co - University of Genova

10

Laboratory

Luca Co - University of Genova

11

Luca Co - University of Genova

12

Luca Co - University of Genova

13ResultsInitial tests with different spheres falling in water

Luca Co - University of Genova

14ResultsInitial tests with different spheres falling in water

Luca Co - University of Genova

15ResultsOverlap 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

16ResultsOverlap 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

17

Conclusions and future applications