# Surface tension acting on DNA molecules

1. Dec 7, 2007

### Don Carnage

Hi.

We have done some experiments on stretching of DNA strands. We want to expand our report with some physical modelling but we are having some problems understanding surface tension. We have read some introductive book about fuild dynamics and used wiki alot but we still have problems calculating simple things :(

A simple thing we want to start calculation is how the surface tension act on a cylinder(DNA molecule) with radius 'r' when pulled up of a solution containing only water.. (see figure)
Or more precise we what to calculate the force which we then should be able to compare
with the elastic force of DNA.

Does someone have an idea of what to do.?

Thx.
Peter

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2. Dec 7, 2007

### Staff: Mentor

Here's a start: The vertical force on the tube due to surface tension depends on the surface tension of water (which is about 0.072 N/m at 25C; see: http://hyperphysics.phy-astr.gsu.edu/hbase/surten.html#c3"), the circumference of the tube (both inside and out, since both sides are in contact with the water), and the angle the water surface makes with the sides of the tube (which depends on what the tube is made out of).

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3. Dec 7, 2007

### Don Carnage

Ok so lets say the cylinder is 100% wet, then i must give something like;

F = y_(H2O,Air) * 2 * pi* R

or if the cylinder is hollow then;

F = 2 * y_(H2O,Air) * 2 * pi* R

Last edited by a moderator: Apr 23, 2017
4. Dec 7, 2007

### Staff: Mentor

Looks reasonable to me.

5. Dec 7, 2007

### Don Carnage

Ok; The radius of DNA is around 1.5nm and Y_H20 is around 72mN. This gives a force with the size of Ångstrøm. The problem is just that a force with sizes around pN can break a DNA strand, so this is a really bad estimate(Because the strands does not break, not even close to). Estimating that DNA is 100% wet is actually pretty close to the
truth (used in various articles/reports) since it is highly hydrophilic. My intuition tells me that it has something to do with the capillary effects, as seen in figure nr. 2.

What happens is that DNA binds to the wafer which has been dipped down into the DNA/Buffer solution and when we lift the sample (for stretching the DNA strands) the buffer-solution does as shown in the picture(the curved surface) - due to some capillary effects.

Can this effect really reduce the Force with 1e-3N or more? Would it be hard to implement
this effect? Does this effect have an influence on the force acting on the DNA strands at all?

Thx

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6. Dec 8, 2007

### Don Carnage

Ok I found a better reference and now i varies a couple of 100%. Its better but still there must be some pretty heavy effects which can chage this result that much..

Last edited by a moderator: Dec 8, 2007
7. Mar 9, 2011

### kapil.dev

Could you please share your finding with us?

rgrds,
Kapil