reported last year<\/a>, which described this new \u201cphotomolecular effect\u201d but only under very specialized conditions: on the surface of specially prepared hydrogels soaked with water. In the new study, the researchers demonstrate that the hydrogel is not necessary for the process; it occurs at any water surface exposed to light, whether it\u2019s a flat surface like a body of water or a curved surface like a droplet of cloud vapor.<\/p>\n<\/p>\nBecause the effect was so unexpected, the team worked to prove its existence with as many different lines of evidence as possible. In this study, they report 14 different kinds of tests and measurements they carried out to establish that water was indeed evaporating \u2014 that is, molecules of water were being knocked loose from the water\u2019s surface and wafted into the air \u2014 due to the light alone, not by heat, which was long assumed to be the only mechanism involved.<\/p>\n<\/p>\n
One key indicator, which showed up consistently in four different kinds of experiments under different conditions, was that as the water began to evaporate from a test container under visible light, the air temperature measured above the water\u2019s surface cooled down and then leveled off, showing that thermal energy was not the driving force behind the effect.<\/p>\n<\/p>\n
Other key indicators that showed up included the way the evaporation effect varied depending on the angle of the light, the exact color of the light, and its polarization. None of these varying characteristics should happen because at these wavelengths, water hardly absorbs light at all \u2014 and yet the researchers observed them.<\/p>\n<\/p>\n
The effect is strongest when light hits the water surface at an angle of 45 degrees. It is also strongest with a certain type of polarization, called transverse magnetic polarization. And it peaks in green light \u2014 which, oddly, is the color for which water is most transparent and thus interacts the least.<\/p>\n<\/p>\n
Chen and his co-researchers have proposed a physical mechanism that can explain the angle and polarization dependence of the effect, showing that the photons of light can impart a net force on water molecules at the water surface that is sufficient to knock them loose from the body of water. But they cannot yet account for the color dependence, which they say will require further study.<\/p>\n<\/p>\n
They have named this the photomolecular effect, by analogy with the photoelectric effect that was discovered by Heinrich Hertz in 1887 and finally explained by Albert Einstein in 1905. That effect was one of the first demonstrations that light also has particle characteristics, which had major implications in physics and led to a wide variety of applications, including LEDs. Just as the photoelectric effect liberates electrons from atoms in a material in response to being hit by a photon of light, the photomolecular effect shows that photons can liberate entire molecules from a liquid surface, the researchers say.<\/p>\n<\/p>\n
\u201cThe finding of evaporation caused by light instead of heat provides new disruptive knowledge of light-water interaction,\u201d says Xiulin Ruan, professor of mechanical engineering at Purdue University, who was not involved in the study. \u201cIt could help us gain new understanding of how sunlight interacts with cloud, fog, oceans, and other natural water bodies to affect weather and climate. It has significant potential practical applications such as high-performance water desalination driven by solar energy. This research is among the rare group of truly revolutionary discoveries which are not widely accepted by the community right away but take time, sometimes a long time, to be confirmed.\u201d<\/p>\n<\/p>\n
Solving a cloud conundrum<\/strong><\/p>\n<\/p>\nThe finding may solve an 80-year-old mystery in climate science. Measurements of how clouds absorb sunlight have often shown that they are absorbing more sunlight than conventional physics dictates possible. The additional evaporation caused by this effect could account for the longstanding discrepancy, which has been a subject of dispute since such measurements are difficult to make.<\/p>\n<\/p>\n
\u201cThose experiments are based on satellite data and flight data,\u201c Chen explains. \u201cThey fly an airplane on top of and below the clouds, and there are also data based on the ocean temperature and radiation balance. And they all conclude that there is more absorption by clouds than theory could calculate. However, due to the complexity of clouds and the difficulties of making such measurements, researchers have been debating whether such discrepancies are real or not. And what we discovered suggests that hey, there\u2019s another mechanism for cloud absorption, which was not accounted for, and this mechanism might explain the discrepancies.\u201d<\/p>\n<\/p>\n
