Slip Flow Measurement: Advice & Ideas

[cecil_hk]

Dear all,

I am hereby to get advices.
My research topic is appointed to be slip flow measurement. I have read a lot of papers in this area. So far, there is no clear physics definitions on slip length and slip velocity, which are from micro viewpoint instead of macroscale. And there is no convincing experiment result, in fluid dynamics area, to clearly prove the existence of non-slip boundary condition for Newtonian fluid. Some physics researchers use Atomic Force Microscope to measure the hydrodynamic resistance force between the tip and the substrate, and they draw some conclusions on slip length. The measured slip length is at nanometer scale, it is comparable with surface roughness. I am aimless now. Should you have any experience or ideas on this topic, please share or talk with me. Thanks.



Cecil

 

[Clausius2]

Dear all,
I am hereby to get advices.
My research topic is appointed to be slip flow measurement. I have read a lot of papers in this area. So far, there is no clear physics definitions on slip length and slip velocity, which are from micro viewpoint instead of macroscale. And there is no convincing experiment result, in fluid dynamics area, to clearly prove the existence of non-slip boundary condition for Newtonian fluid. Some physics researchers use Atomic Force Microscope to measure the hydrodynamic resistance force between the tip and the substrate, and they draw some conclusions on slip length. The measured slip length is at nanometer scale, it is comparable with surface roughness. I am aimless now. Should you have any experience or ideas on this topic, please share or talk with me. Thanks.

Cecil

Ahhh, this is a very interesting topic. I don't have any idea about that by the way. I always believed the non-slip condition turns out to be something ideal. So is there some kind of "atomic friction" or something like that?.

 

[cecil_hk]

Non-slip boundary is an ideal condition. The fluid molecule near wall could not be fixed there.
Some think that there are many nanobubbles at surface, especially hydrophobic surface. So the fluid slips at this air-liquid interface. The surface force measurement has ever proved a long-range force at some surfaces. And this may be the evidence to testify the existence of nanobubbles.
Another kind of explanation is that the liquid viscosity at surface is lower than that in bulk. This can be proved by MD simulation.

 

[FredGarvin]

This level of measurement is way smaller than anything I deal with. I must say that after doing some research into nanotechnology, the area is fascinating. I have noticed that the AFM is the mainstay of instrumentation at this level. I have seen manufacturing processes done with them as well. How one would actually measure a flow with one is beyond me.

 

[Danger]

How one would actually measure a flow with one is beyond me.

I don't understand the whole topic of this thread, but wouldn't using a stationary tip over a flow be equivalent to scanning the tip over a stationary surface? If so, the rate at which the 'bumps' appear would measure the flow. Maybe... hmmm... I think I'll go back to work now.

 

[FredGarvin]

I have read papers of some people using AFMs to create pico level indentations. I wonder if something like that could not be used in conjunction with the AFM as a measuring tool to look at the pressure at the bottom of an extremely small hole at the wall interface.

 

[Danger]

You lost me on that one, Fred. Can you elaborate?

 

[FredGarvin]

Well, my thinking is that, like on the macro scale, if the non slip assumption does not hold true, then there should be some kind of static pressure drop (no matter how incredibly small). An extremely small indentation could be used as a type of static pressure tap. I do know things are very different when working on that scale. You are starting to look at individual molecules on the nano scale. So the correlaries between this scale and the macro scale can't hold exactly true. But maybe there is an analagous situation that can be exploited. I'm just thinking out loud.

 

[Danger]

I'm afraid that even with your explanation, this is way over my head. By 'pressure tap', do mean something like the static inlet on a plane?

 

[FredGarvin]

Yeah. Sorry. That's probably terminology that doesn't get used outside of fluids and measurement. A static tap is a hole in the wall of a vessel that you use to measure the static pressure of a fluid (moving or stationary). As you know, you measure the dynamic pressure with a pitot tube. Some aircraft have separate static ports while others have the static ports somewhere on the pitot tube assembly.

Basically, the OP wants to measure down on the smalles scale possible to see if the idea that, at the wall interface, a fluid does have a zero velocity.

 

[Cyrus]

Hey Fred,

I gota question. Let's say the velocity is zero. Then wouldent the velocity detected be the random speeds due to the molecular movement of the atoms, what one would expect with a gas in a bottle that's stationary. So wouldent he need a type of (kalman?) filter to take away the random noise of the gas movements due to being a stationary gas, to detect any net movement in the direction of the flow? I am just doing a whole lota guessing. Please enlighten me!

