New Photomolecular Effect Solved Water’s BIGGEST Mystery!

Water evaporation has always been a straightforward concept in science—heat from the sun hits a body of water, molecules gain energy, and eventually, they escape as vapor. But what if we told you that this isn’t the full story? What if light alone, without heat, could cause water to evaporate? Researchers at MIT discovered a groundbreaking phenomenon they’re calling the photomolecular effect^[1,2].

This post explains what the photomolecular effect is, how it works, and how it challenges our conventional understanding of evaporation. It also explores potential applications of the phenomenon, like light-powered desalination and cooling technologies. This discovery from MIT opens up a world of new possibilities for climate science, desalination, and even industrial processes. Keep reading to find out how.

The traditional understanding of evaporation

Our understanding of evaporation isn’t new. We’ve studied water since science was born and its evaporation was always linked to heat.

The science of heat-driven evaporation

Evaporation is a process that has long been understood as being primarily driven by heat. When heat is applied to a body of water, it causes the water molecules to gain kinetic energy. As these molecules move faster, they begin to break free from the liquid’s surface, transitioning into the gaseous phase. This process is what we commonly refer to as evaporation.

The key to this process lies in the hydrogen bonds that hold water molecules together. These bonds are relatively strong, which means a significant amount of energy—referred to as the latent heat of evaporation—is required to break them. This energy must be supplied in the form of heat, and it is precisely this energy input that limits the rate of evaporation under normal conditions.

The thermal limit and its implications

The rate at which water can evaporate is capped by what scientists refer to as the thermal limit. This limit is determined by the amount of energy required to break the hydrogen bonds between water molecules. As heat is applied, the temperature of the water rises, increasing the kinetic energy of the molecules. However, there’s a maximum rate at which these molecules can escape into the air, dictated by the available energy in the form of heat.

This concept is fundamental in various natural and industrial processes. For instance, in meteorology, the thermal limit is a critical factor in understanding the rate of water evaporation from oceans, lakes, and other bodies of water, which in turn influences weather patterns and the global water cycle. In industrial settings, the thermal limit governs the efficiency of processes such as drying, cooling, and even desalination.

Evaporation beyond heat: A rare phenomenon

While heat is the primary driver of evaporation, there have been instances where water evaporation has been observed under conditions where heat is not the dominant factor. However, these cases have largely been viewed as anomalies, often attributed to specific environmental conditions or the presence of certain materials that might alter the energy dynamics on the water’s surface.

For decades, these instances were not fully understood and were typically explained away by minor variations in environmental factors, leaving the core understanding of evaporation unchallenged—until now.

MIT’s groundbreaking discovery: The Photomolecular Effect

It’s not common for new scientific discoveries to challenge long-standing theories about matter. But when these breakthroughs happen, they change our perception of what is possible. For example, the photoelectric effect was just one piece of evidence among an ever-growing set of experimental observations that challenged classical physics, spawning quantum mechanics.

The photomolecular effect builds on the same principle to challenge our understanding of how water works.

Challenging the conventional wisdom

In a groundbreaking study conducted at MIT, a team of researchers led by Gang Chen discovered a new phenomenon that challenges the very foundation of our understanding of evaporation. They first observed this phenomenon on water gels^[3] but later realized that it happens to all water bodies.

They coined it the photomolecular effect, and it reveals that light—specifically visible light—can directly cause water to evaporate without the need for heat^[4]. This finding upends the traditional notion that heat is the sole driver of evaporation and suggests that our understanding of the interaction between light and water is far more complex than we previously thought.

The implications of this discovery are vast, affecting not only our theoretical models of evaporation but also practical applications across multiple fields, from climate science to industrial processes. By showing that light alone can trigger evaporation at rates far exceeding those possible through heat alone, MIT researchers have opened up a new frontier in the study of phase transitions in water and other liquids.

