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

2

What surface texture?

• Natural and artificial

• Macro and micro

3

What’s old is new again

Surface texturing to improve performance is an old hat

4

Roughening airplanes

737 wings textured with vortex generators to

reduce drag and increase lift, especially at low

air speeds

5

Other applications of surface texturing

• Increase optical absorption

• Control hydrophobicity

• Reduce friction and wear

• Manipulate growth of algae/bacteria/cells

• …….

6

Coatings (chemical texturing)

Freyman et al, Surf. Coatings Technol. 201, 164 (2006)

CHx CHx + 5 % S

0

0.005

0.01

0.015

0.02

0.025

0 10 20 30 40 50 60

Relative Humidity [%]

Co

eff

icie

nt

of

Fri

ctio

n

undoped film

5 a/o sulfur-doped film

7

Surface texturing methods

Two broad categories involving chemical and

physical methods:

• Lithography

• Direct write

8

Lithography

Old standby: photolithography

9

So

ft L

ith

og

rap

hy

Dong Qin, Younan Xia & George M

Whitesides Nature Protocols 5, 491 (2010)

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)

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.

12

Micro-texturing stainless steel.3

Developing the pattern

• The steel substrate was put into an etchant under sonication,

consisting of 70% w/v FeCl3 1.37 M HCl. Etching time: 30 -

120 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.

13

Micro-texturing stainless steel.4

15

Direct Write

Three broad approaches:

• Mechanical

• Printing

• Optical

16

Direct Write: Mechanical

Diamond- or vibro-machining

Work Piece

Piezo Cell Tool Bit

17

Example of Mechanical Direct Write.1

Tian, Saka, and Suh (1989) showed the texture effect on

friction by first machining grooves in Ti:

18

Example of Mechanical Direct Write.2

52100 sliding on “undulated” Ti with mineral oil as lubricant

19

Example of Mechanical Direct Write.3

52100 sliding on “undulated” Ti with oleic acid as lubricant

20

Direct Write: Mechanical

Micro-rolling and -stamping

• Mechanical analog of soft lithography

• Texturing fidelity depends on speed and mechanicalproperties

• Major features: fast, can texture internal surfaces

21Cao, J., Zhou, R., Ehmann, K, “Desktop Deformation-based Micro Surface Texturing System”, patent pending

First

pass

Rotate the sample

by 90° and roll the

second pass

Mechanical direct write: example

Alternatively

22

Direct Write: Printing

Traditional ink-jet

• Relatively fast

• Dot size ~ tens of microns

23

Direct Write: Printing

E-jet printing: pushing the limits

• Dot size ~ 0.1 micron, line width ~ 0.5 micron

• 500 microns/sec

24

Direct Write: OpticalLaser texturing

Nanoscribe

25

Example: Laser texturing a mechanical seal

Test geometry

Etsion 2010

26

Example: Laser texturing a mechanical seal

Test results

Etsion 2010

27

Effect of texturing on lubrication

Patir and Cheng (J. of Lub Technol 100, 12 (1978)) showed

that surface texture affects average lubricant film thickness

28

Effect of texturing on lubrication

Fishbone grooves Fishbone dimple Sinusoidal Triangular Honeycomb

0

0.0002

0.0004

0.0006

0.0008

0.001

0.0012

Sm

ooth

Fish

Ga

Fish

Gb

Fish

Gc

Fish

Gd

Fish

Da

Fish

Db

Fish

Dc

Sine

a

Sine

b

Sine

c

Sine

dTr

i aTr

i b

Tri C

Hon

ey D

Hon

ey G

Distribution pattern

Film

th

ickn

ess (

hm

/a)

Line of reference

Ren, N., Nanbu, T., Yasuda, Y., Zhu, D. and Wang, Q., Tribology Letters 28, 275 (2007)

Feature size ~ 58-140 µ; depth ~ 4-6.5 µ

29

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

30

CMS-0619284

CMII-0923000

31

Laser texturing: how far can we push?

• 200-micron diameter dimples

• 5% coverage density

• Depth ~ 7 microns

Texturing a curved surface

32

Laser texturing: how far can we push?

Stainless steel

20-micron channel length

2-micron channel width

2-micron channel spacing

1.5-micron channel depth

Resolution

33

Surface Texture and Life

Surface texture affects cell growth in several ways:

Morphology

Functionality

Mortality

34

Surface Texture and Life: Morphology

Stupp et al., Soft Matter 5, 1228 (2009)

Alignment of human mesenchymal stem cells

Stem cells tend to align with the

substrate texture

35

Surface Texture and Life: Functionality

Lim and Donahue, Tissue Engrg 13, 1879 (2007)

Stem cell differentiation

Fat cells need little space!

36

Surface Texture and Life: Mortality

Chen et al., Science 276, 1425 (1997)

Apotosis of capillary endothelial cells

Little space means early death

37

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

38

A page from Mythbusters

Smooth car: 26 mpg

Dimpled car: 29.65 mpg!

(http://dsc.discovery.com/videos/mythbusters-dimpled-car-minimyth.html)