Surface Properties Testing

Question 1

How is Wettability measured?

Answer 1

Using the water contact angle.

Hydrophilic surfaces have low contact angles (<90°), good adhesiveness and good wettability. Super hydrophilic molecules have <20° angle

Hydrophobic surfaces have high contact angles, poor adhesiveness and poor wettability. Super hydrophobic molecules have >150° angle

Question 2

Why is wettability important?

Answer 2

Nonstick surface Teflon (PTFE) contains fluorocarbons which are environmentally persistent, so there is a desire to move away from fluorocarbons

Use can be antibacterial surgical implants, ship hull coatings, clothing/body armour and crude oil separation

Question 3

What is Scanning electron microscopy (SEM) used for?

Answer 3

Electron microscope that scans the surface with a focused beam of electrons. The electrons interact with atoms in the sample producing various signals that contain information about the surface topography and composition of the sample. Samples typically have to be gold sputtered to increase their electrical conductivity and to stabilise them so that they can withstand the high vacuum conditions and the high energy beam of electrons

Some SEMs can achieve resolutions better than 1 nanometer

Question 4

How does Scanning Electron Microscopy (SEM) work?

Answer 4

Electron microscope that scans the surface with a focused beam of electrons. Specimens are observed in high vacuum in a conventional SEM, in low vacuum or wet conditions in a variable pressure or environmental SEM when using wet samples and at wide range of cryogenic or elevated temperatures with specialized instruments

Question 5

What is Energy Dispersive X-ray Spectroscopy (EDX) used for?

Answer 5

Used together with an SEM to generate more information about a sample than an SEM can generate alone.
Used to pick up a specific element on the surface of the sample. It generates information about the chemical composition of a sample including what elements are present as well as their distribution and concentration

Question 6

How does EDX work?

Answer 6

The electron beam hits the inner shell of an atom, knocking off an electron from the shell. When the electron is displaced, it attracts another electron from an outer shell to fill the vacancy. As the electron moves from the outer higher-energy to the inner lower-energy shell of the atom, this energy difference can be released in the form of an X-ray. The energy of this X-ray is unique to the specific element and transition.

Question 7

What is AFM used for?

Answer 7

Form of surface probing microscopy that uses interatomic forces to image topography on the nanometer scale. Has the ability to measure intermolecular forces between atoms

Can image almost any type of surface (polymers, glass, composites, biological samples etc.)
Rough surfaces (with ridges etc.) are more likely to grow unwanted bacteria but also better at cellular integration.

Advanced imaging modes can provide quantitative data on; friction, electrical forces, capacitance, magnetic forces, conductivity, viscoelasticity, surface potential and resistance.

Question 8

How does AFM work?

Answer 8

The atomic force microscope works by scanning a sharp probe over the surface of a sample in a raster pattern. By monitoring the movement of the probe, a 3-D image of the surface can be constructed.

AFMs are sensitive to external vibrations which generally show up as horizontal bands in the image where the scanner skips

Question 9

What is Small Angle X-ray Scattering (SAXS) used for?

Answer 9

Nanoscale density differences in sample can be quantified, can determine nanoparticle size distributions, resolve the size and shape of monodisperse macromolecules, determine pore size and characteristic distances of partially ordered materials.

Better for biological macromolecules as crystalline sample is not needed like in crystallography

Question 10

How does SAXS work?

Answer 10

Analyses the elastic scattering behaviour of x-rays when travelling through material and recording the scattering at small angles

Question 11

What is FTIR-ATR used for?

Answer 11

Attenuated total reflection (ATR) is used alongside traditional IR spectroscopy which allows samples to be in either solid or liquid state without additional prep (prevents the destruction of sample that normally occurs in FTIR prep).

Can be used to analyse biological samples cheaper and more economical

Question 12

How does FTIR-ATR work?

Answer 12

An ATR accessory quantifies the changes that happen to an internally-reflected IR beam, once it comes into contact with the chosen sample. to do this, an IR bean is focused onto a crystal with a high refractive index at a set angle.

Question 13

What is Raman spectroscopy used for?

Answer 13

Used to characterise materials, measure temperature, and crystallographic orientation of a sample
Can be used to identify polymorphic forms is more than one exist of the API
Used as a non-invasive technique for real-time in situ biochemical characterisation of wounds and healing process

Question 14

How does Raman spectroscopy work?

Answer 14

a laser is fired at the sample which bombards the sample with many photons at once, molecules reach the vibrational energy level (an excited state of the molecule where it is more likely to vibrate), and the light is reflected or scattered. The scattered light is recorded to find properties. The frequency difference/shift between each peak in a Raman spectrum and the laser excitation corresponds to the vibrational frequency of a specific molecular bond, hence molecular structure can be determined (similarly to IR)

Question 15

What are the components of an Atomic force microscope?

