THE IMPORTANCE OF INTERFACES Roger Horn

1. THE IMPORTANCE OF INTERFACESRoger Horn Ian Wark Research Institute University of South Australia Adelaide, Australia

2. Interfaces • The boundary between two materials is called an interface. • The two materials could be any combination of solids, liquids and gases (e.g. solid/liquid, liquid/liquid, liquid/gas…). • A huge number of products, and almost all technologies that I can think of, rely on the properties of interfaces. • Look around you…

3. How to slide a rug over the floor, and how to bend a wire… Demonstration

4. Let’s look at the atoms in a perfect crystal

5. 5 10 15 20 = T “Edge dislocation” 10 5 15 19 Let’s look at the atoms in a realistic crystal

6. “Glide” of edge dislocations

7. “Glide” of edge dislocations

8. “Glide” of edge dislocations ?

9. “Glide” of edge dislocations

10. “Glide” of edge dislocations

11. “Glide” of edge dislocations

12. “Glide” of edge dislocations

13. “Glide” of edge dislocations

14. “Glide” of edge dislocations

15. “Glide” of edge dislocations

16. “Glide” of edge dislocations

17. “Glide” of edge dislocations

18. “Glide” of edge dislocations

19. “Glide” of edge dislocations

20. Interface Interfaces block the movement of edge dislocations • Glide of edge dislocations allows a material to deform easily. • This is common in metals, which are ductile. • The presence of interfaces within the metal blocks the dislocations. • Hence internal interfaces make the metal harder.

21. Ceramics do not bend like wire… • Ceramic materials (including glass) are not ductile, they are brittle. • Ceramics break easily, and the break always starts at an interface such as a scratch, a void or a flaw in the material. • The strength of a ceramic is at least 100 to 1000 times less than it would be if there were no flaws present. • Instead, the strength of a ceramic material depends on the size of its largest flaw. • To break glass where you want to, make a scratch in the surface… … then wet it – this reduces the interfacial energy and makes it easier for the scratch to grow into a crack which rapidly grows bigger until the glass breaks. “Brittle” demonstration

22. Si Microelectronics • A silicon atom has four “valence” or bonding electrons. • Each Si atom is bonded to four others in a diamond structure. • A bond consists of two electrons, one from each atom at the ends of the bond. A 2D representation of tetravalent bonding

23. Si Microelectronics • A silicon atom has four “valence” or bonding electrons. • Each Si atom is bonded to four others in a diamond structure. • A bond consists of two electrons, one from each atom at the ends of the bond. A 2D representation of tetravalent bonding

24. Microelectronics • A phosphorous atom has five “valence” electrons. • If we substitute a phosphorous atom into the silicon lattice, it bonds to four neighbours, using four of its electrons. • The P’s fifth electron is free to roam through the lattice. • The roaming electrons can carry a current in the semiconductor. Silicon doped with a small amount of phosphorous is called an n-type semiconductor

25. e– Microelectronics • A boron atom has three “valence” electrons. • If we substitute a boron atom into the silicon lattice, it bonds to four neighbours, but one electron is missing. • The missing electron is called a hole. • The location of the hole can migrate through the lattice, creating the impression that the holes carry a positive charge. Silicon doped with a small amount of boron is called a p-type semiconductor

26. Microelectronics • Now consider an interface between p-type and n-type regions of a semiconductor. • This is called a p-n junction. n-type A small excess of “impurity” electrons act as charge carriers p-type A small deficit of “impurity” electrons  “holes” act as charge carriers

27. p-n junction in a semiconductor Reverse bias Current carriers are depleted and the current stops flowing

28. p-n junction in a semiconductor Forward bias Current carriers are replenished and the current keeps flowing

29. Microfluidics • Microfluidic devices are typically the size of a credit card. • Fluids flow in narrow channels in the device. • This enables chemical functions like mixing, reacting, analysing... • “Lab on a chip” Science and Technology Research Institute, University of Hertfordshire, UK http://strc.herts.ac.uk/mm/micromixers.html http://www.aip.org/tip/INPHFA/vol-9/iss-4/p14.html

30. Microfluidics Science and Technology Research Institute, University of Hertfordshire, UK http://strc.herts.ac.uk/mm/micromixers.html

31. Microfluidics Science and Technology Research Institute, University of Hertfordshire, UK http://strc.herts.ac.uk/mm/micromixers.html

