Taking the example of the Wilhelmy plate, what do you mean by 'the area of free liquid surface' ? the one in the meniscus?

These instruments are calibrated in certain conditions, and the measurement is done by measuring a force. If they measure smaller surface tension it is because the pulling down force is lower than expected, which could be due to the fact that the menisci the expected ones.

I mean the horizontal cross sectional area of the vessel with the fluid.

is this reduction enough to change the shape of the menisci attached to the Wilhelmy plate? If so, you have the answer

Let it be so. I don't know. But why do the menisci reduce?

Because there will form another one in the other static surface, the one of the vessel.

Have a look at the ugly sketch in attachment

Thank you very much for answers and attachment!

But in my case distance between Wilhelmy plate and wall of vessel much more than menisci. Menisci is not more than 5 mm, but distance is not less than 5 cm, for example.

Perhaps a stupid question, but did you ensure to have identically pure liquids in each of your measurements? As you surely know, the surface tension of water is greatly influenced by additives or impurities. Also, could you please provide a text book reference that you referred to? I cannot imagine that the menisci plays a role in your case.

Regards,

Stephan

You may get confinement effects of boundary usually for sizes in micrometer range. But 400 cm2 is very big to expect this. I suggest you to find surface tension for different free areas and plot surface tension as a function of free area to see how it approaches saturation at 73 mN/m.

regards

Govind

Stephan, I can not be sure that surface has no impurities. But liquid is the same for all areas. There is no Langmiur effect. I am sure. In last experiment l was increasing the area of surface. Books I try to tell later.

Dear A. S., a agree with you in plotting a graph, but I think it was made many times in the past by other researchers.

Try to measure the surface tension by different method, for instance pendant drop technique, if you have the possibility, and then compare the results. The obtained result should be the same for the same measured solution, in the same conditions (purity of the water, temperature and so on).

Vasile, thanks, but I do not need to use all methods. I am interested only with methods with big area of luquid surface. Using different methods it is not an answer.

Surface tension is supposed to be an intensive property, independent on size. Therefore your results are strange. Could you please elaborate on the liquids tested ? Are they pure substances or solutions of surfactants, or even blends of surfactants ?

For a pure substance, I would suggest checking for impurities indeed. For surfactant solutions, you may face the 'Langmuir-Blodgett trough' effect: a Langmuir-Blodgett trough is a device allowing to reduce surface exposed to air of a surfactant solution, in order to study the evolution of surface tension with available surface—surface tension decreases with the amount of molecules adsorbed per m².

In your case, depending on the availabkle surface and surfactant concentration, you may get different results according to surface area.

https://en.wikipedia.org/wiki/Langmuir%E2%80%93Blodgett_trough

I just noticed that my answer would explain a phenomenon that goes the other way around from your observations...

Check for particles/substances precipitating at the surface near the blade ?

Pascal, but if I increase the liquid surfase, may the Langmuir effect appear?

This observation was made during films of Langmuir investigation. Thanks. I know about this effect.

 Experimental results.

Pascal, you was true. It is really Langmuir effect.