The experimental breakthrough that led to discovering the Photomolecular Effect

The discovery of the photomolecular effect was not just a theoretical exercise; it was grounded in rigorous experimentation. The MIT team conducted a series of experiments where they exposed water surfaces to visible light under controlled conditions. Remarkably, they found that the evaporation rate under these conditions could exceed the traditional thermal limit by as much as 4X.

What made these findings particularly compelling was the precision with which they conducted the experiments. The researchers measured the evaporation rates using various light sources, including LEDs emitting light at different wavelengths, and found that the effect was most pronounced with green light, around 520 nanometers in wavelength^[4]. This wavelength-specific response further underscored the role of light—rather than heat—in driving the evaporation process.

Photomolecular effect and its relationship to angle of incidence and wavelength

(Image source)^[4]

They also found that polarization and incidence angle played an important role. Transverse-magnetic polarized light produces a much stronger effect than unpolarized light, and the effect also maximizes at a 45° angle relative to the water’s surface.

Implications of the Photomolecular Effect for science and technology

The discovery of the photomolecular effect has profound implications for both scientific understanding and technological innovation.

For one, it forces scientists to reconsider the fundamental principles of thermodynamics as they apply to phase changes in water. Traditionally, the idea that energy could be transferred to water molecules and break them apart in the absence of heat seemed impossible, yet the photomolecular effect demonstrates that photons—particles of light—can impart enough energy to break intermolecular bonds and induce evaporation.

On the technological front, this discovery could revolutionize a wide range of industries. For instance, in the field of water desalination, where energy efficiency is a constant challenge, the ability to use light instead of heat to evaporate water could lead to significant cost and energy savings. Similarly, in industries where drying processes are critical, the photomolecular effect could enable faster and more energy-efficient methods, reducing both time and resource consumption.

How it works: Breaking the thermal limits

The photomolecular effect operates on a principle that is analogous to the well-known photoelectric effect, where photons knock electrons off a metal surface. In the case of the photomolecular effect, photons from visible light strike the surface of water and impart enough energy to break the hydrogen bonds between water molecules. However, instead of knocking off single molecules, these photons can cause clusters of molecules to break free from the surface.

This cluster-based ejection is what allows the photomolecular effect to exceed the thermal limit of evaporation.

Here’s how that works. Normally, to evaporate a single water molecule, heat must break the hydrogen bonds connecting it to other molecules. On average, you must break two hydrogen bonds per water molecule. This requires a significant amount of energy, which limits the rate of evaporation.

However, when entire clusters of water molecules are ejected together, the energy requirement is drastically reduced. This is because the bonds you need to break are fewer, and the energy is concentrated on breaking the bonds at the edges of these clusters.

The Photomolecular Effect defies thermodynamic expectations

At first glance, the photomolecular effect seems to defy the laws of thermodynamics, specifically the law of conservation of energy. If light is supposed to have a fixed amount of energy, how can it cause water to evaporate at a rate that suggests more energy is being used than is available?

The answer lies in how energy is distributed during the evaporation process. In the photomolecular effect, the energy from photons is not used to increase the temperature of the water, as it is in thermal evaporation. Instead, it is directly used to break the bonds holding water molecules together.

Image showing the hydrogen bonds in liquid water

(Image source)^[5]

Additionally, when these clusters of water molecules are ejected into the air, they interact with surrounding air molecules. It’s those air molecules that provide the rest of the energy needed to break apart the clusters. How do they know this? Because they measured a drop in air temperature just above the water’s surface, where the clusters were being broken apart by the air.

This second transfer of energy from the air to the water molecules is what enables the evaporation process to continue at such an accelerated rate and what balances out the missing energy we need for the First Law of Thermodynamics to hold.

The role of light in exceeding the thermal limit

One of the most intriguing aspects of the photomolecular effect is its wavelength dependence. The MIT researchers found that the evaporation rates were highest when the water was exposed to green light, specifically at a wavelength of 520 nanometers. This suggests that the energy of the photons at this wavelength is particularly well-suited to breaking the hydrogen bonds between water molecules.

