Sticky water?

[gene]

It is the rainy season in Singaopre now. This question crossed my mind as i was sitting on a bus. It was raining outside and i was looking out of the window. I realise that water droplets tends to stick on the glass surface. Instead of falling down and off the glass surface due to gravity, it did not. I find this really surprising. And after that, i read a book "The stuff of life" describing water and its properties and stuff. Two words caught my attention, "surface tension". I was wondering has it anything got to do with that.

 

Need someone to enlighten me.

Thanks.

[YT2095]

yes, it has everything to do with surface tension

[gene]

But what actually is surface tension. The book did not explain it. :x

[YT2095]

it`s the forces off attraction between the molecules in the water.

[swansont]

Water is a polar molecule, so there are attractive forces between them. On surfaces they will be asymmetrical (along the surface and inward, but not outward)

 

If the drops are small enough, the force is sufficient to make them stick to things, and also accounts for why drops tends to be small.

 

Detergent tends to destroy surface tension - you can float a razor blade or paper clip on water and then sink it with a drop of liquid dtergent. Also works with water strider bugs (or so I'm told

[YT2095]

LOL, Swansont yeah it does

 

the term is "Surfactant" as seen on many washing powder boxes, it`s job is to break this surface tension as Swansont said

 

there is a neat experiment to demonstrate this also! you`ll need to fill a bath with plain water (a few inches will do) then make yourself a little boat (anything that floats really, a bit of polystyrene will do), then at the back of the "boat" add a small shaving off a bar of soap and stick to the back (maybe make a split in it and push the shaving between it).

when the water is settled and still put your little boat in the water, it SHOULD move forwards as it breakes the surface tension and gets pulled along by the water molecules that haven`t been effected yet

[VendingMenace]

i would like to chime in, if i may...

 

First off, another great question gene! It is awsome to see someone obseve the world around them and then become interested in it. I would say that you have curiosity to become a great scientist

 

 

Now for the question...

 

As far as water dropplets on glass go (or really droplets of any liquid on anything goes), there are really two things that you must consider.

 

First there are the cohesive forces. These are the forces that people have been talking about so far. Cohesive forces are the forces that attract moelcules in a liquid to themselves (hence, if some one makes an argument that holds together, we say it is cohesive).

 

Secondly, there are the adhesive forces. These are the forces that attract molecules in a liquid to a solid surface. (Hence, something like tape, that sticks to something else, is called an adhesive).

 

 

Anyways, in order to understand why a drop of liquid, or a surfaceof a liquid, looks the way is does, we just need to consider these forces.

 

 

First off, we should consider the boundry between liquid and gas. That is, where the liquid ends and the gas begins. The gas (think "air") is not really a solid (hence gas, duh). Thus, there is not anything stable for the liquid to interact with. Thus, we pretty much ignore any adhesive forces bewteen the molecules of the liquid and those of the air. Becuase of this, we find that there are only cohesive forces when we consider this boundry. That means that there are only forces holding the molecules of the liquid together, not pulling them appart. (or as swansont put it, there are only forces "inwards" and not "outwards"). These forces will pull inwards constantly. Thus, the boundry between gas and liquid will assume a shape so as to be minimal. This is why a drop of a liquid will become spherical (a sphere has the smallest surface area to volume ratio). Also, this is exactly why we observe the property of surface tension. The surface of a liquid will resist any change that tries to increase its surface area. Again, this is due to the presence of coehsive forces and the essentially absence of adhesive forces.

 

Next, to consider is the boundry between a liquid and a solid. Here we can no longer ignore the adhesive forces (the attractive forces between the molecules in teh solid and the molecules in the liquid). Thus, we find that there are three possible states. The adhesive forces are either stronger than, equal to, or less than the cohesive forces.

 

If the adhesive forces are greater than the cohesive forces, we will find that a droplet of liquid will be flattened against the solid surface, as the molecule will try to maximize this interface. This is becuase there is now a dominate "outward" force).

