Lecture 10: Fibres Part 2

Question 1

Microscopic techniques for fibres

Answer 1

• Sterescopic
• Comparison
• Polarised
• Reflected light
• Fluorescence
• Thermal
• Darkfield
• Brightfield
• Multispectral
• Scanning and transmission electron microscope
• Crystallography and diffection
• Some methods are used more than others and some are destructve.

Question 2

Micrscopic characteristics

Natural fibres

Answer 2

• Cotton is characterised by its twisted shape
• flax has characteristic nodes which is brought out in polarised light
• hemp has no nodes but has irregulations in terms of shape
• jute has some nodes, narrower, tapering at the ends (unique identifying feature), colourisation changes as it rotates
• silk has characteristic ribbon shape
• wool has scales which are characterizing

Question 3

Microscopic characteristics

Synthetic fibres

Answer 3

• Synthetic fibres are uniform, more consistent but might have surface textures from the manufacturing process that allows us to identify them.
• Longitudinal Appearance - Texture; Crimping; Pigment. All can be unique to a manufacturing process or particular machine.
• Tri-lobal shape is the most common nlyon synthetic fibre as its designed to maximise its shape.
• Modification ratio is a good way of numerically desigining tri lobal fibres in order to identify and compare them
• Dye penetration and presence of crystalline regions (Crystalline structure will interact with the light more and has an identifiable region) or gas voids (defects along the surface) are key microscopic characteristics.
• It is easy to differentiate between natural and synthetic fibres based on their gross chraracteristics.
• Synthetic fibres are more regular.
• Cross sectional area can give you information about the fibre and its manufacturing process.

Question 4

Modification ratio

Answer 4

MR = R / r
R = longer axis

Question 5

How do we characterise nanostructures?

Answer 5

Done with a combination of techniques to study their chemical and physical properties… We care about 2 main types here:
1. Surface Microscopy - Scanning and transmission electron, atomic force
2. Bulk diffraction - x-ray powder, optical

Question 6

Electron microscopy

Fibres

Answer 6

• The resolution of light microscopy is limited by the illuminating wavelength.
• Higher resolutions are achievable using electrons instead of light.
• Non-destructive analysis of very small quantities of material possible this way (although beam damage CAN occur for sensitive samples).
• Allows for the rapid accumulation of results in high resolution.
• Can even give elemental composition of a material.
• Can be non destructive depending on material and method, sometimes there is damage caused to sample.
• Quick technique to allow us to pick something out.

Question 7

Properties of a light microscope

Answer 7

• 200nm resolutilon
• Low depth of focus
• Good field of view
• Easy and rapid sample preparation
• Relatively low cost (£35+)

Question 8

SEM microscope properties

Answer 8

• 10 nm resolution
• High depth of focus
• Good field of view
• Easy and quite rapid specimen preparation
• High relative cost (£120K+)
• Takes longer for you to set up and prepare sample
• Valuable for trace analysis

Question 9

TEM microscope properties

Answer 9

• 1 nm resolution
• Medium depth of focus
• Limited field of view
• Skilled and slow sample preparion required
• £300K+ cost

Question 10

SEM for fibre analysis

Answer 10

• Can reveal surface features unseen in optical microscopy e.g. Scale-like features on the outer surface of Cashmilon bicomponent acrylic fibres.
• Or fibre-end fracture morphology of a polyester fibre in a woven fabric coat to complement optical damage analysis.
• Elemental analysis of fibres reveals more information.

Question 11

Equal refractive indexes

Answer 11

In the event that the two refractive indices are equal, the light passing through the particle does not deviate at all, and the particle remains invisible.

Question 12

Different refractive indexes

Answer 12

• When the refractive indices are far apart, the light passing through will change direction substantially.
• If refracted sufficiently, they miss the objective lens and these areas of the particle become dark, resulting in high contrast.
• RI is one of the most useful ways to identify a material
• Depending on the difference of RI of sampe and the mounting media, it will affect how the light transmit through into the cone and how it responds based on refraction, diffraction, snells law, etc.
• If theres high contrast that means there’s a high different in RI between media and sample so a lot of refraction of light (light gets pushed out away from the cone)

Question 13

Measurement of refractive index - Becke line

Answer 13

• Take a particle and put it into focus on one plane and we move our focal point up and we see whether or not the light is focused inwards or outwards away from the sample in order to help identify it. Depending on whether the light has the halo effect or is pushed in, it allows us to determine if the RI difference between the sample amd mounting media is above or below it.
• Being able to qualitatively assess RI difference between the mounting medium and a particle is useful, but it’s also desirable to know whether the particle has a RI higher or lower than that of the mounting medium.
• This can be achieved using the Becke Line Test.
• Small particles behave much like lenses, refracting light in a manner that depends on the relative RI values of the particle and mounting medium

Question 14

A particle with a higher refractive index

Answer 14

A particle with a higher RI mounted in a medium of lower RI, will focus axial illuminating rays toward a point above the particle.

