Bioinspired materials 1 Flashcards
Derive Young’s eq.
Draw drop on surface, three interfaces: solid-liquid, solid-gas and liquid-gas. The angle theta goes on the inside the bubble and then just add vectors of the surface tensions.
gamma_sg = gamma_sl + cos(theta)*gamma_lg
–> cos(theta) = (gamma_sg - gamma_sl)/gamma_lg
Explain how hydrophilic/phobic is related to the angle theta in Young’s eq. and define a superhydrophobic surface.
A surface is hydrophilic when theta 90° and the liquid dewets the surface, ie. don’t want to be on the surface. A superhydrophobic surface means that theta is almost 180°.
Explain the wenzel eq. and how a rough surface affects the character of the surface.
Wenzel eq.
cos(theta_app) = R * cos(theta)
Theta_app is the apparent contact angle when the surface is rough this increases the hydrophobic effect or the hydrophilic effect.
That is cos(theta_app) increases for both cases, but for theta > 90° does it mean that theta decreases and
Discuss the Cassie eq. when one medium is air.
Cassie eq.
cos(theta_app) = f1cos(theta_1) + f2cos(theta_2)
if medium 2 is air, this means that theta_2 = 180° and f2=1-f1
–> cos(theta_app) = f1*cos(theta_1) + f1 - 1
This makes a linear relationship crossing the y-axis at -(1-f1) and from this one can find the real theta if one has measure theta_app and knows the volume fraction.
But Cassie eq. only describes the hydrophobic case.
Explain the concept contact angle hysteresis.
It is possible to have two different contact angles on the same contact surface. When a drop hangs on a window for example.
The lower angle is larger than the upper, the lower is called advancing angle and the upper receding angle. There is an equilibrium between the gravitation pulling the drop down and the capillary force pulling it upwards.
Explain what a self-cleaning surface is based on superhydrophobicity.
When a surface is superhydrophobic due to structure and wax-like coating, water drops roll on the surface and pull dirt with when rolling.
If superhydrophobic surface is not self-cleaning the hydrophobicity will decrease when dirt stucks in the pores.
Name one application of superhydrophobic self-cleaning surfaces.
Transparent and hydrophobic self-cleaning film on solar cells.
Self-cleaning is also good on car windscreens, or windows etc.
Give an example of a self-cleaning surface that breaks down dirt on the surface.
When surface covered in TiOx, when illuminated by UV-light the surface becomes superhydrophobic, because when it is electronically excited by the incoming photons it can make O2 and OH-ions into radicals. These radicals will react with the carbon chains of the dirt on the surface and break them down. Which divides large dirt particles into smaller parts –> thin film of dirt instead on surface which is see-through.
Give examples of how to create a superhydrophobic surface.
Ex. 1: Etch on the surface using plasma to create voids –> increasing surface area and roughness.
Ex. 2: Creating structure with pillars on the surface by lithography. Array of silicon nano-sized pillars, sharpening the tips by thermal oxidation. After hydrophobic coating is the contact angle around 175°.
Explain how a self-healing material using the capsule approach works.
There are small hollow areas inside the material that are filled with a healing agent and around in the material there are also small catalysts. When there is a crack in the material, one of the capsules breaks and lets the healing agent out and when it meets with a catalyst it will make the polymer network of the healing agent stiff.
Explain how a self-healing system works that is based on hollow fibres.
For example on fibre containing resin and one hardener system, or hardener system as small capsules outside the fibres. It is based on the same principle, the cracks breaks the fibre and resin comes out and when it meets the hardening it will be solid.
