Assume we have a very hydrophobic surface. My current understanding is that water on this surface will form distinct “beads”, as in this picture
My understanding (from googling) is that this occurs because the water is more attracted to itself than to the surface (& that the bead shape is from the water molecules on the surface not being attracted to molecules above them).
My question is: If the water is attracted to itself, why are the beads not joining up in one (or a few) large beads/sheets?
Perhaps it is because the distance between the beads? If so, does that mean that the following are true?
if two beads were moved close enough (touching?) they would join? And, in the extreme, if all the beads were made close enough (touching), they would form one giant bead (sheet)?
If water was poured continuously onto this surface on the same spot, the water would form a giant bead (sheet)? (assuming no splatter).
(I think this second bullet is equivalent to asking if the reason the linked image has many separate beads is from water landing making contact with the surface in different locations (as opposed to the same constant location)
Water isn’t “attracted” to itself in the sense of two droplets exerting a force on each other without contact. But if you put two droplets in contact with each other, yes, they will merge into one.
Assuming the surface on which a collection of water beads exists is fairly flat and level, yes, if you add more water to it, the beads will merge to form one very large bead. However, water pressure scales with depth, and surface tension doesn’t, so the giant water bead will not be of the same proportions as a small bead; it will collapse under its own weight until the pressure at the bottom is low enough that the surface tension at the edges can keep the droplet from spreading/collapsing further. The smaller the bead (and more hydrophobic the surface), the more spherical the bead will be; very large beads will be extremely oblate, more like a sheet with rounded edges.
Sorry, I was trying to say that the water molecules are attracted to eachother. But again, what little I know is from google so even that may not be correct
Sorry, just want to try to put this in my worldview to make sure I understand. Basically, you are saying that as water is added there is more water “on top” (of a given point/level), which means more pressure at that given point/level. When this is larger than surface tension it will flatten the giant water bead until water pressure & surface tension are in equilibrium.
I am by no means an expert, or even a novice, in the technicalities of fluid dynamics. But sometimes, when I’m bored, I do play with water droplets. That said, I’m fairly confident that the answers are: yes, yes, and yes. If you have a car, the carwash is a good way to see how a spray beads up on the windshield, but afterward, you can pour it gently to make large globs. A toothpick is a great way to drag beads toward each other/
Let’s compare a bead (I think “puddle” might be a better word) containing 25 ml of water, and another of 50 ml. In my imagination, both have a totally flat and level surface, until you look near the rounded edges. Further, I imagine that both are equally tall, so that the surface area of the large one would be exactly double that of the small one, if you ignore the area near the rounded edge.
The point I think is that a bead can only get so high, before the pressure at the bottom is greater than the surface tension. Thus, while smaller beads can exist, beyond a certain volume, the water pressure at the bottom is stronger than the hydrophobic property of the surface it is on - so the bead’s lower surface expands until this is again in balance. So beads can be so high, but no higher. Add more water to th bead, its area expands. expand it enough, you get a continuous sheet. The least imperfections in the surface levelness will encourage the large bead to separate - the bead is thinnest at that point, so more hydrophobic tendency and less pressure to stay flat and connected.
Basically the flat surface below is pushing the water away, and the pressure of the water above is pushing the water bead down, and surface tension is preventing the water from spreading further along its perimeter. A largish bead forms where these are in balance. smaller beads are a result of the surface tension predominating.
I had not thought about surface imperfections & thickness of a bead! Seems to me like this plays a part in why pouring water on a hydrophobic surface can create many small beads instead of a big one.
Thank you for the thorough explanation & this is why I love the Straight Dope message boards!
Now, what if we had many beads of soda water and beads of vinegar and started bringing them together? Or simply beads of different colored water? When they join would they mix or tend to form a large beard with different colored zones? If I had thought of this back when I was working with the fluid physics guys I could have gotten a grant.
a “bead” of water, in the sense of a sort of roundish body of water sitting on a hydrophobic surface, can vary in size. For example, condensation of moisture on a cold surface is not actually a continuous sheet of water; it starts off as a multitude of extremely small droplets, each of which gains mass/size as the condensation process continues. When two of these droplets grow large enough to make contact, they coalesce into a single larger droplet. If you have a horizontal surface, then iterations of this coalescence process can result in a collection of very large, oblate droplets. If you have a vertical surface, then the droplets only get so big before gravity compels them to run downward.
FWIW, describing a 25-ml bolus of water as a “bead” is probably not good terminology. That’s 1.5 cubic inches, for which “puddle” is probably a good word: as you describe, such a large body of water will have a large flattish surface with rounded edges, with a thickness of maybe a couple of millimeters. A puddle with 50 ml of water will likewise have rounded edges and a large flat middle area of the same thickness as the first puddle, albeit with about twice as much surface area.
Now imagine instead a bead of water that’s only 1 microliter, a cubic millimeter of water. This will be much closer to spherical, and barely half the height of your puddles’ 2-mm thickness. Smaller droplets will be even closer to spherical in shape.
I read in the literature some years back… watch that video, and notice that the center of the big drop has much less distortion than at the edges. Edges have height changing, funny angles, etc that bend light every which way. The center of the puddle has very flat bottom, very flat top, consistent over a large area, so all light is bent the same way. A lot of small drops means lots of edges, so hard to see through. Same water in a puddle or swimming pool is easy to see through.
Apparently some people are working to make extreme hydro philic surfaces, instead of phobic. With a phobic surface, the water easily clumps into beads that can easily move and merge. Decent results. A philic surface pulls so hard the water squishes, flattens, and spreads out really thin right away. Ideally water “instantly” spreads into a few-molecule thick layer over an entire windshield. Great results.
Re water, vinegar, olive oil, etc. Any non-uniformities (different colors, different salt content, w’ever) will spread out and mix, eventually giving a uniform solution. How fast it mixes? No idea. Talk to a specialist with a good computer.