 

If the adhesive force is less than the cohesive force, then the liquid will form a "bead" like shape on the solid. (think water on wax). This is becuase there is still a dominate "inward" force, so that the liquid will attempt to form a sphere. It is of course prevented form forming a perfect sphere, however, by the presense of some cohesive, or, "outward" forces.

 

If the adhesive and cohesive forces are balanced, we will find that the liquid dropplet will form the intermediate between a sphere and a flat shape. Wich makes sense.

 

 

Now, it just so happens that for water on glass the adhesive forces are greater than the cohesive forces. So, for an unaltered glass surface, water on it will look "flattened". That is to say water dorplets on glass will not really bulge out. (you can see this for yourself if you look at water on a glass stove, or if you look at the windows of your house when it rains).

 

Also, another cool effect of this is the "minescus" that we observe when water is in a glass tube, such as a graduated cylinder. That is to say, since the the adhesive forces to glass are stronger than the cohesive forces, the water will tend to "crawl" up the sides of the cylinder. Becuase of this, we will observe that the liquid surface looks carved out or bowl-like (if you have seen this, you know what i am talking about). Of course, no all liquids must have this minescus. Liquids that have a higher cohesive force than adhesive force to glass (such as mercury) will have an "inverted" miniscus. That is to say the liquid's surface will be domed out, or will form a bulge.

 

Well i hope that helps out some. This is just a cursory introduction to cohesive/adhesive forces. There are many many types of these forces, all of which have their varying causes and it is a cool thing to think about. But for the purposes of this post and your question, i think this is prolly good enough. If you want more information, just ask!

[wolfson]

Cohesive and adhesive forces in a liquid

Liquid spreads across a surface>>> liquid wets the surface. A drop of liquid wets the surface, or retains its spherical shape, depends on: Cohesive forces = intermolecular forces, like molecules. Adhesive forces = intermolecular forces between different types of molecules. When cohesive >>> adhesive retain shape adhesive >>> cohesive liquid spreads out. Water wets glass meniscus is concave. Bottom of meniscus is below water–glass contact line Adhesive forces >>> cohesive forces Mercury does not wet glass meniscus is convex Top of meniscus is above mercury–glass contact line Cohesive forces >>> adhesive. So basically when water wets a surface we see a “drop down”, but when Mercury wets a surface we see a “standing up” distinction. By the way some very good points Vending, are you a chemist???

[VendingMenace]

yuppers, i am a chemist. Or rather, i am studying to be on, i suppose.

 

What about you, what do you do to saty out of trouble?

[gene]

Vending. Thanks for the effort.

Well, after reading your post, i have quite some of an understanding. I got it best when you gave the minescus example. Yeah. i think i got it. Thanks a lot dude.

 

So, what exactly is the term use to describle an object's (let's say acid) adhensive force or cohensive force? Like how powerful or not powerful it is?

[gene]

You guys are really knowledgeble! How old are you guys? You people seem to know a lot. I mean, do you read about them or what???

[VendingMenace]

sup gene!

 

You guys are really knowledgeble! How old are you guys? You people seem to know a lot. I mean, do you read about them or what???

 

Well, perhaps i only seem really knowledgable. Its a funny thing, but the more you learn the more you realize that there is so much more to learn. But of course that is cool, as long as you enjoy learning

 

Personally, i am 24, pretty young, really, as far as the scientific world goes. And really, this shows in my understanding of chemistry. While i do know a bit of general knowledge, i am completely ingnorant still when it comes to what i research, but then again, i still have 4 more years to learn it

 

As far as how we know what we know. It is a combination of several factors. First, i have been in school for almost 20 years now, so you would expect i would konw something. Secondly, i have read some stuff, but then again, i have been alive for 24 years, so that is to be expected to. Thirdly (and prolly most important) i have not hesitated to ask questions. If i feel that someone might be able to answer a question that i have, i ask them, and i pay attention to their answer and try to understand what they are saying. Then i ask more questions, and so on and so on...

 

Of course you must always question what you are told and you must trust those that give you answers, but after a while (24 years) of asking questions you will be just as knowledgable (prolly more, really) as i appear to be.

