Why does water break its flow profile to bond to a solid surface? Have you ever noticed how there is an attraction with running water and a solid object somewhat in its flow profile? For example, if you put your toothbrush under a running faucet and begin to pull the toothbrush away, the water is attracted to the toothbrush and flow down the stem vs. flowing straight down with the pull of gravity. What causes this attraction? Covalent bonding? Surface tension?
Well, the property is called adhesion, but that’s really just putting a name to the ignorance. Fluid mechanics is a bit outside my expertise, so I’ll leave this one to one of the other physicists we have onboard.
Meanwhile, is this related to any Staff Report posted on this web site? If so, could you please post a link to the column in question, so we’re all on the same page? If it’s not, then this message was put in the wrong forum, and if you let myself, C K Dexter Haven, or any of the other moderators know, we’ll be glad to move it over to the General Questions forum for you.
Oh, yeah, and welcome to the SDMB!
Moving this to GQ, at the e-mailed request of the OP
At this risk of getting this moved to CCC <evil laugh> it seems to me that this column (and its discontents) Why does the shower curtain blow up and in instead of down and out? is somewhat related to the question at hand.
damn. apparently my undergraduate degree in fluid mechanics didn’t prepare me for this question. (caveat - I switched majors in grad school & have done little in the way of fluid mechanics over the past few years)
but rather than posting a totally useless message, let me speculate for a moment…
The water leaving the faucet approximates laminar (smooth) flow. When the flow strikes the toothbrush, the laminar flow lines are disrupted into turbulence. Turbulent flow goes every which way and therefore interacts more with the surface of the solid (friction, absortion, etc.). Of course, the bristles of the toothbrush make for a nice high-friction/increased flow path environment. The result? The smooth flow from the faucet gets caught up in the obstructing toothbrush and some of the flow is diverted. In the case of the toothbrush, I don’t think there’s any particular bond other than water’s affinity to wet a surface (in large quantities, water acts as a lubricant but in small quantities it can act as an adherent).
now let’s see if I put my foot in my mouth…
The way I see it, there is a weak attractive force between molecules that has nothing to do with chemical bonding of molecules. Its called van der Waals force. The water molecules immediately adjacent to the toothbrush are attached in this manner and experience 0 velocity. Water molecules further away are more influenced by gravity and pick up speed. This force is still acting between water molecules, however the vertical component is overcome by gravity . That is why the angle of the toothbrush retaining full flow is inversely proportional to the volume of the flow.
[wavey lines]
ah yes, the “boundary layer”…it’s all coming back to me now
[/wavey lines]
Part of the answer is the heat factor.
Water will not break on hot hard surfaces, like a griddle, but will bead up.
I don’t think it’s a fluid mechanics question. The force direction of water leaving the faucet and the force of gravity are both vertical. If this truely is only a fluid mechanics problem, then there would need to be some horizontal force to push the fluid to the toothbrush as the angle of the toothbrush increases. Even though the boundary layer does get larger at the point of contact with the toothbrush and the running water, the water will continue to run down the toothbrush to the end of the brush that is outside the boundary layer of running water. There is some attraction or adhesion that causes the fluid to stick to the toothbrush to overcome the pull of gravity. I think it’s a question of hydrogen attraction to a solid surface. The van der Waals forces is probably the answer, however if not mistaken, this is a description of many intermolecular forces and is to vague to explain the reason for the true attraction. (i.e. Hydrogen bonding? Surface tension?) Is temperature truley a factor? Does is matter what the fluid is or what the solid material is made out of for this attraction?
Grienspace-
I don’t understand the assumption that the angle of the toothbrush is inversely proportional to the volume of the flow. Do you have an equation that I could review and/or more information to back up the van der Waals force theory?
This is a little off topic, but there’s a trick in all the junior science books.
In the winter, when static electricity is high, run a comb through your hair and put it near but not touching a thin stream of water from the faucet.
The stream will bend to meet the comb.
A great crowd pleaser.
This is something that has puzzled me for awhile. I thought I would post it again to see if anyone would want to add their comments or review the question again. I’m not convinced we have a gotten a correct or full answer.
That’s the Liedenfrost effect. The water drop is supported by the thin layer of water vapor produced when some of it turns to steam.
As far as the comb trick, my guess is that water being polar by nature, is simply being attracted by the static electrical charge on the comb.
And other objects, uncharged as they may be, still have a polar nature of their own, which is enough to make water stick. Take glass for instance. Sneeze on a window, and the spray stays there. It sticks due to Van der Waals forces, hydrogen bonding or whatever. “Weak Forces” let’s call it. Spit on a Teflon frying pan, and you can pour the spit out with nary a trace. Teflon doesn’t lend itself to these “weak forces” of attraction.
So, the object in the path of the water allows water to stick to it, thereby slowing down the boundary layer of flow, and thus slowing down subsequent layers enough to bend the flow.
honkeytonkwilly- thats more or less as I remember the explanation to this effect from high school. Water is polar, and thus is attracted to other polar substances (the details of which might be as has been posted here, I don’t know). I also don’t remember specifically, but similarly to the comb, there are some substances which will repell the water, and the stream will move away from the object.
I seem to remember something about glass rods (attracting) and granite, perhaps, repelling?
Thanks for the post guys. I will look further into the Liedenfrost effect. This is something that I have not heard of before or have forgotten. Although I am convinced the reason the water sticks to the toothbrush is do to some Van der Waals force, I would like to learn more about this attraction.
However,some things that have been posted don’t make physical since. For instance, this attraction can’t be do to water vapor turning to steam, because the running water in this example is at ambient conditions. Water doesn’t change it’s state until temperatures are above 212 deg F.
Outside of giving this attraction a title (Van der Waals force, weak forces, or adhesion), what is really happening?
Does some of the water break the boundary layer and flow down the toothbrush, or does the boundary layer change do to the obstruction?
My answer is the first. I think this attraction is great enough to break the boundary layer.
Also, everyone knows what water is made of, but what about the toothbrush. It is some kind of polymer, but what part of the compound is attracting the water?