 

[Danger]

Okay... I'm going to make a couple of assumptions here and see if I'm on track. Please correct me before I get too far out of line.
1) non-slip boundary condition is a layer of fluid that clings to the wall of a pipe without moving?
2) to make a nano-indentation, you are thinking of using the AFM to remove a small group of atoms from that layer?
3) random thermal fluctuations would have to be averaged out in order to get an accurate reading?

 

[FredGarvin]

Hey Fred,
I gota question. Let's say the velocity is zero. Then wouldent the velocity detected be the random speeds due to the molecular movement of the atoms, what one would expect with a gas in a bottle that's stationary. So wouldent he need a type of (kalman?) filter to take away the random noise of the gas movements due to being a stationary gas, to detect any net movement in the direction of the flow? I am just doing a whole lota guessing. Please enlighten me!

I don't think I can enlighten you on this. Like I said before, this is a scale that I have no experience in! My experience lies in the bulk world. The measurement of large flows, not down to this level. The little bit of research I have done on this level was in terms of manufacturing with nano-sized dimensions. The addition of the flowing aspect has me stumped. I'm just kind of thinking out loud here...

I think you just pointed out another reason why this research would be very tedious and difficult. I hope the OP comes back and discusses what he/she has done so far in this area.

 

[cecil_hk]

Dear all,

Thanks a lot for your talking about my question.
Researches on this problem can be divided into two parts: simulation and experiment.
For simulation, usually MD simulation, some papers have ever shown the fluid density distribution in a 100 nm (not quite remembered) width channel. It is clear that the density near the wall is smaller than that in bulk. But it should be noticed that the definition of density is not the same as in macroscale. Maybe it is a kind of trick to get that result. The definition of density for one molecule is another physics problem. Anyway, the simulation result proves the low density assumption. And the slip boundary does exist.
For experiments, it can be divided into three directions. 1. Micro-PIV measurement, by Meinhart in UCLA. His paper has been cited more than 100 times. But be frank, few people trust the result because the working fluid is a kind of solution (partilces in water). 2. Flow rate measurement. No cogent results have been shown now. Because the pressure and flow rate are hard to be measured accurately. Also the little change in temperature will cause the fluctuation in density. This always brings errors to the experiment. Another important flaw is that the microchannel should be changed once during the experiment, say, a hydrophilic one to a hydrophobic one. So I think the sysem can not be absolutely same. 3. AFM measurement. When the system tip approaches the surface, the fluid between the tip and surface will be drained. And this will cause a drainage force. Based on a model developed by a Russian researcher in 1995, the slip length can be calculated. For this model, surface roughness is neglected.
In experiments, the slip length is usually found on hydrophobic surfaces. Some believe that there would be plenty of nanobubbles covering the hydrophobic surface. So the slip in MD simulation and in experiment are not the same mechanism.
I wish my review could help you to understand more about this search.
For my research, I plan to use DC field to change the wetting ability of the surface, that is, from hydrophobic to hydrophilic. The advantage is that the microchannel needn't be changed during the experiment. But one professor points out that DC field will change the fluid properties. So this method is not reliable. My group has no AFM. And the AFM for public use in my university can not do the experiment, in which the tip should be immerged into fluid.
My project is appointed to be slip flow measurement. It is a project that my supervisor's PhD friend in Germany couldn't accomplish ten years ago. That guy quited his PhD study after about 3 years' efforts on this experiment. My situation is similar with him. More than two years have been spent, but there is no progress so far. There is no available instruments , and my supervisor can not give me any instruction. I am doing another project now, which can learn something on MEMS. But this project is not mine. After serious consideration, I decide to drop out. I am applying for some european universities in these days. It is quite sad to seek a position during the Christmas. I wish I could get at lease one addmission this year, which could give me a chance to leave here.
Thanks again.

Best wishes,

Cecil

 

[FredGarvin]

Wow. Thanks for nailing down the extreme difficulties in doing this. I knew things were different on that scale, but I didn't realize just how much.

Sorry to hear about your decision to leave the program. It must have been very frustrating.

Best of luck to you.

 

[Danger]

I knew that advanced education was difficult, but how on Earth can someone be assigned an impossible task and then flunk out for not achieving it? Something seems wrong about that.