Question 15

A particle with a lower refractive index

Answer 15

Lower RI particle in higher RI medium will direct light in opposite direction. Moving the line outside the particle.

Question 16

Becke line immersion measurements

Answer 16

• if the light bends inwards or outwards, allows us to understand if the RI index is above or below it.
• Becke Line immersion measurements can be made by mounting the substance in media of varying RI’s until little change is observed
• This has limitations as will only be true for one wavelength of light at a time (so averaged for white light) need a more precise method
• Light travels at different speeds through a material depending on the wavelength and the RI of the material.
• The variation method is more precise for isotropic materials.

Question 17

Normal light

Answer 17

Waves vibrating in every direction perpendicular to the direction of travel.

Question 18

Linearly polarised light

Answer 18

• Waves vibrating in one direction (it goes to a particular polarised plane of light comin in)
• When it passes through anisotropic sample, theres going to be a change as it passes through bc of Snell’s law, etc.

Question 19

Polarised light microscope

Answer 19

• Only particular polarisation of light will actually be transmitted through a sample that we’re interested. Not every polarisation of light will be transmitted through every sample.
• Normal light can become polarised if it passes through a material that only allows transmission of rays in a particular direction, such as a crystal, or a film
• Polarisation is useful in forensic microscopy when applied to anisotropic substances which nearly all fibres are.

Question 20

Anisotropic substances

Answer 20

• Having a physical property which has a different value when measured in different directions
• Nearly all fibres are anisotropic
• Its only polarised in one plane because with any anisoptropic material theres going to be some polarisation. We can analyse this and measure it.

Question 21

Uniaxial

Answer 21

Uniaxial, theres one axis and then another axis

Question 22

Let’s consider a uniaxial material that allows rays to vibrate in two axes, ω (blue) and ε′ (red) depending on it’s orientation
Let’s illuminate that material with light that has been polarised so that it is vibrating the the E-W plane on the sample.

Answer 22

In this case we polarise the light from west to east, we have a material of interest that has two planes which let different polarisations of light through. We can see how they interact. If our epsilon plane is aligned with east to west polarised light it means all of the light transmitted is going to be responding to the RI and the material properties of the epsilon material. SO the RI we measure is going to respond to that physical feature.

Question 23

Pleochroism

Answer 23

• This property in anisotropic materials causes it to show different absorption colours when it is exposed to polarised light coming from different directions
• It’s relatively easy to compare the pleochroism (or RI if more detail required) of fibre found at a crime scene to fibre taken from the suspect thereby suggesting or disproving link between the suspect and the scene.

Question 24

Retardation

Answer 24

• You approach the sample with full white light instead of polarising the light.
• Different polarisaion of light travel through at different speeds. This means all of our white light aren’t going to pass through at the same time so we get retardation as one light is slowed down.
• The white light going in is no longer white light coming out. This is determined by the contribution in our sample of our two axises.
• These features come about because the two components of light travel through the crystal at two different velocities (the exact velocities determined by the values of ω and ε′)
• Because their velocities are different, one ray travels faster than the other while inside the crystal, that means we end up with light that is different colour compared to the white light that came in.
• The difference between the two rays is called retardation.
• Slow ray is said to be retarded behind the fast ray, and the exact distance that the slow ray falls behind is called the retardation (R)

Question 25

Equation for retardation

Answer 25

R(nm)= B x T(um) x 1000 (nm/um)
B = birefringence
T = Thickness

Question 26

How can retardation be measured?

Answer 26

Retardation can be measured by rotating analyser relative to polariser and seeing how much light is going in and coming out.

Question 27

Birefringence

Answer 27

• If thickness and retardation are known then we can calculate the birefrigence of material.
• The Retardation and Birefringence of a fibre allows us to identify it and compare it to a known sample
• We can extract the birefirgence of a material from the retardation and thickness of a material which gives us a unique number. So we can compare this to an unknown.
• Birefringence is really important for optical microscopy analysis.

Question 28

What is birefringence?

Answer 28

the refraction of light in an anisotropic material in two slightly different directions to form two rays.