Ultra-ever-dry, good for aerodynamic reasons too?

[Holm86]

Hi,

Something I can contribute to a little bit as I am just completing a PhD on whether such surfaces can be used to reduce drag in WATER.

The concept of my research is that if you make a surface superhydrophobic and immerse it in water, you can hold a layer of air on the surface. This theoretically has an effect on the viscous drag of the surface due to the change in viscosity between air and water.

To explain this take a basic definition of shear stress (which is directly proportional to the viscous drag)

Shear stress = viscosity x velocity gradient

So when you have water flowing past a superhydrophobic surface, with air trapped on it, the shear stress is proportional to the viscosity of the air rather than the water. You essentially have a lubricating layer of air, which has ~100x smaller viscosity, making your shear stress smaller and hence the reducing the drag.

Thus, these surfaces cannot be used in air in this way as you would be lubricating air with air, which would clearly have no effect.

The problem for using these surfaces in water is that retaining an air layer on the surface is very hard. The air is subject to shearing forces, dissolves into the water over time, is affected poorly by contaminants etc etc. The upshot of my research is that you can achieve a drag reduction in water, but only if you can maintain a thick layer of air on the surface.

I hope that explains a bit, tried to condense my thesis into half a page . Let me know if you have any specific questions.

Cheers
Gee

The friction of air on air could still be lower than the fricton of air on a surface?? Though even if it dosnt do anything friction wise if its able to trap air it could hold on to the boundary layer better than other surfaces. Then it could be usefull in diffusers and underside of wings.

[flyboy2160]

...
The friction of air on air could still be lower than the fricton of air on a surface?? Though even if it dosnt do anything friction wise if its able to trap air it could hold on to the boundary layer better than other surfaces. Then it could be usefull in diffusers and underside of wings.

And your scientific basis for saying this is what? Classical aerodynamic theory already assumes the molecules on the surface are stationary. Do you have evidence that they are not and that this product makes them so?

[Gee]

Does anyone know how ultra-ever-dry surfaces behaves when dirt is thrown at them with high velocity?

I'm asking because at the end of a grand prix the cars always look dirty and are covered with flies and marbles, which of course changes the surface roughness and maybe the airflow over these areas.

@Gee: How durable are the air layers you research in your thesis and with which methods do you produce them?

The surfaces that I have used are not very durable at all. They can hardly withstand a human touch, making the experiments on them all the more fun. There is a huge amount of research into how to make these types of surface coatings, but the main ones I have used are a chemical deposition procedure on copper and Grangers solution, which is a similar product to ever-dry but for clothes.

As you said it would be interesting to have a look at whether they could be used as anti-dirt coatings and prevent a surface becoming rougher during a GP. Airlines/air craft manufacturers are already heavily researching this area for anti-icing coatings.

[aussiegman]

Hi,

Something I can contribute to a little bit as I am just completing a PhD on whether such surfaces can be used to reduce drag in WATER.

The concept of my research is that if you make a surface superhydrophobic and immerse it in water, you can hold a layer of air on the surface. This theoretically has an effect on the viscous drag of the surface due to the change in viscosity between air and water.

To explain this take a basic definition of shear stress (which is directly proportional to the viscous drag)

Shear stress = viscosity x velocity gradient

So when you have water flowing past a superhydrophobic surface, with air trapped on it, the shear stress is proportional to the viscosity of the air rather than the water. You essentially have a lubricating layer of air, which has ~100x smaller viscosity, making your shear stress smaller and hence reducing the drag.

Thus, these surfaces cannot be used in air in this way as you would be lubricating air with air, which would clearly have no effect.

The problem for using these surfaces in water is that retaining an air layer on the surface is very hard. The air is subject to shearing forces, dissolves into the water over time, is affected poorly by contaminants etc etc. The upshot of my research is that you can achieve a drag reduction in water, but only if you can maintain a thick layer of air on the surface.

I hope that explains a bit, tried to condense my thesis into half a page . Let me know if you have any specific questions.

Cheers
Gee

This is very interesting and I remember seeing somewhere in the distant past that there was some use of low pressure air "bubbled" through the hull to reduce drag. It was a modified low-pressure-air hull lubrication system from an ice-breaker which uses it to reduces resistance against the hull, reducing fuel and horsepower requirements as well as hull ablation.

[bhall]

That sounds like supercavitation. The Russian Navy has a torpedo, the VA-111 Shkval, that moves through the water in a pocket of air generated through its nose to reduce drag. Combined with a rocket motor for propulsion, the torpedo can reach speeds in excess of 200 MPH.

[Dragonfly]

But in the case with the torpedo there is interaction between liquid and gas, fluids in different state of aggregation.

[bhall]

I know. My post is a response to the one above it.

[Tommy Cookers]

This is very interesting and I remember seeing somewhere in the distant past that there was some use of low pressure air "bubbled" through the hull to reduce drag. It was a modified low-pressure-air hull lubrication system from an ice-breaker which uses it to reduces resistance against the hull, reducing fuel and horsepower requirements as well as hull ablation.

IIRC there was a Danish? speedboat record breaker (and others) that used this method eg in 1913

Mitsubishi demonstrated a similar ship sized system last year - a 15% drag reduction