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51 Cards in this Set

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Describe the origin of Contact angle hysteresis

If a droplet sits on a tilted surface, the contact angle at the front and the back of the droplet correspond to the advancing and receding contact angles respectively. If the advancing angle is greater than the receding angle, have contact angle hysteresis.

What defines a superhydrophobic surface?


- theta > 160


- low contact angle hysteresis

Describe the principle of the Wenzel interface

A homogeneous solid-liquid interface model predicting that the contact angle of a liquid with a rough surface is different from that with a smooth surface.

Define the roughness ratio factor


Rf=Arough/Aflat


Where Arough is the area of solid-liquid interface and Aflat is the area of a flat plane.

How does the contact angle change with increasing roughness ratio?

Hydrophobic surfaces become more hydrophobic, hydrophilic surface becomes more hydrophilic.

Derive the Young equation

State the Wenzel equation

Describe the principle behind the Cassie-Baxter modelof surface roughness


- the gaseous phase may be trapped in the cavities of a rough surface


- a composite/non-homogeneous solid-liquid-air interface is therefore formed


- 2 sets of interfaces ( solid liquid and composite liquid-air and solid-air)

State an equation defining the Cassie Baxter interface

To calculate contact angle for CB, Wenzel equation is modified by combining the fractional area of wet surfaces and fractional area with air pockets.
fLA= fractional flat geomet4ric area of liquid air interface


To calculate contact angle for CB, Wenzel equation is modified by combining the fractional area of wet surfaces and fractional area with air pockets.


fLA= fractional flat geomet4ric area of liquid air interface

Describe the 3 levels of hierarchical plant surfaces

- 3D wax crystals


- papillose cell


- large hairs

Why is a self cleaning leaf surface useful?

Carbon dioxide diffuses faster through air than water therefore want no water on leaf surface to allow CO2 to diffuse away as a result of photosynthesis

How can an ultra low energy surface be achieved?


-By use of fluorinated solid surfaces.


-Long peflourinated chains are required in order to displace repellency towards low surface tension liquids. (Need enough CF2 groups to act as electron withdrawing groups and draw density from the CF3. Usually 5-7)

What are the advantages of pulsed plasmachemical functionalization?

- small energy required to excite molecules


- small amounts of chemicals used

What is a SAM?

Self assembled monolayer

Why do we used pulsed and not continuous waves in plasmachemical functionalization?


Continuous wave allows many side reactions to occur, resulting in a messy surface.


By using pulsed functionalization, the pulse activates the molecules during the short time on and they add to the surface, however there is less time for secondary reactions to occur, resulting in a much more ordered surface.

Describe the principles of soft lithography.

1) Make a mould using dental wax


2) Functionalize this mould


3) Use mould to create a new material.


4) Remove mould, surface functionalization should be transferred.

Why do saturated polymers exhibit higher levels of surface fluorination than unsaturated polymers?

Hydrogen abstraction by fluorine to form HF.


Fluorine addition at C=C then occurs, giving higher levels of surface fluorination than in unsaturated polymers.

Describe how a honeycomb surface can be created

1) Begin with surface, add polymer, solvent and humidity


2) Evaporate the solvent and allow water to condense


3) Evaporate both solvent and water to leave surface with large scale roughness


The use of a humid cabinet helps to adjust condensation/evaporation levels as required.

Describe how the stenocara beetle collects fog


Hydrophilic bumps along its back with hydrophobic valleys in between. Water collects on hydrophilic regions and droplets then roll down hydrophobic regions towards the beetles mouth.

Describe the process of microarray fabrication to replicate the Stenocara beetle

1) Plasmachemical functionalization to create a superhydrophobic background


2) Clamp a mask to the surface


3) Plasma deposition of superhydrophilic pixels through the mask


4) Removal of the mask

Describe a method of quantifying the fog collecting properties of a surface


Use microcondensation.


Generate a mist and direct towards the sample mounted at 45 degrees to the mist, with a collection vessel beneath. Can measure how much water collects in the vessel.

Name 3 uses of an oleophobic-hydrophilic surface


1) Anti fogging devices


2) Water and oil separation


3) Self cleaning surface

Describe the process of heavy metal capture and release


- can design surfaces for heavy metal ions and regenerate these by washing in mild acid.


- The use of host-guest inclusion complexes.

Give 3 uses of a ZnO surface


1) Photoconductive


2) Switching wettability


3) Antibacterial activity

Describe how a ZnO surface can exhibit photoconductive processes.

ZnO is a transparent semiconductor with a wurtzite crystal structure with bandgap of 3.3 eV.


When ZnO is exposed to UV photon radiation with energy greater than its bandgap, electrons are promoted from the valence band to the conduction band. Therefore formation of electron hole pairs.


Electrons can become trapped by physisorbed oxygen at the surface and react with this to form chemisorbed O2-(ads)


Such O2-(ads) can attract holes from the bulk which migrate to the surface and combine to form a surface vacancy with photodesorption of molecular O2.


Following surface desorption of molecular oxygen, further electrons are promoted to the conduction band and no longer trapped by the oxygen => conductivity

Explain why the conductivity of the ZnO surface decays after termination of the UV signal

When the signal is terminated, the oxygen is readsorbed to the surface and therefore continues to prevent electrons from being promoted to the conduction band.

Describe how a ZnO surface can exhibit photo switching wettability


Photodesorption of O2(g) from ZnO creates surface vacancies which can allow dissociative water molecules to adsorb, therefore giving hydroxyls on the surface which are responsible for the hydrophilic behaviour.


