I need to explain capillary attraction in a seriously non-technical way – an interesting challenge since I’m not at all sure I understand it myself.
Is the phenomenon related to, or at least similar to, surface tension?
AFAIK, it’s 100% due to surface tension. Liquids with an attraction to the capillary “crawl” up the tube, while liquids that are repelled don’t. The height that the liquid moves up the tube is related to the “minimum energy” final state.
It just seemed so darned unlikely that my intuitive understanding would have anything whatsoever to do with reality!
Celery and food coloring! Takes me back to 1st grade.
I think the term is “capillary action”.
Liquids stay together and don’t evaporate because their molecules are cohesive, liking to stick to each other.
They do this preferentially at a surface where there’s a gas on the other side, because they don’t really have anything to cling to on the gas side. Thus the surface is under some tension. It’s tight. This tension in the surface is called surface tension, or more properly interfacial tension. For water it’s about 75 millinewtons per meter, for ethanol I think about 20.
Liquids that contact a solid that is similar to them - for example, oil sitting on a wax surface - have little tension at the interface between the liquid and the solid, but still have tension in the liquid gas interface. There is, at least in principle, a tension in the solid gas interface too. You can think of the point where the solid and liquid and gas all meet as having sufficient tension in the solid gas interface direction to pull it that way, meaning the oil will spread over the wax. The angle the liquid makes there is called the contact angle, and if it is less than 90 degrees the liquid will tend to spread. We say that liquid is good at wetting that solid, or we say that the liquid has a tension lower than the critical surface tension for that solid.
If the liquid is in a tube of the solid, and the contact angle is small, and it’s good at wetting that solid, then unless something else holds it out, the liquid will creep into the tube, the same way water creeps into cotton or a sponge.
Male capillaries are only interested in female capillaries with big breasts. Female capillaries are only interested in male capillaries with flashy cars.
Here’s the Google page for capillary action. It has articles that run from childishly simple to scientificly in depth.
That’s wrong.
Water is attracted to glass. Water sticks to glass. Simple as that.
Where surface tension comes in is that water is also attracted to itself. This actually counteracts capillary action. Since water would rather be attracted to itself than stick to the glass, then it’ll bunch up in a droplet. In small tube, however, there is so much glass (surface area) for water to be attracted to, that self-attraction is overpowered. Some other liquids that don’t have so much cohesion (self attration) will crawl up the sides of flat glass (no small tubes needed).
This is what the first paragraph of Wikipedia means when it says:
Okay, I may be asking the wrong question, then. ('Twouldn’t be the first time – nor the last, I’m sure.)
What I am looking for is an explanation of why, in dirt, water drains out of big pores between particles (because of gravity), but stays put in smaller pores. Is that due to surface tension or capillary action (or, as the person I’m trying to translate put it, capillary attraction)?
>Water is attracted to glass. Water sticks to glass. Simple as that.
Sorry, Alex, but it isn’t as simple as that. You’re missing the point.
Water is attracted to candle wax, too. Hold a candle overhead, horizontally. With your other hand, wet your fingers and flick them at the underside of the candle. Little droplets that hit the candle will stick to it. Go try it if you want, and come back.
Liquids typically are attracted to any solid surface they touch. Liquids whose molecules look more similar to the solid, at least at the surface, have a greater affinity and will try harder to flatten themselves against the surface to maximize the contact area. But they will all do it at least a little bit.
The point here is what the angles between the solid-liquid, solid-gas and liquid-gas interfaces do. If the solid is flat, the solid-liquid and solid-gas interfaces meet at a 180 degree angle (on the solid side). The liquid-gas interface meets this surface at some angle. If the angle does not balance out all the interfacial tensions, the point of contact of the three will slide along the solid surface until it does.
If the angle between the solid-liquid and liquid-gas interfaces is less than 90 degrees, the liquid likes wetting that solid well enough that it will try to cover the whole thing, if it can do so without making the angle too tight.
In a small hole, a capillary, it only takes a little bit of liquid to let the column creep along the entire length of the hole, even if it has to fight gravity to do so.
You can even calculate the pressure the column of liquid can attain in trying to fill the hole, from the cosine of the angle, the circumference of the hole, and the differences between all the interfacial tensions.
