# Tag Info

15

My guess will be that it's mainly the effect of capillarity forces due to the porous quality of the stone surface. This might also be a good read.

12

The pressures at A and B are indeed equal. However, the pressure in the fluid immediately below the curved meniscus is equal to $p_{atm}-hdg$ as a result of surface tension. So the pressure at A is $$p_A=p_{atm}-hdg+hdg=p_{atm}=p_B$$That is, there is a discontinuous change in pressure across the meniscus as a result of the surface tension in combination ...

11

This effect is called capillarity and is not that straightforward. The contact between water and a solid surface is determined by the chemical bonds. It is macroscopically observed in the contact angle that the water/air surface makes with the solid surface. This angle depends on the strength of the bonds between the solid and the water molecules. You can ...

11

No, it won't overflow. That should be obvious since doing so would create a constant flow, constantly using energy, but without any energy input. Put another way, that would be a perpetual motion machine, one you could actually extract free power from. The same force that pulls the water along the inside of the capillary tube also holds it there when it ...

10

It's not only atmospheric pressure which is involved in water delivery to a tree, but mainly the capillary action and osmosis. Osmosis and Hydrostatic Pressure Roots take advantage of "pressures" when water and its solutions are unequal. The key to remember about osmosis is that water flows from the solution with the lower solute concentration (the ...

6

The formula for capillary rise that most people know is easily derived through a pressure balance between the capillary pressure and the hydrostatic pressure. The hydrostatic pressure equals $$\Delta P_h=\rho g h$$ whereas the capillary pressure is $$\Delta P_c=\frac{2\gamma}{R}=\frac{2\gamma \cos \theta}{r}$$ So balancing these we get our 'famous' equation:...

6

$h$ is the average height. The equation you wrote comes from balancing the forces on the liquid in the tube. The two forces are the downward force of gravity with the upward force of the liquid being attracted to the wall of the tube. The downward force of gravity is simply $mg$ where $m$ is the mass of the liquid in the tube and $g$ is the acceleration of ...

5

For the case that you have drawn, the behavior of the drop is actually the exact opposite of what you mention: it will move from right to left. This is caused by surface tension and the curvature of the droplet caps which creates a larger pressure in the drop at side B than at side A. To make it more quantitative. Let's assume that the funnel is ...

5

As you've indicated in your title, the correct question is "why doesn't this work". The system, as described would continue to produce energy indefinitely without any being added to it (perpetual motion, violation of conservation of energy...) . So you can be sure there's a problem. I believe that part of the trouble here is in the assumption that the ...

5

In the west the vast majority of towels are made from cotton, and cotton is basically cellulose. The surface of cellulose is fairly reactive (the bulk isn't unless you're a termite!) and will react with water to produce surface hydroxyl groups and negatively charged groups. Both of these lower the contact angle of water on the fibres and hence increase ...

5

No, this sadly will not work. You can of course get water up this way, but it won't disconnect from the glass tube. As capillary forces result from surface tension, to make the water fall back down, you will need to overcome this surface tension. This turns out to cancel the "won" energy. With a colleague I already discussed a more sophisticated way to try ...

5

The speaker actually claims it works by "interstitial suspension". I think he is referring to capillary action, which you are aware is involved here. He does not claim this method is faster, only that it is more environment-friendly. Shaking before wiping is a crucial factor. Folding has 2 advantages : it reduces excess towel at the edges which is not used, ...

5

The equation you have derived is essentially the Lucas-Washburn-Rideal equation which is given by $$\rho \pi R^2 \dfrac{d}{dt}\left( h \dfrac{dh}{dt} \right) = 2\pi R \sigma \cos(\theta) - \pi R^2\rho g h - 8\pi \mu h \dfrac{dh}{dt}$$ In your equations it seems you have assumed the contact angle to be zero ($\theta =0$) and your ...

5

$p_A$ is equal to $p_B$ here. The disparity is arising due to the fact that pressure just outside the meniscus is greater than the pressure inside. This is due to the curvature of the meniscus and surface tension. This difference is compensated by $hdg$ to make $p_A=p_B$.

