New microscope technique reveals details of droplet nucleation


MASSACHUSETS – Nucleation is a pervasive phenomenon that governs 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 the process to be observed directly in detail, which could facilitate the design of improved and more efficient surfaces for a variety of these processes.

The innovation uses conventional scanning electron microscope equipment, but adds a new processing technique which can increase overall sensitivity up to ten times and also improve contrast and resolution. Using this approach, the researchers were able to directly observe the spatial distribution of nucleation sites on a surface and follow its evolution over time. The team then used this information to derive an accurate 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 described today in the journal Cell Reports Physical Science, in an article by MIT graduate student Lenan Zhang; visiting scholar Ryuichi Iwata; professor of mechanical engineering and department head Evelyn Wang; and nine more at MIT, the University of Illinois at Urbana-Champaign, and Shanghai Jiao Tong University.

“A really powerful opportunity”
When the droplets condense on a flat surface, such as on condensers that recycle steam from power plants back to water, each droplet requires an initial nucleation site, from which it forms. The formation of these nucleation sites is random and unpredictable, so the design of such systems relies on statistical estimates of their distribution. According to the new findings, however, the statistical method used for these calculations for decades is incorrect and another should be used instead.

The high-resolution images of the nucleation process, as well as the mathematical models developed by the team, allow the distribution of nucleation sites to be described in strictly quantitative terms. “The reason it’s so important,” Wang explains, “is because nucleation occurs in just about everything, in many physical processes, whether it’s natural or man-made materials and systems. For this reason, I think understanding this in a more fundamental way is a really powerful opportunity. “

The process they used, called Phase-Enhanced Environmental Scanning Electron Microscopy (p-ESEM), looks through electron fog caused by a cloud of electrons scattering from moving gas molecules on the surface. surface to be imaged. Conventional ESEM “can image a very large sample of material, which is quite unique compared to a typical electron microscope, but the resolution is poor” because of this electron scattering, which generates random noise, Zhang explains.

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 collides with something. This phase delay information is extremely sensitive to the slightest disturbance, down to the nanoscale, explains Zhang, and the technique they developed allows these electron-wave phase relationships to be used to reconstruct a more detailed picture. .

Using this method, he says, “we can get much better imaging contrast enhancement, then we are able to reconstruct or directly image electrons down to a few microns or even at a submicron scale. This allows us to see the nucleation process and the distribution of the large number of nucleation sites. “

This breakthrough allowed the team to investigate fundamental issues regarding the nucleation process, such as the difference between site density and the closest distance between sites. It turns out that the estimates of this relationship that have been used by engineers for over half a century are incorrect. They were based on a relationship called the Poisson distribution, for both site density and the nearest neighbor function, when in fact new work shows that a different relationship, the Rayleigh distribution, describes more precisely the closest neighbor relationship.

Zhang explains that this is important because “nucleation is a very microscopic behavior, but the distribution of nucleation sites at this microscopic scale actually determines the macroscopic behavior of the system.” For example, during condensation and boiling, it determines the heat transfer coefficient and, during boiling, even the critical heat flux, ”the metric that determines how well a boiling water system may heat up before triggering a catastrophic failure.

The results are also about much more than the simple condensation of water. “Our finding about the distribution of nucleation sites is universal,” says Iwata. “It can be applied to a variety of systems involving a nucleation process, such as the splitting of water and the growth of materials.” For example, he says, in water separation systems, which 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 key to their overall performance and is largely determined by the nucleation process.

Iwata adds that “it looks like the division of water and the condensation are very different phenomena, but we have found a universal law among them. So we’re so excited about it.

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

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

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

Nenad Miljkovic, professor of mechanical sciences and engineering at the University of Illinois who was not associated with this work, says: “Nucleation governs a plethora of phenomena such as boiling, condensation and deposition. chemical vapor phase (as mentioned in the article) but also a multitude of other phase transitions such as fouling of precipitation, initiation of corrosion, as well as the solidification of ice or changing materials. phase in energy storage.

He adds, “Classically, previous work has relied on Poisson statistics to obtain an estimate of nearest neighbor spacing, which can then be used in models to predict heat transfer, growth rates or the dynamics of reaction as a function of the process in progress. The work in progress is fundamentally changing this classic point of view.

The research team included Lin Zhao, Shuai Gong, Zhengmao Lu, Yang Zhong, Samuel Cruz, and Kyle Wilke of MIT; Jinlong Zhu, at the University of Illinois at Urbana-Champaign; and Zhenyuan Xu and Ping Cheng at Shanghai Jiao Tong University in China. The work was supported by the Singapore-MIT Alliance for Research and Technology, Toyota’s central R&D laboratories and the National Science Foundation of China and the National Science and Technology Major Project.

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