MadSci Network: Physics
Query:

Re: How does capillary action work in space?

Date: Mon Jan 15 01:03:35 2001
Posted By: Bradley Kelley, Grad student, Mechanical Engineering, Colorado State University
Area of science: Physics
ID: 978633392.Ph
Message:

Dear Pio,

I looked up capillarity in my trusty Physics book and found an equation 
describing capillary action.  (Physics for Scientists and Engineers, 2nd 
edition by Douglas Giancoli pg. 302).  The equation is as follows:

h=(2*y*cos(phi))/pgr

where:
h= height of capillary action in a tube
y= surface tension of the liquid
phi= angle of contact of liquid
p= density of liquid
g= gravitational constant
r= radius of capillary tube

Now for water contacting a glass tube, phi is essentially = 0, so the 
equation becomes:

h=(2y/pgr)

Now for your problem you don't really need the numbers for these, but I 
will give them anyway in an example from the book.  Say the xylem (nutrient 
carrying tube in a plant) has a diameter of .001 cm, how high can capillary 
action pull water at sea level?  At sea level g=9.81 m/s^2 and we will 
assume the xylem acts like glass with phi = 0.  Surface tension of water is 
given as y = .072 N/m.  Density of water is 1000 kg/m^3.  Making sure we 
have all of the units right, our equation becomes:

h = (2*.072N/m)/(1000kg/m^3 * 9.8m/s^2 * .00001m) = 1.5m is how high 
capillary action will pull the water.

Now you don't have to understand all of that, it is just an example in case 
someone who looks at this happens to need it.  No, in your case it is 
a case of mathematical limits.  From the second equation h=(2y/pgr), if 
everything stays the same except the gravity "g", then we get a limit 
equation.  As g approaches zero (as in space), that means h will approach 
infinity.  So, if we have the right set of circumstances, capillary action 
would work indefinitely given the right set up.

The right set of circumstances is important.  In space, there would be no 
reason for the water to head up a tube instead of having the surface 
tension bond with itself and float off as a blob.  So it isn't like you 
could just have an aquarium full of water on the Space Shuttle and stick a 
glass tube in it.  There would have to be some containment bag, kinda like 
those silver Mylar grape juice cartons that come in some lunches.  Also, 
you would still need air pressure since water will evaporate at zero 
pressure regardless of temperature.  Lastly, you couldn't get any air 
bubbles in the tube since the surface tension on one end of the column of 
water would counteract the effect on the other end.  However, if all those 
criteria are met, I can't see why capillary action wouldn't move in a tube 
for a very long distance in space.

Hope this explanation helps a little.  I looked to see if NASA had anything 
on this but was unsuccessful in finding any.  Maybe you can suggest it as 
an experiment!  Anyway, if you have any other questions, please contact Mad 
Scientist and they can forward it too me.  Best of luck!
BK



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