# Capillary force when immersed in liquid

1. Aug 4, 2010

### sir_manning

Hi

I have a question about the capillary force between two hydrophilic surfaces. I am working with small cantilevers (5-50 um long, 5 um wide, 200 nm thick). If L represents the lever, which is hydrophilic, and X represents some bulk material (also hydrophilic), a profile of the lever looks like:

LLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLLL
XX
XX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX
XXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXXX

The levers are immersed in water and then dried slowly. My question is, when does the lever (L) begin to experience an attraction to the material (X)? Will the capillary force start when it is immersed, or only when there is a small amount of water between the lever and the material? Basically, I'm wondering if I can get around any capillary forces by keeping the cantilevers in water.

Thanks.

2. Aug 5, 2010

### Andy Resnick

I think you are confused about applying the capillary force (which occurs due to the presence of a three-phase line) and interfacial energy (which is the interface between two phases.

Immersing the entire thing in water means there is no three-phase line. Assuming we can ignore differences in the polarizibility between air and water (which could affect the interaction energy between 'L' and 'X'

The presence of a three-phase line means there is a force acting on the substrate- this is not an attraction to 'X', but a consequence of the different energy between a water/'L' interface and an air/'L' interface.

Or am I not understanding your question?

3. Aug 5, 2010

### sir_manning

So, a 3-phase line is needed for the capillary force to work, since (as you said) the force is a result of the different energy between the L/air and L/water interface.

If there are just two phases, there is still an interface energy, but since there is just one interface, there is no energy difference and therefore no force.

Is this a correct summary of the situation? Thanks for the reply.

4. Aug 5, 2010

### Andy Resnick

Don't forget the water/air interface at a three phase line, but basically, that is correct.