SuperHydroPhobic: How can a droplet sit on a surface
Most of the real surfaces are rough. There two options for the water droplet.
i) To follow the roughness of the underlying substrate. When the substrate is hydrophilic the surface roughness reduces the water contact angle and when it is hydrophobic it enhances it.
So the parameter r (the ratio between the true surface area over the apparent one) controls θ* contact angle (Wenzel equation):
cosθ*= rcosθ
θ: Young’s contact angle
ii) The droplet does not follow the roughness of the surface
In this case the droplet is like a fakir. It is sitting upon a patchwork of solid and air. The more it is sitting on air, the higher the measured water contact angle.
The contact angle θ* is an average between the angle on the solid (cos θ) and the one on the air (cos180o = -1):
cosθ*= -1+fs(cosθ+1)
fs: fraction of the liquid that contacts the solid
The fakir state is also called Cassie – Baxter state.
A surface is called super-hydrophobic when water contact angle exceeds 1500. In this case usually a water droplet can bounce on the surface and also can roll-off on it with a tilt of less than 50. In other words the surface is water-repellant. This kind of surfaces is also called self-cleaning. On a hydrophilic or on a hydrophobic surface the pollutants adhered to the water droplet, but the water droplet sticks on the substrate. So the water remains and evaporates on the surface and so the pollutants remain on it. Consequently the pollutants do not remove from the substrate. On a super-hydrophobic substrate the water rolls-off and leaves the surface taking the dirt with it. So the water cleans the surface. This is a very useful application especially for glasses of buildings. The self-cleaning mechanism is presented in the next figure:
Click for larger image
11 comments
I’m having trouble reading the formulas. You think you might help me out by fixing that issue?
Hi There,
How much less friction is there with water flow over a superhydrohobic coated surface compared to a hydrophilic surface…anyone know? And are there any published tests con firming this? Cheers
Hello! I am a graduate student giving a talk on superhydrophobic surfaces. May I have your permission to use your images and reference your work? Thank you.
-Alexis Lee
University of New Orleans
Is there any ready-made product to spray on surfaces?
How to use?
Dear Sir
please give me a quotation for Super-hydrophobic all surface
Regards
masoumi
Please make your question more clear.
Answering to Benny Ho…
yes, there are many products that can be sprayed. a solution of PERFLUORODECYLTRIMETHOXYSILANE in ethanol can be sprayed over any surface, making it superhydrophobic (after drying).
the problem of this technique is that if ther is no chemical reaction between the surface and the PERFLUORODECYLTRIMETHOXYSILANE, the product will wash away after some uses.
for enhacing the reactivity between the surface and the perfluorodecyltrimethoxylane 20 second plasmas treatment is enough…
hope I´m helpfull..
can u give me what chemical that have been used to make the surface superhydrophobic? thank you. email me, ;D
Can the PERFLUORODECYLTRIMETHOXYSILANE apply on the glass surface, paint and plastic surface?
I have used fluorine based coating with 5% SiOinside the content. The water repellent effect is quite good on the painted surface, but won’t last long. Is that the fluorine is not a good material for chemical bonding to the paint?
I want to know how are super-hydrophobic surfaces made…..??
kindly reply,
thanking you,
Neha.
What about the hydrophobic material that might stick to the surface? As they are hydrophobic, they wont interact with water and hence cleaning them is not possible in case of self cleaning coating too.
Thank you
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