32. Microfluidics – “H-filter” University of Washington, USA http://faculty.washington.edu/yagerp/microfluidicstutorial/tutorialhome.htm

33. Microfluidics – “T-sensor” Sensor for “green” reaction product This only gives a positive system if A is present in the sample stream A + B  C (green) A B University of Washington, USA http://faculty.washington.edu/yagerp/microfluidicstutorial/tutorialhome.htm

34. Microfluidics • We understand fluid flow rather well, at least for large-scale systems. • But do fluids in very narrow channels flow in the same way? Poiseuille flow The fluid adjacent to the walls does not move. Plug flow The fluid slips past the walls, and the flow resistance is reduced.

35. Poiseuille flow profile (calculated) http://faculty.washington.edu/yagerp/microfluidicstutorial/tutorialhome.htm

36. Microfluidics Poiseuille flow The fluid adjacent to the walls does not move. “no-slip” boundary condition Plug flow The fluid slips past the walls, and the flow resistance is reduced. “slip” boundary condition

37. Slip or no-slip at the walls?

38. Slip or no-slip at the walls? OUCH ! OUCH ! OUCH ! THAT’S BETTER !

39. A drop of liquid spreads on a surface. • We say the liquid is “wetting”. • A drop of liquid beads up on a surface • We say the liquid is “non-wetting”. Slip boundary conditions and wetting • Scientists are still researching whether the boundary condition for microfluid flow should be slip or no-slip. • The answer may depend on the wetting properties of the surface. • What do we mean by wetting?

40. Wetting • Wetting is characterised by a contact angle, q. • A wetting liquid has a low contact angle; a non-wetting liquid has a high contact angle • Wetting is important in many areas: • surface coatings (paint, magnetic tape, hard disks, …) • washing materials (detergents, laundry, shampoo). • adhesives. q Wetting demonstration (let’s hope it works…)

41. = “surfactant” molecule (like soap or detergent) Wetting then de-wetting (“autophobicity”) mica • Water wets mica, so it spreads to a flattish drop with a low contact angle. • (Actually, the water has surfactant molecules in it.)

42. Wetting then de-wetting (“autophobicity”) 2. The surfactant molecules adsorb to mica, which gives it an “oily” coating.

43. Wetting then de-wetting (“autophobicity”) 3. The water does not wet the oily coating, so it retracts to form a high contact angle.

44. Friction Normal force, N Frictional force, F Tribology • Tribology is the study of friction, lubrication and wear. • These are all important properties of interfaces. • Tribology is tremendously important in many technologies, particularly machinery. … but aren’t forces in perpendicular directions supposed to be independent of each other? The law of friction has been known for 300 years, but scientists are still trying to understand it fully.

45. Micro-electromechanical systems (MEMS) • MEMS are tiny devices with moving parts, engineered using similar technology to microelectronics. • Examples are • accelerometers that trigger the airbags in a modern car, • arrays of tiny mirrors in a data projector. www.memx.com/products.htm

46. Micro-electromechanical systems (MEMS) • What would happen if a moving part in a MEMS device should get stuck to a nearby part? • In a data projector, one pixel goes dead (stays black, usually). • In an airbag sensor, you don’t want to even think about it. • It is important to understand when and why two materials adhere to each other. • Automotive airbag sensors are designed with many parallel MEMS accelerometers (about 80) to make sure they are fail-safe.

47. How do we measure adhesion (and other interface properties, including friction)? • There are many scientific instruments to measure various interfacial properties. • I just want to mention two that are designed to measure adhesion, friction, and forces between materials. • Atomic force microscope (AFM) • Surface force apparatus (SFA) “AFM” demonstration

48. AFM images The surface of graphite, showing individual atoms! Two polymers that do not mix (like oil and water). The surface of a hair fibre. From www.quesant.com/Gallery/gallery_contents.htm

49. AFM images The surface of a DVD. The surface of a hard disk, imaged using a magnetic force sensor. “Quantum wires” fabricated on a silicon wafer. From www.quesant.com/Gallery/gallery_contents.htm

50. Surface force apparatus • Unlike the AFM, the surface force apparatus cannot produce images of surfaces. • Like the AFM, the SFA can measure • adhesion, • friction, • electrostatic and other forces between two materials, • structure of liquids adjacent to solid surfaces, • flow properties of liquids in very thin films (right down to molecular dimensions). “SFA” demonstration