Surface Tension measurement
Surface Tension measurement
(OP)
How does one go about measuring Surfuce tension? I have a substance which I must check that the Surface tension is between 20-25 dynes/cm.
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Surface Tension measurement
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Surface Tension measurementSurface Tension measurement(OP)
How does one go about measuring Surfuce tension? I have a substance which I must check that the Surface tension is between 20-25 dynes/cm.
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RE: Surface Tension measurement
"If a liquid of density d rises a height h in a tube of internal radius r, the surface tension is:
T = rhdg/2
The surface tension will be in dynes/cm if r and h are in cm, d in g/cm3, and g in cm/s2."
Note that surface tension is dependent on the interface fluid (usually it's air, and assumed to be air unless specified otherwise), and there are some who believe it can depend on the tube wall material as well...
RE: Surface Tension measurement
m777182
RE: Surface Tension measurement
RE: Surface Tension measurement
Books say that a more accurate capillarity equation for the estimation of surface tension would have to bring into account the difference between the densities of the liquid and the vapor at the top of the column (ρl-ρv), and the fact that at the top of the liquid column the surface is not flat, meaning that the radius of curvature of the meniscus should be accounted for when measuring h.
The more accurate equation with highlighted corrections would then be:
While the typical capillarity equation generally in use:
where θ is the angle of wetting. With a "perfectly wetting liquid" this angle is zero (cos θ =1); most common liquids wet glass having the angle ~zero, and the formula becomes equal to that brought by trueblood.
For example: at 20oC, ethyl acetate (EA) considered a glass-wetting liquid, rises 4.12 cm in a capillary tube of radius 0.01294 cm. The density of EA at this temperature is 0.9005 g/cm3.
The estimated surface tension, applying the more "accurate" formula, assuming θ = 0, g = 9.81 m/s2, and ρair=0.00117 g/cm3:
The value, as estimated by the simplified equation, would be:
Which tells us that if the liquid in hand "wets" the capillary, trueblood's advice on the measuring procedure would be applicable and the results accurate enough.
RE: Surface Tension measurement
RE: Surface Tension measurement
For example, if a given capillary tube is cut down to half the height of the liquid rise, the rising liquid wouldn't overflow the shorter tube, but would stop at the rim forming a new angle of contact whose cosine would be proportional to h, in this case, 0.5.
If the original angle was zero with its cosine = 1, the new meniscus would be flattened to a contact angle whose cosine = 0.5, meaning the new θ = 60 degrees.
Thus, to avoid having to cope with measuring θ , when dealing with a glass-wetting liquid, it would be wise to use a sufficiently long (longer than the liquid rise) capillary tube.
RE: Surface Tension measurement
Janet