This wavelength-specific effect also opens up new possibilities for controlling and optimizing the evaporation process. By selecting light sources that emit at specific wavelengths, it may be possible to maximize the efficiency of evaporation in industrial applications. Furthermore, this discovery raises new questions about the interaction between light and matter at the molecular level, potentially leading to new breakthroughs in fields as diverse as photochemistry, material science, and even quantum physics.

Potential implications: A new era for water and climate science

The photomolecular effect has important implications in how we understand the natural water cycle and, by extension, the weather.

Rethinking the global water cycle

The discovery of the photomolecular effect forces us to reconsider our understanding of the global water cycle, a fundamental component of Earth’s climate system^[6]. Traditionally, the water cycle has been viewed through the lens of thermal evaporation, where heat from the sun drives the transition of water from liquid to vapor. This vapor then forms clouds, which eventually precipitate back to the surface as rain or snow. However, if light alone can significantly accelerate evaporation without heat, it introduces a new dynamic to this cycle.

The water cycle which could be impacted by the photomolecular effect

(Image source)^[6]

This new understanding suggests that regions receiving intense sunlight, particularly in the visible spectrum, could experience much higher rates of evaporation than previously estimated. This could lead to faster cloud formation, potentially altering precipitation patterns and influencing everything from agricultural productivity to water availability in regions already vulnerable to climate change.

Climate sensitivity and weather forecasting

One of the most critical aspects of climate science is understanding climate sensitivity, which refers to how much the Earth’s climate will respond to changes in greenhouse gas concentrations. Climate sensitivity is directly linked to the water cycle, as evaporation plays a crucial role in cloud formation and the distribution of heat in the atmosphere.

The photomolecular effect could be the missing piece that explains certain discrepancies in current climate models. For example, models that underestimate the rate of evaporation could lead to inaccurate predictions of cloud cover, which in turn affects predictions of global temperatures and weather patterns.

By incorporating the photomolecular effect into these models, scientists might gain a more accurate understanding of how the climate will respond to ongoing increases in atmospheric carbon dioxide.

Environmental impacts: A double-edged sword

While the photomolecular effect presents exciting new possibilities for understanding and harnessing the power of light, it also raises concerns about its environmental impact. If this effect is indeed widespread in nature, it could mean that certain ecosystems are more sensitive to changes in sunlight than we realized. For example, increased evaporation rates could deplete water sources more quickly in arid regions, exacerbating drought conditions and putting additional stress on already fragile ecosystems.

On the other hand, this discovery could also be leveraged for environmental benefits. By understanding how light influences evaporation, we could develop new strategies for water conservation, particularly in areas where water scarcity is a pressing issue. Additionally, this knowledge could be used to design more efficient systems for capturing and recycling water in agricultural and industrial settings, helping to reduce overall water consumption.

The mystery of clouds: Solving a climate puzzle

Clouds play a crucial role in regulating the Earth’s climate by reflecting sunlight back into space and trapping heat in the atmosphere. However, despite their importance, clouds remain one of the least understood components of the climate system. One long-standing puzzle in climate science is why clouds and fog absorb more sunlight than theoretical models predict.

This discrepancy has significant implications for climate modeling. Understanding the exact mechanisms behind cloud formation and behavior is therefore essential for accurate climate predictions.

The photomolecular effect as a missing link

The discovery of the photomolecular effect offers a potential explanation for this cloud mystery. If light can directly cause water molecules to evaporate more efficiently, it could mean that clouds are absorbing light in ways we haven’t fully accounted for. This could lead to increased light absorption explaining the anomalous observations.

This effect could also help explain why some clouds appear to absorb more sunlight than others. Variations in the size and density of water droplets within clouds could lead to differences in how they interact with light, influencing their ability to reflect or absorb sunlight. By incorporating the photomolecular effect into cloud models, scientists may be able to resolve these discrepancies and improve the accuracy of climate forecasts.

