THE IMPORTANCE OF INTERFACES Roger Horn Ian Wark Research InstituteUniversity of South Australia...

THE IMPORTANCE OF INTERFACESTHE IMPORTANCE OF INTERFACES

Roger HornRoger Horn

Ian Wark Research Institute University of South Australia Adelaide, Australia

Ian Wark Research Institute University of South Australia Adelaide, Australia

InterfacesInterfaces

• 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…

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

DemonstrationDemonstration

Let’s look at the atoms in a perfect crystalLet’s look at the atoms in a perfect crystal

5 10 2015

5 10 1915

“Edge dislocation”

= T

Let’s look at the atoms in a Let’s look at the atoms in a realisticrealistic crystal crystal

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

?

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

““Glide” of edge dislocationsGlide” of edge dislocations

Interfaces block the movement of edge dislocationsInterfaces block the movement of edge dislocations

Interface• 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.

Ceramics do not bend like wire…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” demonstrationBrittle” demonstration

MicroelectronicsMicroelectronics

• 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.

Si

A 2D representation of tetravalent bonding

MicroelectronicsMicroelectronics

• 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.

Si

A 2D representation of tetravalent bonding

MicroelectronicsMicroelectronics

• 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

MicroelectronicsMicroelectronics

• 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

e–

MicroelectronicsMicroelectronics

• 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

p-n junction in a semiconductorp-n junction in a semiconductor

Reverse bias

Current carriers are depleted and the current

stops flowing

p-n junction in a semiconductorp-n junction in a semiconductor

Forward bias

Current carriers are replenished and the

current keeps flowing

MicrofluidicsMicrofluidics

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

• 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”

MicrofluidicsMicrofluidics

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

MicrofluidicsMicrofluidics

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

Microfluidics – “H-filter”Microfluidics – “H-filter”

University of Washington, USA http://faculty.washington.edu/yagerp/microfluidicstutorial/tutorialhome.htm

Microfluidics – “T-sensor”Microfluidics – “T-sensor”

University of Washington, USA http://faculty.washington.edu/yagerp/microfluidicstutorial/tutorialhome.htm

Sensor for “green” reaction product

A B

A + B C (green)This only gives a

positive system if A is present in the sample

stream

MicrofluidicsMicrofluidics

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.

• 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 profile (calculated)Poiseuille flow profile (calculated)

http://faculty.washington.edu/yagerp/microfluidicstutorial/tutorialhome.htm

MicrofluidicsMicrofluidics

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.

“no-slip” boundary condition

“slip” boundary condition

Slip or no-slip at the walls?Slip or no-slip at the walls?

Slip or no-slip at the walls?Slip or no-slip at the walls?

OUCH ! OUCH !OUCH !

THAT’S BETTER !

Slip boundary conditions and wettingSlip 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?

• 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”.

WettingWetting

• Wetting is characterised by a contact angle, .

• 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.

Wetting demonstrationWetting demonstration

(let’s hope it works…)(let’s hope it works…)

Wetting then de-wetting (“autophobicity”)Wetting then de-wetting (“autophobicity”)

= “surfactant” molecule (like soap or detergent)

1. Water wets mica, so it spreads to a flattish drop with a low contact angle.

(Actually, the water has surfactant molecules in it.)

mica

Wetting then de-wetting (“autophobicity”)Wetting then de-wetting (“autophobicity”)

2. The surfactant molecules adsorb to mica, which gives it an “oily” coating.

Wetting then de-wetting (“autophobicity”)Wetting then de-wetting (“autophobicity”)

3. The water does not wet the oily coating, so it retracts to form a high contact angle.

TribologyTribology

• 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.

FrictionFrictionNormal force, N

Frictional force, F

NF

… 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.

Micro-electromechanical systems (MEMS)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

Micro-electromechanical systems (MEMS)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.

How do we measure adhesion (and other How do we measure adhesion (and other interface properties, including friction)?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” demonstrationAFM” demonstration

AFM imagesAFM images

From www.quesant.com/Gallery/gallery_contents.htm

The surface of graphite, showing individual atoms!

Two polymers that do not mix (like oil and water).

The surface of a hair fibre.

AFM imagesAFM images

From www.quesant.com/Gallery/gallery_contents.htm

The surface of a DVD.

The surface of a hard disk, imaged using a magnetic force sensor.

“Quantum wires” fabricated on a silicon wafer.

Surface force apparatusSurface 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” demonstrationSFA” demonstration

Modified SFA to measure fluid dropsModified SFA to measure fluid drops

mercurycapillarytube

water

thin silver

mica

The mica can be moved up and down with nm control

200m

50nm

100nm

150nm

200nm

250nm

mica

mercury

mica

Mica has a negative surface charge in water

Mercury also has a negative surface charge, so the drop is repelled by the mica surface

50nm

100nm

150nm

200nm

250nm

mica

mercury

mica

Mica has a negative surface charge in water

This time mercury has a positive surface charge, so the drop is attracted to the surface

The mercury drop jumps into contact with the mica

SummarySummary

• Interfaces are all around us…

… and very important in countless technologies and products.

• I have tried to illustrate this with a few examples – – properties of materials (metal ductility/hardness, brittle fracture of

ceramics),

– p-n junctions which are fundamental to microelectonics devices,

– microfluidic devices,

– wetting,

– tribology,

– micro-electromechanical systems (MEMS).

• And I have briefly described two instruments (AFM and SFA) for measuring adhesion and other forces between materials.

Thank you for your attention !Thank you for your attention !

• Microelectronics– p-n junctions– soldering

• Microfluidics– slip BCs; wetting (and sensitivity to a few angstroms at surface)– autophobic demo

• Tribology– friction, adhesion– MEMS

• Colloids?• Measurements

– AFM demo – topology (+ images)– surface map of adhesion– adhesion – BOX demo– mercury films

MicrofluidicsMicrofluidics

Posner Research Group, Arizona State University, USA

http://microfluidics.asu.edu/