Large-scale chemical and physical texturing of surfaces for engineering and biomedical applications

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1 Large-scale chemical and physical texturing of surfaces for engineering and biomedical applications Yip-Wah Chung Department of Materials Science and Engineering Department of Mechanical Engineering Northwestern University 1

2 What surface texture? Natural and artificial Macro and micro 2

3 What s old is new again Surface texturing to improve performance is an old hat 3

4 Roughening airplanes 737 wings textured with vortex generators to reduce drag and increase lift, especially at low air speeds 4

5 Other applications of surface texturing Increase optical absorption Control hydrophobicity Reduce friction and wear Manipulate growth of algae/bacteria/cells. 5

6 0.025 Coatings (chemical texturing) Coefficient of Friction CHx undoped film CHx + 5 % S 5 a/o sulfur-doped film Relative Humidity [%] Freyman et al, Surf. Coatings Technol. 201, 164 (2006) 6

7 Surface texturing methods Two broad categories involving chemical and physical methods: Lithography Direct write 7

8 Lithography Old standby: photolithography 8

9 Soft Lithography Dong Qin, Younan Xia & George M Whitesides Nature Protocols 5, 491 (2010) 9

10 Micro-texturing stainless steel.1 Preparing the steel substrate Nail polish was diluted in acetone in a 1:2 ratio. The solution was spin-coated on the steel surface at 3000 RPM to create a thin polymer layer. Making the stamp Use lithographic technique to produce a PDMS stamp with the required texture (2-micron lines) 10

11 Micro-texturing stainless steel.2 Making the pattern A drop of acetone was put on the stamp, and the substrate was placed on top. Where the high features of the stamp contact the steel surface, the acetone dissolves the nail polish, thus replicating the stamp pattern. 11

12 Micro-texturing stainless steel.3 Developing the pattern The steel substrate was put into an etchant under sonication, consisting of 70% w/v FeCl M HCl. Etching time: seconds Excess etchant was washed away with water. The remaining nail polish was washed with acetone, and the substrate was then washed with isopropyl alcohol. 12

13 Micro-texturing stainless steel.4 13

14 Three broad approaches: Mechanical Printing Optical Direct Write 15

15 Direct Write: Mechanical Diamond- or vibro-machining Piezo Cell Tool Bit 16

16 Example of Mechanical Direct Write.1 Tian, Saka, and Suh (1989) showed the texture effect on friction by first machining grooves in Ti: 17

17 Example of Mechanical Direct Write sliding on undulated Ti with mineral oil as lubricant 18

18 Example of Mechanical Direct Write sliding on undulated Ti with oleic acid as lubricant 19

19 Direct Write: Mechanical Micro-rolling and -stamping Mechanical analog of soft lithography Texturing fidelity depends on speed and mechanical properties Major features: fast, can texture internal surfaces 20

20 Mechanical direct write: example First pass Rotate the sample by 90 and roll the second pass Alternatively Cao, J., Zhou, R., Ehmann, K, Desktop Deformation-based Micro Surface Texturing System, patent pending 21

21 Direct Write: Printing Traditional ink-jet Relatively fast Dot size ~ tens of microns 22

22 Direct Write: Printing E-jet printing: pushing the limits Dot size ~ 0.1 micron, line width ~ 0.5 micron 500 microns/sec 23

23 Laser texturing Direct Write: Optical Nanoscribe 24

24 Example: Laser texturing a mechanical seal Test geometry Etsion

25 Example: Laser texturing a mechanical seal Test results Etsion

26 Effect of texturing on lubrication Patir and Cheng (J. of Lub Technol 100, 12 (1978)) showed that surface texture affects average lubricant film thickness 27

27 Effect of texturing on lubrication Fishbone grooves Fishbone dimple Sinusoidal Triangular Honeycomb Film thickness (hm/a) Line of reference 0 Smooth Fish Ga Fish Gb Fish Gc Fish Gd Fish Da Fish Db Fish Dc Sine a Sine b Sine c Sine d Tri a Tri b Tri C Honey D Honey G Distribution pattern Feature size ~ µ; depth ~ µ Ren, N., Nanbu, T., Yasuda, Y., Zhu, D. and Wang, Q., Tribology Letters 28, 275 (2007) 28

28 Laser texturing: how far can we push? Speed Fundamental limit: ~ one dot/ps for ps lasers Practical limit: texture density and depth Resolution Far field limit: diffraction Practical limit: optics and material Substrate curvature Flat vs curved 29

29 CMS CMII

30 Laser texturing: how far can we push? Texturing a curved surface 200-micron diameter dimples 5% coverage density Depth ~ 7 microns 31

31 Laser texturing: how far can we push? Resolution Stainless steel 20-micron channel length 2-micron channel width 2-micron channel spacing 1.5-micron channel depth 32

32 Surface Texture and Life Surface texture affects cell growth in several ways: Morphology Functionality Mortality 33

33 Surface Texture and Life: Morphology Alignment of human mesenchymal stem cells Stem cells tend to align with the substrate texture Stupp et al., Soft Matter 5, 1228 (2009) 34

34 Surface Texture and Life: Functionality Stem cell differentiation Lim and Donahue, Tissue Engrg 13, 1879 (2007) Fat cells need little space! 35

35 Surface Texture and Life: Mortality Apotosis of capillary endothelial cells Chen et al., Science 276, 1425 (1997) Little space means early death 36

36 Concluding Thoughts Many techniques available for large-scale surface texturing Surface texturing affects biological, mechanical, and physical properties Scientific challenge Understanding the surface texture-property relationship Engineering challenge Faster and cheaper texturing with high fidelity 37

37 A page from Mythbusters Smooth car: 26 mpg Dimpled car: mpg! ( 38