MadSci Network: Physics

Re: capillary action in space

Date: Sun Jan 14 10:08:50 2001
Posted By: Arjun Kakkar, Undergraduate, Mechanical Engineering, Tata AutoComp Systems
Area of science: Physics
ID: 979056316.Ph

capillary.txt.html If you put a tube into water in space, would the water go up the tube and why?

Answer: Yes, assuming the tube is of a "normal enough" material like glass and the tube is inserted in a spaceship, a case of presence of air but no gravity.

Why: There are some basic things to understand before we go ahead with understanding why this would happen.

Firstly, we need to understand a phenomenon called "WETTING" which is observed on the interface between solids and liquids. When a liquid is brought in contact with a solid, a curvature is observed in the liquid surface (meniscus) near the solid surface. This curvature is characterised with an angle called "ANGLE OF CONTACT" (call it ). One can easily observe it in a glass tumbler with some water in it. The water rises a bit upwards in this case - this is the case of an "acute angle of contact" (<90 deg) which is made by WETTING LIQIDS. An acute angle of contact indicates that a force in the direction of the open end of the tube is being applied to the water. This is the force that makes the water rise in a capillary (thin tube). On the other hand, if the angle of contact is obtuse (>90 deg), a downward force is applied to the liquid thereby pushing the liquid (a dense one, like mercury) down.

The origin of this phenomenon is related to SURFACE TENSION, and in turn on cohesive and adhesive forces. One can find details on this in any standard good physics book: Resnick and Halliday used to be one of my favorites.

Now, in this respect let us compare things in space vis-a-vis that at earth. There are two cases:

1. in a spaceship (no gravity, normal pressure). In this case since there is only an absence of gravity, the attractive force in the direction of the open end of the tube shall still exist. Thus the water will rise TILL THE TOP OF THE TUBE. At the top it shall slightly flatten out, id est will not have a speherical shape as expected.

2. outside the spaceship (no gravity, almost zero pressure). In this case, the surface tension of water shoots up. Since the surface tension of the solid-vacuum interface has almost the same value, I THINK that that the water will hardly rise. It would behave somewhat like a very cohesive liquid. Still I am a bit unsure about this. If you want a further expln. it is given in below. If you don't want to read too much mathematics right now, you needn't go further.

The pressure gradient introduced as a result of wetting is given by:

p2-p1=2*gamma/R (1)

where gamma is the surface tension of the interface and R is the radius of curvature of the spherical shape the liquid takes up in the capillary.

Now refer to the figure (FIG1) attached. Three gamma values are important at the solid-liquid-gas interface. These have been indicated in the figure. The equilibrium condition is:

gamma_sol_gas = gamma_sol_liq + gamma_liq_gas * cos(theta) (2)

This relation determines the value of theta. If gamma_liq_gas becomes very high, as in the present case (in space outside the spaceship), theta becomes close to 90 degrees. This means that R in equation 1 becomes very high, thus making the pressure differnce very small because of which the water hardly rises.

REFERENCES: Try R&H, as indicated above. Or try one of the Russian books on Physics they are rather good. For advanced readers, you can go to books on fluid statics and dynamics and look up the index for surface tension.

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