## Fluids

#### Microfluid surface tension

• Shabi1992
Subscriber
Hello.
I have a question about simulating microfluidics.
The water moves in a sudden contraction because of surface force between water and wall which is glass.
How Can I simulate such condition? I mean how can I create a surface tension between fluid which is water and the wall arround that with surface tension coefficient?
Thanks alot
• DrAmine
Ansys Employee

How many fluids do you have in your system?

• Shabi1992
Subscriber
I have a specific volume of water at the start of tube and at t=0 sec the water start to move continuously because of surface forces.
Thanks
• Karthik R

Hello,

Do you have more than one phase in your system? Are they immiscible? If so, you could use the volume of fluid (VOF) multiphase model to simulate this flow. Here are some examples you could use to model a capillary driven flow. Please let us know if this is not what you want.

CFD Fluent tutorial - Ink Jet drop formation | VOF model

Slug Flow CFD tutorial using Multiphase VOF model | Fluent tutorial

I hope this gets you started.

Thank you.

Best Regards,

Karthik

• Shabi1992
Subscriber
Actually I want to calculate the time that lasts which my fluid reach the end of the geometry.
I just have water in a glass tube and one geometry is sudden expansion and one geometry is sudden contraction.
The fliw moves continuesly due to surface force.
I do not know how simulate such cases ?
Thank you very much for your guidence.
Now can you please help me about how I can enter the kind of the solid wall which is glass and Affect on surface force to move water?
Thanks alot
• DrAmine
Ansys Employee

Does it mean that the whole tube is full with water at t= 0 ? If yes then you need to implement a momentum source term which would rely on the wall effects (expansion and contraction) ( I am explicitly not using the word " surface tension" as the latter is property of liquids in contact with other fluids and not alone). If the stress at the wall is caused by temperature gradients then you can go for the Marangoni Stress B.C. If you have your own theory then you can use UDF.

• Shabi1992
Subscriber
Actually at t=0 the specific section of the domain at the geometry is full of water and then we want to calculate the final time that this fluid move to reach the end.

We have not temperature as well.

So you mean the substance of the wall which is glass is not important for moving the fluid?

So you mean the other section of the geometry should not be vacume and I should assume air in the other part ? So we have water and air and by this way we have 2phases?
• Karthik R

Hello,

If I understand your problem right, you seem to have two fluids - water and air. As water moves further into the domain, air exits.

Is this accurate? If this is the case, you might want to use VOF to model this flow. Please have a look at the examples I shared. You can search for them on Youtube and you should be able to understand the detailed set-up using these videos.

If this is not accurate, you might want to provide a sketch so we are all on the same page.

Thank you.

Best Regards,

Karthik

• Shabi1992
Subscriber
Actually I do not have inlet or outlet in boundary confition. I have some water in a closed tube that moves continuously because of surface tension. The material of tube is glass as well and I do not know how to apply the material of the tube or surface tension coefficient in my geometry.
Thankd
• Karthik R

Hello,

Surface tension force is cohesive force which becomes dominant only at the surface of a fluid. The imbalance in the inter-molecular forces becomes dominant when you have two fluids at an interface (because of the difference in the forces of attraction between the respective fluids). You can read more about this using the following link

Is your glass tube open at either ends?

You really have to help us understand your problem clearly. Again, a schematic of the problem  would be very helpful.

About material properties of glass, you can open the material property assigner, select solid, and see if you can assign glass. As far as surface tension assignment is concerned, you have to select a multiphase model and surface tension selection can be found in the 'Phase interaction' panel. Please watch the two youtube videos I shared with you in my earlier post.

Thank you.

Best Regards,

Karthik

• Shabi1992
Subscriber
I will attach my geometry and boundary condition right now.
Thanks
• DrAmine
Ansys Employee

Once again is the whole case filled with water or do you have air in the parts where the water is not there at the beginning? If the tube if full of water then you want to track the momentum and this suction/depression due to contraction/expansion of at the glass walls need to be accounted for via UDF. If you have air or any other gas kremella has already talked about almost everything.

• Shabi1992
Subscriber
Thanks.
Imagine a tube with length of 100 and the water at t=0 is from x=0 to 30. So the start of the tube is full of water from 0 to 30 and in this part we just have water.
On the other hand I want to simulate and calculate the time which the water needs to achive the x=100 or at the end of the tube.
I just know that the water moves because of surface tension and I do not know from 30 to 100 (at first) is vacuum or full of air I should assume and I do not know that how.
• DrAmine
Ansys Employee

So you have multiphase case. You can switch on the VOF model define your phases: phase 1 will be air (or near vaccum that would depend on pressure) and phase-2 water. In the phase interaction panel you can then chose a surface tension interaction model (both are fine), give your surface tension coefficient and enable wall adhesion. Now at your walls you can give a contact angle (static or dynamic: dynamic only via UDF). After initialization with all water in the whole region, you can define a cell register for the region between 30 and 100. This region can be patched with volume fraction of water equal to zero.