[bashir2008 (OP)]

Hello everybody
This is my first topic here and i would like to share knowldege with all of you
my question is about surface tension
if we do have a film ( two different plastic film ) and then we put a drop of water on the first one and another drop on the second one , the water drop might do one of these:
1- the drop will stay as a drop on the film
2- the drop will spread on the film

1- Why the drop stay as a spherical drop in first part of the question and why it spreads in the second part ?
2-Could you explain to me what is the realation of the surface tension in the two questions ?
3-is the surface tension a property for the water or even for the solid (plastic film) ?

I am looking forward to hear from you

Regards

[RD]

The contact angle of the drop is dependent on the properties of the liquid, the solid (flat) surface, and the gas (vapour) surrounding them ... 

 [ Invalid Attachment ]

http://en.wikipedia.org/wiki/Contact_angle

[lightarrow]

Hello everybody
This is my first topic here and i would like to share knowldege with all of you
my question is about surface tension
if we do have a film ( two different plastic film ) and then we put a drop of water on the first one and another drop on the second one , the water drop might do one of these:
1- the drop will stay as a drop on the film
2- the drop will spread on the film

1- Why the drop stay as a spherical drop in first part of the question and why it spreads in the second part ?
2-Could you explain to me what is the realation of the surface tension in the two questions ?
3-is the surface tension a property for the water or even for the solid (plastic film) ?

I am looking forward to hear from you

Regards

Simplyfying, in the second case you say that the water "wets" the surface, in the first that it doesn't. Example: water on clean paper wets it, if you make the surface dirt with oil, it doesn't anylonger. This because when there are attractive interactions between the molecule of the liquid and those of the solid surface, the liquid wets the surface; when there aren't, it doesn't.

[bashir2008 (OP)]

Thanks for replying but it is still not very clear for me
1- if the drop wets the surface: does that mean the surface tension of the liquid or of the solid is bigger ? or is it equal ?
2- if the drop does not wet the surface: does that mean the surface tension of the water is bigger than the solid ?
Please try to make it easier and clear

Thank you very much

[lightarrow]

Thanks for replying but it is still not very clear for me
1- if the drop wets the surface: does that mean the surface tension of the liquid or of the solid is bigger ? or is it equal ?
2- if the drop does not wet the surface: does that mean the surface tension of the water is bigger than the solid ?
Please try to make it easier and clear

Thank you very much

What changes in the two cases is just the contact angle (see the good draw made by RD). That angle is less than 90° if the liquid wets the surface and greater than 90° if it doesn't. The liquid's surface tension doesn't change a bit if you use different surfaces but with the same liquid. Of course, the resultant forces *do* change, instead (force is a vector).

[bashir2008 (OP)]

Ok:: i just want to make sure that i understand what you are trying to say here
1- the onely thing might change is the contact angle between the drop and the surface.. but how we explain that when someone says he needs to measure the surface tension of his liquid sample with different solids.. what we conclude when the drop wets and when it doesnot wet ? you might say the molecule interaction does it..

2- How we can calculate the surface tension if we do have the contact angle ?

Thanks a lot

[lightarrow]

Ok:: i just want to make sure that i understand what you are trying to say here
1- the onely thing might change is the contact angle between the drop and the surface.. but how we explain that when someone says he needs to measure the surface tension of his liquid sample with different solids..

It doesn't seem correct to me, to say that.

2- How we can calculate the surface tension if we do have the contact angle ?

To measure the surface tension you don't need the contact angle, because you can put the liquid in a rectangular metallic frame with one side long L moving freely, and measure the force F you need to keep it in equilibrium. Surface tension is F/2L. The 2 because you have 2 layers in this case.
Another way to measure surface tension is capillarity: the liquid goes up a glass (for example) thin pipe. You measure the contact angle and the height of the liquid and then you equate the weight of the liquid column to the vertical component of the surface tension, knowing the pipe's internal circumference.

[bashir2008 (OP)]

Dear lightarrow
i have some more questions:
1- if i do have different solid materials and need to make them as films and then see if the liquid (water drop) wets the surface of not, how would that be ? in other words :it is an idication to see if the surface is hydrophilic or not..
2- Do you think this is an indication for the surface tension or not ? if yes: could you explain it please ?

Best regards

[lightarrow]

Dear lightarrow
i have some more questions:
1- if i do have different solid materials and need to make them as films and then see if the liquid (water drop) wets the surface of not, how would that be ? in other words :it is an idication to see if the surface is hydrophilic or not..

Yes, I'd say that if the water wets the surface, this is an indication of the surface' hydrophilic nature.

2- Do you think this is an indication for the surface tension or not ? if yes: could you explain it please ?

If you always use the same liquid (for example water) then it's NOT an indication for the surface tension. If, instead, you would use different liquids and the same solid surface, you would notice different shapes for the drops, which is due to the different hydrophilicity AND to the different surface tensions and different densities. If you could have two different liquids with the same density and to which the solid surface has the same hydrophilicity, then the different surface tensions would be apparent by the different drops shapes: the one with less surface tension would be more flattened by its own weight.
Hope to have helped you...