Chen says he recently spoke about the phenomenon at an American Physical Society conference, and one physicist there who studies clouds and climate said they had never thought about this possibility, which could affect calculations of the complex effects of clouds on climate. The team conducted experiments using LEDs shining on an artificial cloud chamber, and they observed heating of the fog, which was not supposed to happen since water does not absorb in the visible spectrum. \u201cSuch heating can be explained based on the photomolecular effect more easily,\u201d he says.<\/p>\n<\/p>\n
Lv says that of the many lines of evidence, \u201cthe flat region in the air-side temperature distribution above hot water will be the easiest for people to reproduce.\u201d That temperature profile \u201cis a signature\u201d that demonstrates the effect clearly, he says.<\/p>\n<\/p>\n
Zhang adds: \u201cIt is quite hard to explain how this kind of flat temperature profile comes about without invoking some other mechanism\u201d beyond the accepted theories of thermal evaporation. \u201cIt ties together what a whole lot of people are reporting in their solar desalination devices,\u201d which again show evaporation rates that cannot be explained by the thermal input.<\/p>\n
The effect can be substantial. Under the optimum conditions of color, angle, and polarization, Lv says, \u201cthe evaporation rate is four times the thermal limit.\u201d<\/p>\n<\/p>\n
Already, since publication of the first paper, the team has been approached by companies that hope to harness the effect, Chen says, including for evaporating syrup and drying paper in a paper mill. The likeliest first applications will come in the areas of solar desalinization systems or other industrial drying processes, he says. \u201cDrying consumes 20 percent of all industrial energy usage,\u201d he points out.<\/p>\n<\/p>\n
Because the effect is so new and unexpected, Chen says, \u201cThis phenomenon should be very general, and our experiment is really just the beginning.\u201d The experiments needed to demonstrate and quantify the effect are very time-consuming. \u201cThere are many variables, from understanding water itself, to extending to other materials, other liquids and even solids,\u201d he says.<\/p>\n<\/p>\n
\u201cThe observations in the manuscript points to a new physical mechanism that foundationally alters our thinking on the kinetics of evaporation,\u201d says Shannon Yee, an associate professor of mechanical engineering at Georgia Tech, who was not associated with this work. He adds, \u201cWho would have thought that we are still learning about something as quotidian as water evaporating?\u201d<\/p>\n<\/p>\n
\u201cI think this work is very significant scientifically because it presents a new mechanism,\u201d says University of Alberta Distinguished Professor Janet A.W. Elliott, who also was not associated with this work. \u201cIt may also turn out to be practically important for technology and our understanding of nature, because evaporation of water is ubiquitous and the effect appears to deliver significantly higher evaporation rates than the known thermal mechanism. \u2026 \u00a0My overall impression is this work is outstanding. It appears to be carefully done with many precise experiments lending support for one another.\u201d<\/p>\n<\/p>\n
The work was partly supported by an MIT Bose Award. The authors are currently working on ways to make use of this effect for water desalination, in a project funded by the Abdul Latif Jameel Water and Food Systems Lab and the MIT-UMRP program.<\/p>\n<\/div>\n","protected":false},"excerpt":{"rendered":"
It\u2019s the most fundamental of processes \u2014 the evaporation of water from the surfaces of oceans and lakes, the burning off of fog in the morning sun, and the drying of briny ponds that leaves solid salt behind. Evaporation is all around us, and humans have been observing it and making use of it for […]<\/p>\n","protected":false},"author":2,"featured_media":0,"comment_status":"closed","ping_status":"closed","sticky":false,"template":"","format":"standard","meta":{"footnotes":""},"categories":[96,95,94,93,97,79,85,52,59,92],"tags":[],"_links":{"self":[{"href":"http:\/\/localhost:8888\/sawberries\/wp-json\/wp\/v2\/posts\/64"}],"collection":[{"href":"http:\/\/localhost:8888\/sawberries\/wp-json\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/localhost:8888\/sawberries\/wp-json\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/localhost:8888\/sawberries\/wp-json\/wp\/v2\/users\/2"}],"replies":[{"embeddable":true,"href":"http:\/\/localhost:8888\/sawberries\/wp-json\/wp\/v2\/comments?post=64"}],"version-history":[{"count":0,"href":"http:\/\/localhost:8888\/sawberries\/wp-json\/wp\/v2\/posts\/64\/revisions"}],"wp:attachment":[{"href":"http:\/\/localhost:8888\/sawberries\/wp-json\/wp\/v2\/media?parent=64"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/localhost:8888\/sawberries\/wp-json\/wp\/v2\/categories?post=64"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/localhost:8888\/sawberries\/wp-json\/wp\/v2\/tags?post=64"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}