The high contact angle recovers over a matter of weeks after UV extinction as hydroxyls are thermodynamically displaced by readsorbed molecular oxygen.

Explain how a ZnO surface can exhibit antibacterial activity

ZnO has a bandgap that allows electrons to be promoted from the valence band to the conduction band with UV radiation, leading to electron hole pair formation.


If electrons and holes migrate towards the surface of the semiconductor without recombination, they can participate in various REDOX reactions with adsorbed species such as water, O2etc.


The valence band hole oxidises water on the surface to form hydroxyl radicals.


The conduction band electrons can reduce oxygen on the surface to form peroxide radicals.


Hydroxyl/peroxide radicals are powerful oxidisers and therefore easily oxidise any organic species to water/CO2

Give two uses of titanium dioxide surfaces


1) SElf cleaning windows


2) hydroxyl/peroxide decontamination of water


(same mechanisms as for ZnO)

Describe 3 problems which may arise from biofouling.

1) Damage of medical devices


2) Cause of diseases


3) Marine biofouling resulting in increased energy consumption

List the three forces which govern the adorption of a protein onto a solid surface

1) HYDROGEN BONDS - between polar groups contained in the protein and on the surface


2) HYDROPHOBIC FORCES - arise due to formation of a water depletion zone at the interface between hydrophobic regions on a protein molecule and a hydrophobic surface


3) ELECTROSTATIC INTERACTIONS - associated with solvated charged groups on the protein surface and the solid substrate.

Explain the dependence of protein adsorption on free energy


Protein adsorption is governed by free energy change.


Loss in conformational entropy of the protein due to fixation of some parts is compensated for by a gain in enthalpy due to interaction of distinct chemical groups with the surface.

Give 3 surface functionalization approaches to making protein resistant surfaces


1) Chemisorption - solution of molecules with a linker group which reacts with the substrate to form a SAM


2) Physisorption - add to surface and sticks via V der W


3) Surface Initiated polymerization

Describe the two types of surface initiated polymerization and their advantages and disadvantages.


- grafting from reactions


Only small molecules need to reach reactive surface sites => higher grafting densities


-grafting onto reactions


bulky polymer chains must diffuse to the surface, may be shielded by previous polymer chains BUT polymer chains can be characterized before hand therefore have more control

Give 5 ways of making protein resistant surfaces


1) Adsorption of Serum Albumin


2) Polyethylene oxide (PEO)/Polyethylene glycol (PEG)


3) Phospholipids


4) Polyacrylamide groups


5) Saccharide groups

Explain how Bovine serum albumin can be used to decrease protein adsorption


simple strategy in which adsorption of SBA onto the surface can suppress protein adsorption.


Not very robust, absorbed proteins vulnerable to displacement by more surface active proteins due to the Vroman effect

What is the Vroman Effect?

First abundant proteins of low affinity adsorb reversibly to the surface and later scarcer proteins with high affinity displace the preadsorbed proteins.

LCST/UCST

Lower critical solution temperature


Upper critical solution temperature

How can PEO/PEG resist protein adsorption?


Use as a benchmark for minimizing protein adsorption.


Non fouling character attributed to high levels of polymer chain hydration as well as conformational flexibility of the polymer


Intrinsic disadvantages - PEO susceptible to oxidative degradation and chain cleavage in aqueous environments

How can phospholipids minimise protein adsorption to a surface?


High level of hydration of the zwitterionic headgroup


Hydration layer ensures that proteins which come into contact with the surface do so reversibly and without deformation


Relies on the ability to self assemble as supported lipid bilayers


Relies on weak van der Waals forces

How do Saccaharide groups resist proteins?


These are highly hydrated hydrophilic surfaces therefore are protein resistant.


E.g. dextran (much better than PEO/PEG)

Describe how polyacrylamides resist protein adsorption


Usual example is the thermoresponsive polymer poly(N-isopropylacrylamide)


Behaves as a protein resistant material below its LCST and protein adsorbant above its LCST


At T<LCST, forms an expanded structure in aqueous solution with a random coil configuration.


T>LCST, collapses into a globular structure where hydrophobic interactions dominate, polymer precipitates out of water.


Hydration-dehydration switching above LCST due to entropy gain during amide group dehydration.

Give 2 ways of preparing poly(N-isopropylacrylamide) surfaces


1) surface initiated polymerization


2) surface grafted pre formed polymer

List the 4 main models in which proteins can be resisted


1) Mobility model


2) Water barrier model


3) Entropy model


4) Counterion release model

Describe the mobility model

''wagging off'' of protein molecules due to high polymer chain mobility

Describe the water barrier model of protein resistance

tightly bound water on to of layer/along chains prevents interactions between polymer and chain

Describe the entropy model of protein resistance

Protein adsorption not favourable due to entropy loss caused by compression of the restricted volume of polymer chains by proteins

Describe the counter ion release model of protein resistance

Depending on ionic strength of surrounding medium, protein adsorption occurs

Give 5 disadvantages of slow release biocides which are currently used as antimicrobials


1) Only offer protection from specific forms of bacteria


2) Don't kill viruses


3) Have limited life span


4) Costly impregnation methods


5) Environmental problems from biofouling/biocorrosion

Give three advantages of the use of polymeric quaternary ammonium salts for antimicrobials


1) broad spectrum of antibacterial activity


2) High kill rate


3) Non-toxic to mammals

Describe the mechanism of cell death using PQAS


1) Diffusion of antimicrobial through cell wall


2) binding to cytoplasmic membrane


3) Disruption of the cytoplasmic membrane


4) Release of cytoplasmic constituents such as K+, DNA, RNA


5) Cell death (spills out contents)