Hi Veena,

I suppose you use the Wilhelmy-plate method using a roughened platinum-plate, correct? Other possibility is that you are using a Du Nouy-ring. My explanation should work for both.

First you should check whether the correct circumference of the plate or ring have been entered on the instrument. If others are using the same instrument with different probes, it may be that they changed this setting. Please note the circumference is two times the plate with AND two times the plate width. You may check this by measuring the surface tension of pure water. This should give 72.8 mN/m, if you find a much higher value, there is something wrong with the settings or the instrument is malfunctioning. If you measure 72.8 ± 1 mN/m, read on.

Second, if your sample is very viscous this may contribute to the high value you are measuring. You can check this, if your instrument allows this to be done, by drastically lowering the speed with which the plate or ring is pulled from the surface or interface.Alternatively you can dilute your sample with pure water in a few discrete steps, plot the data an next extrapolate to zero water addition.

I hope this helps, kind regards, Leo

Hello Prof Leo Vleugels,

Thank you so much for your valuable suggestion. I will try to do this and compare the results. 

Also I request you to share any published article for the same .

Regards,

Veena.

Hi Veena,

I found nothing on sewage sludge, but on oils. The link below refers to an article that shows the importance of withdrawing speed in contact-angle measurements using a Du Nouy ring. As you know contact angle can be related to surface tension by Young's equation assuming contact angle is zero (see article equation 4).

http://people.eri.ucsb.edu/~keller/papers/offline/Abstract72.pdf

Oh and I am not a Professor, I am a PhD-student, just started late in my career.

Kind regards, Leo