Potential applications: Light-powered desalination and cooling

The discovery of the photomolecular effect has opened up exciting possibilities for developing new technologies that leverage light instead of heat to drive evaporation. This could lead to more efficient and sustainable methods for desalination, drying, and cooling. Let’s explore how these applications could transform industries and daily life.

A light-powered desalinator/dryer

When most people think of solar desalination, they envision a process where sunlight is used to heat water, causing it to evaporate, like in this solar dome desalination plant.

(Image source)

The vapor is then condensed to produce fresh water, leaving salts and other impurities behind. This process is known as photothermal desalination^[7], and it relies on converting light into heat, which then drives the evaporation process.

However, the photomolecular effect offers a fundamentally different approach. Instead of converting light into heat, this effect would allow light to directly evaporate water without raising the temperature. This method could solve several of the challenges associated with traditional desalination.

For one, it would significantly reduce the energy lost as heat to the environment, making the process more efficient.
Additionally, it could minimize the fouling of absorbent surfaces by salt and other contaminants, a common problem in traditional solar desalination systems.

How would we build a light-desalination plant?

The concept of a light-powered desalination plant is not just a futuristic idea—it could be built using technologies we already have. While the researchers at MIT didn’t provide a detailed blueprint, we can piece together what such a system might look like based on their findings.

#1 The light source

First, the system would need a source of green light, as the photomolecular effect is most efficient at a wavelength of 520 nanometers. This could be achieved using green LEDs or green diode lasers, both of which emit light at or near this wavelength.

Another approach could involve filtering sunlight to isolate the green spectrum, although this would result in the loss of other wavelengths, reducing overall efficiency.

#2 Polarization

Next, the light would need to be transverse-magnetic, or TM-polarized to maximize the photomolecular effect. Polarization ensures that the magnetic and electric fields of the photons oscillate in the same plane, which enhances the interaction with water molecules^[8].

Light polarization modes: transverse electric and transverse magnetic polarization

(Image source)

This can be tricky because using a linear polarizer with a regular light source typically results in the loss of about 50% of the light. However, laser diodes, which naturally emit polarized light, could be an effective solution.

Although lasers are typically less efficient, with some achieving only around 30% efficiency, more advanced options could push this higher under ideal conditions.

To further optimize light polarization, we could use a birefringent crystal^[9] to split the light into two perpendicularly polarize beams. We could then run one beam through a Faraday Rotator^[10] to twist it by 90° before recombining them. This method could minimize light losses, making the system more efficient.

#3 Water surface area

For the water itself, the design would need to maximize the surface area exposed to the light. This could be done using thin films of water or even dispersing the water into tiny droplets, ensuring that as much of the water as possible is exposed to the polarized light.

Controlling the temperature and airflow would also be crucial to carry away the evaporated water efficiently, preventing it from recondensing before it is collected.

Would this system be more efficient than current desalination technologies? Based on the research, it seems likely. Traditional desalination methods only achieve about 7–16% of the thermodynamic limit. With the photomolecular effect, there is potential to exceed this significantly, making light-powered desalination not only possible but potentially revolutionary.

New light-based cooling devices

The potential of the photomolecular effect doesn’t stop at desalination. Another exciting application lies in cooling technologies. Remember, when light knocks clusters of water molecules into the air, these clusters collide with air molecules. As they break apart, the air molecules lose some of their heat, which results in a cooling effect.

This phenomenon opens up possibilities for developing new types of cooling devices that use light instead of traditional refrigerants. For instance, imagine air conditioners that operate by shining light on water, creating a cooling effect as the water evaporates and cools the surrounding air. Such devices could be more energy-efficient and environmentally friendly, as they would not rely on harmful refrigerants or use high power resistive elements.

The possibilities are truly endless, and as research continues to advance, we may soon see a new generation of cooling devices that harness the power of light, offering sustainable and efficient solutions for both personal and industrial cooling needs.