Answer 15

Scanner – needs fine control, most AFMs use piezoelectric materials

Tip – originally diamond, now silicon/silicon nitrate, generally pyramidal or tetrahedral, around 5-15nm radius around the apex

Cantilever – contact mode needs cantilevers to deflect easily without damaging the sample or tip, and high resonant frequency to avoid vibrational instability, 0.3-2um thickness, 100-200um length. A laser is focused onto the back reflective surface of the cantilever and reflects in a photodetector to determine deflection of the cantilever

Question 16

What is Contact mode in AFM?

Answer 16

The tip contacts the surface through an adsorbed fluid layer on the sample surface. The cantilever deflection is set by the user and is related to how hard the tip pushes against the surface so that the user controls how aggressive the interaction between the probe and sample. The feedback circuit adjust the probe height to try to maintain constant force and deflection

Has higher scan speeds, and rough samples can be scanned easily.

Shear forces may distort image, may get strong capillary forces due to adsorbed fluid layer in ambient conditions, reduced resolution, may damage tip or sample

Question 17

What is Non-Contact mode in AFM?

Answer 17

Oscillation slightly above resonant frequency, van der Waals and other long-range forces decrease the resonant frequency and causes the amplitude of oscillation to decrease. In ambient conditions the adsorbed fluid layer is often significantly thicker than the region where van der Waals forces are significant (water is also attracted to the van der Waals forces). Lots of fluid can affect how the probe moves so ultra-high vacuum conditions are preferred

Normal and lateral forces minimised so no damage is done, better for very soft samples, can get atomic resolution in ultra-high vacuum environment.

Adsorbed fluid layer may be too thick in ambient conditions for measurements, slower scan speed than both

Question 18

What is Tapping mode in AFM?

Answer 18

Most commonly used mode. Cantilever oscillates at or slightly below its resonant frequency, the tip lightly “taps” the surface during scanning. Oscillation is also damped when tip is closer to the surface

Nearly no lateral (shear) forces, higher lateral resolution, lower forces leave less damage.

Slower scan speed than contact mode

Question 19

Describe the tip surface interactions in AFM

Answer 19

The diamond tip contacted the surface directly, with the interatomic van der Waals forces providing the interaction mechanism

When the tip is brought close to the sample, several forces may operate:

• Coulombic interaction – strong short-range repulsive forces, repulsion increases as separation decreases
• Van der Waals interaction – longer range attractive forces

Question 20

Describe the process that occurs in an AFM force displacement curve

Answer 20

• When the cantilever is approaching the surface, if there are attractive or repulsive forces between the tip and the sample, those could be measured in this segment by the respective down or up bending of the cantilever.
• “Snap-in”: the cantilever snaps into contact with the sample. This is due to tip-surface interactions, like capillary forces.
• Repulsive portion: the tip and sample are in contact and bends up upon further movement of the piezo. This section of the process is referred to as the net-repulsive portion.
• Repulsive portion on withdrawal: the tip is now unbending while being withdrawn from the surface.
• Pull-out: the tip gets “stuck” in an adhesive dip before it is able to emerge from the adhesion at the interface.
• After this the cantilever returns to its unperturbed state while the piezo further increases the tip sample distance.

Question 21

How do you detect the deflection of the cantilever in AFM?

Answer 21

The most common way to detect the deflection of the cantilever is using a laser.
A diode laser is focused onto the back reflective surface of the cantilever and reflects onto a photodetector.
The vertical deflection of the cantilever is determined by the difference in light intensity measured by the upper and lower sectors

Question 22

What can affect the feedback in AFM?

Answer 22

When the tip contacts the surface directly, the tip or surface may be damaged. The tip may be blunt, reducing the imaging capability of AFM. Soft surfaces like biological samples can be easily damaged by contact

affected by:
- Setpoint – value of deflection or amplitude that the feedback circuit attempts to maintain (the gap kept in noncontact and tapping modes). Usually set such that the force on the cantilever is small but probe remains engaged with the surface
- Feedback gains – the higher these are set, the faster the feedback system of the computer will react, if gains are too high then the feedback circuit can become unstable and oscillate causing high frequency noise in the image
- Scan rate – scanning the probe over the surface more slowly gives the feedback circuit more time to react and results in better detail, but this increases the time needed to acquire an image

Question 23

What are some artifacts that can be seen in AFM?

Answer 23

• hysteresis - piezo response not linear to voltage. To fix this use a non-linear voltage waveform
• scanner creep - if applied voltage suddenly changes when the scanning position is moved, the piezo scanners response is not all at once, causing distortion
• Bow and tilt - prove tilts due to sample/scanner non-planarity and scanner movement. to fix remove artifact in post-processing software