The new microscope technique reveals the details of droplet nucleation

Nucleation is a ubiquitous phenomenon that regulates the formation of droplets and bubbles in systems used for condensation, desalination, water splitting, crystal growth and many other important industrial processes. Now, for the first time, a new microscopy technique developed at MIT and elsewhere allows you to directly observe the process in detail, which could facilitate the design of improved and more efficient surfaces for a variety of such processes.

The innovation uses conventional scanning electron microscope equipment, but adds a new processing technique that can increase overall sensitivity up to tenfold and also improves contrast and resolution. Using this approach, the researchers were able to directly observe the spatial distribution of nucleation sites on a surface and monitor how it has changed over time. The team then used this information to derive a precise mathematical description of the process and the variables that control it.

The new technique could potentially be applied to a wide variety of research areas. It is featured in the magazine today Cell Reports Physical Science, in an article by MIT graduate student Lenan Zhang; visiting researcher Ryuichi Iwata; mechanical engineering professor and department head Evelyn Wang; and nine others at MIT, the University of Illinois at Urbana-Champaign, and Shanghai Jiao Tong University.

“A truly powerful opportunity”

When droplets condense on a flat surface, such as on condensers that conduct steam in power plants back to water, each droplet requires an initial nucleation site, from which it accumulates. The formation of these nucleation sites is random and unpredictable, so the design of such systems is based on statistical estimates of their distribution. According to the new findings, however, the statistical method that has been used for these calculations for decades is incorrect and a different one should be used instead.

The high-resolution images of the nucleation process, together with the mathematical models developed by the team, allow the distribution of the nucleation sites to be described in rigorous quantitative terms. “The reason this is so important,” says Wang, “is because nucleation occurs in practically everything, in many physical processes, whether it is natural or in engineered materials and systems. For this reason, I think understanding it in a way more fundamental is a truly powerful opportunity. “

The process they used, called advanced-phase environmental scanning electron microscopy (p-ESEM), allows them to peer through the electron fog caused by a cloud of electrons dispersing from gas molecules moving across the surface to be imaged. Conventional ESEM “can display a very large sample of material, which is very unique to a typical electron microscope, but the resolution is poor” because of this scattering of electrons, which generates random noise, Zhang says.

Taking advantage of the fact that electrons can be described as particles or waves, the researchers found a way to use the phase of electronic waves and the delays in that phase generated when the electron hits something. This phase delay information is extremely sensitive to the slightest perturbations, down to the nanoscale, Zhang says, and the technique they developed makes it possible to use these electron-wave phase relationships to reconstruct a more detailed image.

Using this method, he says, “we can achieve a much better improvement in image contrast, and thus we are able to directly reconstruct or visualize electrons at a few microns or even at a submicron scale. This allows us to see the nucleation process. . and the distribution of the enormous number of nucleation sites “.

The progress allowed the team to study fundamental problems about the nucleation process, such as the difference between site density and the closest distance between sites. It turns out that the estimates of that relationship that have been used by engineers for over half a century were incorrect. They were based on a relationship called the Poisson distribution, for both the site density and the nearest neighbor function, when in fact the new work shows that a different relationship, the Rayleigh distribution, more accurately describes the neighbor relationship. closer.

Zhang explains that this is important, because “nucleation is a very microscopic behavior, but the distribution of nucleation sites on this microscopic scale actually determines the macroscopic behavior of the system.” For example, in condensation and boiling, it determines the heat transfer coefficient and in boiling also the critical heat flow “, the measure that determines how hot a boiling water system can get before triggering a catastrophic failure.

The results also refer to much more than just water condensation. “Our discovery of the nucleation site distribution is universal,” Iwata says. “It can be applied to a variety of systems that involve a nucleation process, such as water splitting and material growth.” For example, he says, in water splitting systems, they can be used to generate fuel in the form of hydrogen from electricity from renewable sources. The dynamics of bubble formation in such systems is critical to their overall performance and is largely determined by the nucleation process.

Iwata adds that “it looks like water splitting and condensation are very different phenomena, but we’ve found a universal law between them. So we’re so excited about it.”

Several applications

Many other phenomena are also based on nucleation, including processes such as the growth of crystalline films, including diamond, across surfaces. Such processes are increasingly important in a wide variety of high-tech applications.

In addition to nucleation, the team’s new p-ESEM technique can also be used to probe a variety of different physical processes, say the researchers. Zhang says it could also be applied to “electrochemical processes, polymer physics and biomaterials, because all of these types of materials are extensively studied using conventional ESEM. However, by using p-ESEM, we can certainly achieve much better performance due to” inherent high sensitivity “of this system.

The p-ESEM system, says Zhang, by improving contrast and sensitivity, can improve signal strength in relation to background noise by up to 10 times.