How water-droplet/dew stick to spider's web? What keeps them there?
[Physics] What makes water-droplet/dew stick to spider’s web and what keeps them there
water
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I'll give somewhat satisfying answer...
Heat - We know that Heat is a form of energy that is transferred by difference in temperature of regions. Here, it's just the amount of energy required by the $H_2O$ molecules in the liquid state to get converted to gaseous state. As we provide heat, it gets absorbed by water to get converted into water vapors.
Convection - It's the transfer of heat through water (proper term is fluid) caused by molecular motion. You see, Convection can be brought to focus by taking normal experiments into account. Particles (even Chalk) would rise and fall in somewhat rotatory motion due to this flow of heat. The water vapor formed at the bottom of the liquid (water) flows to the top in the form of united water vapor bubbles. 'Cause it's a gas and its so hot down there..! (This explains bubble formation when water is boiled.)
Now, why boiling point differ with altitude:
The atmospheric vapor pressure is 1 atm only for us (I mean, at sea level). For higher elevations, the pressure is too low out there in atmosphere. It should also be noticed that the volume change is too high when a liquid changes to gas.
By Ideal gas equation, For 'n' moles of gas $\implies$ $P=\frac{nRT}{V}$ which shows that pressure is inversely proportional to volume. So, we could relate this expansion with pressure. Expanding against low pressure takes somewhat lower energy than expanding against high pressure. And hence the boiling point would be low at higher altitudes. Wiki says it somewhat clearly with a formula.
Using this water boiling point calculator, Boiling point at sea level (1 atm or 29.92 inches of Hg) is approximately 212 °F whereas at a height of about 5000 ft. (24.9 inches of Hg), it is 203 °F.
How was this amount of zoom created by the water droplets on the concave lens of my glasses?
First, the typical magnification one gets from a drop of water is only a factor of ~4-5, not 10s to 100s. It is possible to construct micro-lenses from water droplets as discussed here https://www.nature.com/articles/ncomms14673, however that wasn't done to generate high magnification but rather to focus light from individual locations with a high spatial resolution.
...a single cell was zoomed in to the level where I could see the nucleus in the center, some squiggly organelles outside the nucleus and an irregular cell membrane around it.
Are you sure what you saw wasn't within or on the water droplet? There are lots of smallish bugs/animals in water and on your face/glasses that could be magnified by a water droplet. Since you state the source of the water was rain, it's more likely the small critter was already on your glasses (glasses can be rather nasty afterall).
There are lots of micro-animals like tardigrades, myxozoans, rotifers, nematodes, and loriciferans, many of which can look like a cell. I would guess this is a much more likely explanation than a single water droplet actually generating microscope-level magnification allowing you to see single cells, which are on the micron scale.
It is not possible to take a picture of this phenomenon. Probably the cells are in my eye.
This is unlikely. The amount of light that is reflected off of a water droplet vs transmitted is low (e.g., see https://en.wikipedia.org/wiki/Electromagnetic_absorption_by_water), which is why you can see through them.
It is possible you have some small organisms in your eye, but that the water droplet provides a magnified reflected image is unlikely. The most likely answer is the one I propose above, namely, that there are micro-animals on your glasses that are being magnified by the water.
The maximum angular resolution (e.g., see visual acuity) of the human eye is ~0.47 arc minutes or ~28 arc seconds. Typically ~1 arc minute is considered to be good vision. The smallest objects resolvable by the human eye are limited by the diffraction limit of the pupil and come in around the width of a typical human hair or ~55-75 micrometers (e.g., see https://en.wikipedia.org/wiki/Naked_eye).
So if we use the magnification of ~4-5 I mentioned above, something that is acutally ~0.055-0.075 mm in size would appear to be ~0.22-0.30 mm through an ideal water droplet. To put it another way, assuming the above and ideal conditions, the smallest resolvable objects through a water droplet would be ~0.011-0.015 mm.
With all that in mind, I note that the size range of single-celled organisms is rather large. There are bacteria that have cell diameters of less than a micron while some protozoans can be more than 100 microns.
Thus, in principle, it is possible that a large, single-celled organism was magnified and visible to your naked eye through the water droplet and your glass lense. However, I still maintain the more likely explanation is that there were some micro-animal-like critters on your glasses that were magnified by the water droplet.
Side Note
Depending on where you live in the world can also influence whether there is a significant probability of small micro organisms existing within the water drops from rain fall, i.e., they are inside the rain drops before they hit the ground. So I initially argue this is less likely above, it is not impossible for the rain water itself to contain micro organisms, e.g., see https://www.nature.com/articles/s41545-019-0030-5.
Best Answer
The water droplets on a spider's web are an example of dew. Cold air holds less water than warm air, so if the air has absorbed a lot of water during the day, as it cools overnight the water condenses on anything that can provide a nucleus.
There is an energy barrier to the formation of water droplets from water vapour, so the droplets generally form where there is something to act as a nucleus. This Wikipedia article describes the role of nuclei in formation of clouds, but the same principle applies to condensation onto a spider's web. There's nothing special about a spiders web: dew forms on just about anything. It's just prettier on a spider's web so we notice it more.
The reason you get droplets is due to a phenomenon called Rayleigh Instability. This causes a smooth film of water on a fibre to break up into droplets.
The droplets stick to the fibres of the web due to capillary forces.