 

In short, don't get discuraged or impressed by our knowledge. It is just a by product of our interets and the fact that we have been alive for a while If you keep up asking questions (like you are doing) and pay attention and keep a sense of wonder, you too will gain much knowledge.

 

But enough of that, onto your question (that is the whole point then isn't it?)

 

 

Well, i do not really profess to be a guy that knows alot about liquids, not do i study them extensively. As such i do not really know many of the techincal terms that are used to describe the strengths of adhesive vs cohesive forces.

 

There are many things, however, that i know are talked about that are reflections of cohesive and adhesive forces. i will give a brief run down of them here.

 

First off, as has already been mentioned, there is surface tension. This is a measurement of how much the liquid/gas interface does not wish to be distorted or how much this interface resists applied pressures. The phenomenon of surface tension is a result of the cohesive forces of the liquid and the almost non-existance cohesive forces with the surrounding gas. By measureing the amount of pressure required to break the surface of a liquid, we can measure its surface tension.

 

It turns out that boiling point can also be used to say something about the cohesive forces within a liquid. Liquids evaporate when the individual molecules gain enough energy to escape the reast of the liquid. In another thread, it was mentioned that temperature is really just a measure of the average kenetic energy of the molecules in the thing whose temperature is being measured. Thus, when the temperature of a substance increases, the average energy of the molecules in that substance increases as well. Remembering that cohesive forces are the forces between molecules in a liquid, we see that in order to boil, the molecules in the liquid must gain enough energy to overcome theses cohesive forces. Thus, by measuring the temperature at wich a solution boils (and hence the energy of the molecules required to boil) we can learn sometihng about the strength of the cohesive forces within the liquid (the forces that must be overcome if the molecules are to trasition from liquid to gas).

 

Another commonly used term is viscosity. Viscosity is a term that describes how well a liquid flows. Water is a liquid that flows quite well, these liquids are called "fluid". Molassas or maple syrup (or hoisin sauce) are examples of liquids that do not flow very well, these liquids are termed "viscous." It is not hard to imagine that a liquid in wich the forces between the atoms is strong and not very dynamic (cannot change very quicky) will be quite viscious, where as in a liquid that is fluid the attractive forces are either weak or highly dynamic. Thus, we can learn a bit about the cohesive forces from viscousity (though it is hard to tell the strength from this alone).

 

As far as adhesive forces go, as mentioned before contact angle can be used to determine this. At least qualitatively. That is, if you make a small drop of a liquid on a surface and then measure the contact angle with this surface, it will tell you something about the relative strength of cohesive vs. adhesive forces.

 

 

 

Anyways those are just a few terms that i can remember hearing that migh be able to be used to say something about cohesive vs adhesive forces. I am not quite sure how to make any quantitative statements, but these are at least some qualitative things that could be said/examined.

 

To further understand what might cause adhesive and cohesive forces, you might want to look into the so called "inter-moleculare" forces. For starters, check out london forces, dipole-dipole interactions, and hydrogen bonding. I would talk about them right now, but i have been writting for a while, and now i have a book (The name of the rose by Umberto Ecco) calling.

 

If you have trouble finding or understanding expliantions of these forces, then just ask and i will try to write up something on it. Perhaps i will even write it in word, so that you don't have to put up with all these mis-spelled words :/

 

Cool, well enjoy the learning! Lates.

[wolfson]

Well i have a Bsc (Hons.) chemistry, and i am currently reading Applied science with forensic investigation/Crime scene science. What year you in Vending?

[gene]

Oh so surface tension has a lot to do with the kinetic particle theory. So, now i understand. To confirm, surface tension is a combination of both adhesive and cohesive forces. Cohensive---> for the fact that Hydrogen and oxygen make up the water molecule. Adhesive ---> for the fact that the water molecule sticks to the glass surface. Anyway, does the glass surface have a charge? so that the attraction will occur? This two conbine to be known as surface tension. Right?

 

Oh ya, yesterday i read about surface tension in the alveoli, lungs too.

Oops. I'm kinda of confused after reading so much information.