4

The wicking is not occuring because of a siphon action. Rather, capillary action is responsible. The abrupt stop at the edge of the cup has two likely explanations; one -- all your solvent evaporated before capillary action wicked solute that far, or two -- The cup preserves a higher relative humidity within, which drops abruptly outside the cup, thus ...

4

It is explained very nicely by Olin Lathrop. The surface tension is an inter-facial force. The surface tension force pulls the liquid tangential to the wall of the capillary. When a capillary is dipped in water, it starts rising up due to pulling force from the solid-vapor interface. If the capillary has insufficient length, as the water rises it ...

4

It all depends on what you consider "noticeable". The change in height in a capillary comes about from the curvature of the liquid, and the resulting change in pressure. Simplifying for a moment, the curvature of the surface (for a small capillary) can be approximated to a section of a sphere (the real math is much harder... but the principles are easy to ...

4

Adhesive forces are accounted for when calculating capillary height. My guess is that you think they are not because you read, somewhere, a discussion in which adhesive forces were used to calculate a contact angle, then the contact angle was used to calculate the height. In that case, adhesive forces are being use the calculate the height. They are simply ...

4

I would explain it this way: Adhesion happens because of intermolecular forces of attraction. For e.g.., Water molecules of any aqueous solution contain 2 hydrogen and one Oxygen atoms that are covalently bonded. And in an aqueous medium they could get split into protons and hydroxyl ions. These ions are again bonded by Hydrogen bonding. A glass capillary ...

4

Even though capillary forces may be the dominant force in this situation, hydrostatic (gravitational body) forces are still there. And that results in the equilibrium seen in the left most figure as you start this experiment. So as you bend the tube over, the head (force due to height of the fluid) is reduced and that would allow the surface tension of the ...

4

Surface tension produces a pressure difference across the curved interface between the liquid and the air and it is this difference in pressure which results in the capillary rise. The pressure is atmospheric where the dots are black and less than atmospheric by an amount $\frac {2T}{R}$ where the dots are red. Here $T$ is the surface tension and $R$ is the ...

3

I suggest you build one in your kitchen. Cut a sponge into a J shape and hook it over a pencil, so that the straight edge dangles into a bowl of water. Put piece of tissue paper under the hook to catch drips and wrinkle, in case they come while you're asleep. Wait. I suspect you'll find that the top of the sponge never actually gets wet enough to drip. ...

3

Molten solder has a low contact angle on (clean) copper. So if you looked at a cross section of the pipe joint as the solder was flowing in you'd see something like: The solder is drawn into the joint in exactly the same way as water rises in a capillary tube. Both are correctly described as capillary action.

3

Your question consists of three parts so let's answer them one by one. 1) Surface tension between liquid-solid and air-solid why isn't all strange surface tension phenomenon seen at those surfaces? Just a short note upfront: I don't know what you mean by 'strange surface tension phenomenon' so I will explain in general how surface tension is also ...

3

Perhaps because oils from your fingers have interfered with the wicking, and that is where you held the string.

3

Surface tension is a phenomenon which occurs irrespective of whether a solid surface is in the vicinity. It is the result of the discontinuity in molecular attractive forces present at the free surface. The translates into a situation in which the surface behaves as if there is an elastic membrane embedded within the surface. The membrane force acts ...

3

Yes, the contact angle will change. When the length of the capillary tube is sufficient, we balance the hydrostatic and the capillary pressure to obtain the rise in height $h=\frac{2\gamma\cos\theta}{\rho g r}$ with $\gamma$ is the surface tension, $\theta$ is the contact angle, $\rho$ is the density, $g$ is the acceleration due to gravity and $r$ is the ...

3

The derivation can be thought of as this: Let, Radius of Capillary be $r$ Density of the liquid $\rho$ Height of the liquid be $h$ Surface Tension of Liquid be $T$ Contact angle $\theta$ Weight of liquid inside capillary = Volume * Density * $g$ $$=\pi r^2 h \rho g$$Which is the downward force, and the force that is balancing this is the force due to ...

3

I believe that the force that is keeping those water droplets stationery even when the comb is vertical is the cohesion and adhesion of water molecules as they are attracted to each other (molecules near one teeth to molecules near the other teeth of the comb to form a sort of bridge) and also the attractive forces of sticking to the walls of the teeth of ...

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