Doesn’t the light itself transfer heat? Like with a magnifying glass and an ant, it amplifies and possibly focuses the light to burn the ant. Or even more simply, sunburns.
Light can make water evaporate without heat
171–180 of 198 posts
Re: Light can make water evaporate without heat
#172Earlier quoted context omitted.
> For H20 to move from liquid water to vapor, energy must be added. I think that is a wrong assumption. Liquids will naturally evaporate even with 0 external energy, assuming there is not too much pressure in the surrounding atmosphere.
The equilibrium partial pressure of water vapor with a source of water in another phase is not zero for most temperature and pressure regimes on earth, that is true. But thermodynamics still hold; the water vapor is still in a more energetic state than liquid water or ice.
Energy levels are one thing, but liquids are just a fundamentally unstable state of matter. Absent external pressures, they will either evaporate or freeze depending on temperature. Most likely, with very low atmospheric pressures such as in the vacuum of space, the outer parts will quickly vaporize, consuming energy from the inner parts which will then freeze.
Basically, some amount of a liquid will move to the higher energy state, and other parts will move to a lower energy state. The energy to vaporize some of the liquid doesn't need to come from something external to the liquid.
Re: Light can make water evaporate without heat
#173> The researchers found that the effect varied with color and peaked at a particular wavelength of green light. I wonder if this affects photosynthesis in any way? IIRC plants are green because chlorophyll does not absorb that color. Maybe that’s partially because green light induces too much evaporation?
No. Plants reflect green to balance luminosity extremes. Green is by far the most luminous color in the visible spectrum with dark purple being the least luminous. Plants that reflect purple absorb the most visible light and can produce the most sugars. Most plants must undergo a chemical transition to shift from production to dark mode and by choosing to reflect green maximum energy production is sacrificed for inte…
Whats the mechanism here?
Re: Light can make water evaporate without heat
#174Earlier quoted context omitted.
Yes, although the molecule clusters evaporate rapidly after they leave the surface
Even if they're in a high humidity environment where you could plausibly seed a cloud in the first place?
Thinking about this again, and I'm not sure. On one hand, yes, you could theoretically see a situation where the clusters want to grow because there are so many water molecules around them. But normally we seed clouds with much, much larger aerosols. Larger diameter = different (more favorable, I believe) surface energy.
Re: Light can make water evaporate without heat
#175I can't wrap my head around this story. What does it mean in thermodynamic terms? Isn't there a fixed amount of energy per mass that it takes to convert liquid water into vapor? Why does it matter that the energy comes from light?
Thermodynamics and evaporation are my day job and I think most other explanations here are missing the point. Evaporation normally occurs when individual water molecules have enough thermal energy to break their intermolecular bonds, leaving the bulk liquid and entering the air. In this case, they found strong evidence that water molecules were being removed in groups of several water molecules. Because intermolecula…
Thank you. This is the first thing I've read about this story that is at all coherent.
Re: Light can make water evaporate without heat
#176Re: Light can make water evaporate without heat
#177I dont see the breakthrough. All light causes evaporation, even at near-zero heat. Every light ray has the possibility of knocking off a molecule of water, transfering enough energy to turn it into gas. If the angles are just right, the water molecule vibrating just the right way, it gets lauched off the surface to become gas. Whether it stays gas depends on other things, but i dont see how observing the process is a…
'All light causes evaporation' is not really true. IR heating works because the wavelength resonates with the vibration modes of the intramolecular bonds. Water has low absorption of visible wavelengths, so you would not expect light absorption in the visible spectrum to provide enough energy to knock individual water molecules free.
Water is an interesting substance and a lot of the properties of water come from intermolecular hydrogen bonding and polarity. Even though it's a liquid, there are transient molecular structures that spontaneously occur in bulk due to hydrogen bonding between water molecules. The polarity of water causes interesting effect at interfaces (surface tension, electrical double layer, etc).
It's possible that the water in the hydrogel is forming some hydrogen bonded structure that interacts with green light. Where the individual water molecules won't strongly interact, the larger structure does. That could lead to the ejection of 'packets' of water molecules as discussed in the paper. Why and what these multi-molecular structures are? No idea. But this is a very interesting effect.
Re: Light can make water evaporate without heat
#178From the paper:
For the wavelength-dependent evaporation measurement, we used LED with different wavelengths. LED lamps were purchased from Chanzon with rated power of 100 W and different wavelengths: purple 390nm, blue 440nm, green 520nm, yellow 590nm, red 650nm, and IR 850nm.
Note that rated power is not the same as energy. E=hf. Since green had the most impact on evaporation, this phenomenon is not similar to the photoelectric effect.
Re: Light can make water evaporate without heat
#179> The researchers found that the effect varied with color and peaked at a particular wavelength of green light. I wonder if this affects photosynthesis in any way? IIRC plants are green because chlorophyll does not absorb that color. Maybe that’s partially because green light induces too much evaporation?
Maybe, but evaporation is critical for plants. They don't have a heart to pump blood and supply nutrients, they have to rely on evaporation to move nutrients from the roots up
Plants rely on on capillary action to bring water up: https://www.usgs.gov/special-topics/water-science-school/sci....
There's probably some aspect of evaporation, but the density of water is much, much higher when it's kept in liquid form.
Re: Light can make water evaporate without heat
#180Earlier quoted context omitted.
Maybe, but evaporation is critical for plants. They don't have a heart to pump blood and supply nutrients, they have to rely on evaporation to move nutrients from the roots up
Plants rely on on capillary action to bring water up: https://www.usgs.gov/special-topics/water-science-school/sci... . There's probably some aspect of evaporation, but the density of water is much, much higher when it's kept in liquid form.
That's part of the process, but not all of it. The overall process of water movement through vascular tissue is called transpiration [0]. Yes, capillary action plays a part, but the underlying process is due to differences in water potential. To avoid saturation (which would halt the flow of nutrients), water is released through pores in the leaves called stomata [1], where it evaporates, creating a differential that results in additional water being pulled in from the soil.
[0] transpiration: https://en.wikipedia.org/wiki/Transpiration
[1] stoma: https://en.wikipedia.org/wiki/Stoma
Disclaimer: I'm not a biologist. I'm a third-generation home gardener; my grandfather was the youngest of his family and the to leave the family farm in Kansas to go to college. He always maintained a large garden and loved sharing his knowledge along with his vegetables.
Edit: added links and a disclaimer