The solution is to picture a jar of jam. If you have a small jar and turn it over, nothing falls out. The jam sticks to the jar and it sticks to itself. But if you get a really big jar and turn it over, the weight of the jam will be too much. The jam will still stick to the jar, but its self-stickiness (its cohesion) will be too weak. Most of the jam will fall out, but a layer will stick to the inside and you’ll have a dirty jar (or a wet dirt pore).
Surface tension is caused by self-stickiness, so it’s a related idea. Water forms droplets a lot like jam forms chunks. Capillary action can’t be directly explained by jam, unless you imagine the jam vibrating a lot. It’s the random motion of water molecules that cause them to travel up against gravity, but it’s those two stickiness factors that decide if they stay there.
P.S. angles aren’t an explanation, they’re just a neat way of calculating.
Thanks, Alex, that was helpful, in a delightfully non-technical sort of way.
How are you going to explain of the the jam in the garden?
Ant trap.
>P.S. angles aren’t an explanation, they’re just a neat way of calculating.
Alex, the OP may or may not find angles helpful in explaining the way the forces come together to move the point of contact into or out of a capillary, but the angles, or the forces that define them, are very much at the center of determining whether a liquid draws itself into a capillary. The fact that the liquid sticks to the solid isn’t. Whether the liquid sticks to the solid does not predict whether it will also draw itself into the capillary, whereas the contact angle does predict it, and the relation between the three interfacial tensions does predict it.
I think your explanation “Water is attracted to glass. Water sticks to glass. Simple as that.” is incorrect, because liquids that stick to solids may or may not draw themselves into capillaries in those solids. It is simple, granted, but better questioners should stay confused than assimilate an incorrect answer.
>It’s the random motion of water molecules that cause them to travel up against gravity…
OK, now, whatever are you suggesting here? I’m baffled! You must mean the Brownian motion of the molecules, if you are referring to it as “random”, right? But Brownian or any other random motion is not going to cause a net travel up against gravity. It is not going to cause any net motion. If it did, it would not be random.
The Wikipedia article that beowulff so kindly linked looks generally correct, and the idea of energy states as a means of understanding it looks particularly applicable to the OP. Could you please explain whatever you mean by calling it wrong and suggesting random motion is at work instead? Which of the statements the article makes are incorrect? Can you substitute correct ones instead, with some explanation?
So the understanding I thought I had achieved from what Alex said is not correct?
Sigh.
twickster, I don’t know exactly what sense you made of Alex’s postings, but the most informative and correct information in play here is the Wikipedia article.
beowulff invokes surface tension, and that’s pretty relevant. (S)He’s vague when he refers to “an attraction to the capillary”, but if that means the ability to stick to the solid the capillary walls are made of, then that’s not quite correct because liquids will stick to a great many things that they won’t draw themselves into. There are degrees of stickiness, and several posts including Alex’s are generally correct to say capillary action depends on this stickiness.
The important thing to think about is the way three different interfaces behave in this system. There’s the surface where the liquid and the gas meet, the interface where the solid and the liquid meet, and the interface where the solid and the gas meet. All of these interfaces have tension in them, or, in other words, there is energy bound up in each of the interfaces. All the interfaces want to minimize their area to minimize that energy, just like a tight rubber band wants to minimize its length by retracting to minimize the energy bound up in its tension. That’s why liquid surfaces are smooth and shiny even if the liquid isn’t horizontal (think of wet paint on a wall).
Liquids draw themselves into a capillary if doing so lowers the energy of the system. In other words, if the tension in the solid gas interface is greater than the tensions in the solid liquid interface and liquid gas interface put together. More precisely you have to consider the angle at which things meet, but the idea is there.
People talk about the “contact angle” of such a system, and say that if the contact angle between the solid liquid interface and the liquid gas interface is less than 90 degrees, the liquid would like to spread over the solid. In that case, it would draw itself into a capillary.
In fact, people measure the contact angle at the edge of a droplet to determine this. They use an optical angle measurement microscope called a “goniometer”. You can buy sets of oils or other liquids with graduated values of surface tension (meaning gas liquid interfacial tension). A further detail is that when the drop is getting bigger, the contact angle is usually larger than when the drop is shrinking. People then talk about advancing contact angle and receeding contact angle.
Soaps and detergents are specifically active at these interfaces. You may have heard them called “surfactants”, which is a contraction of “surface active agents”. The big reason to use soaps in cleaning is that they lower the gas liquid interfacial tension and let the liquid penetrate the bond between a dirt particle and the solid it is sitting on, so that it comes off.
Look over the Wikipedia article and see where you get with it. Good luck!