×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Surface Tension measurement
2

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.

RE: Surface Tension measurement

Use a capillary tube.  From my old CRC handbook,

"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

A very simple method is to use a so called stalagmometer: basically it is a pipette with a sort of rounded outlet. For a known volume of a liquid you count the number of drops and then calculate the radius  and surface of a drop: it is proportional to the surface tension of your liquid/air system. It works even with two inmiscible liquids (water/oil,..). The method is somehow obsolete, you will loose your eyes counting droplets, but you may improve the method by employing some optical pulse counting device or similar. It is a good practice to make a comparative test with a substance with well documented surface tension data like water.
m777182

RE: Surface Tension measurement

This is to comment on trueblood's excellent advice.

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 (ρlv), 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:

σ = 0.5 (rg/cos θ)(ρlv)[h + (r cos θ)/3]

While the typical capillarity equation generally in use:

σ = 0.5 rgρlh/cos θ

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:

σ = 0.5(4.12+0.01294/3)(0.9005-0.00117)(981)(0.01294) ~ 23.54 dyne/cm

The value, as estimated by the simplified equation, would be:

σ = 0.5(4.12)(0.9005)(981)(0.01294) ~ 23.54 dyne/cm

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

Thanks for the ego boost 25362 :).  I mentioned the wall effect, which you have described in much more accurate detail.  Wish I'd had that knowledge lo these many years ago, when a colleague (a very cute one too) was trying to measure surface tension, and not getting very repeatable results, nor could she correlate her tests to "book" values.  Turned out she had drawn her own glass capillary tubes, by using a wire as a mandrel.  She had coated the wire with something (silicone?) that affected how the liquid wetted the surface.  Using a cleanly drawn tube worked, as did pre-wetting the tube with fluid.  But it took us awhile to figure it out...hmmm, maybe being ignorant in that case was not so bad...

RE: Surface Tension measurement

BTW, the angle of contact θ, in σ = 0.5 rgρlh/cos θ  is an important factor.

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

(OP)
Thanks very much everyone,
Janet

Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources