New microscope technique reveals details of droplet nucleation
Credit: David L. Chandler/MIT

Nucleation is a ubiquitous phenomenon that governs the formation of both 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, 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 the overall sensitivity by as much as 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 track how that changed over time. The team then used this information to derive a precise mathematical description of the process and the variables controlling 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 a paper by MIT graduate student Lenan Zhang; visiting research scientist Ryuichi Iwata; professor of mechanical engineering and department head Evelyn Wang; and nine others at MIT, the University of Illinois at Urbana-Champaign, and Shanghai Jiao Tong University.

“A really powerful opportunity”

When droplets condense on a flat surface, such as on the condensers that cycle the steam in electric power plants back into water, each droplet requires an initial nucleation site, from which it builds up. The formation of those 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 that’s 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, along with mathematical models the team developed, make it possible to describe the distribution of nucleation sites in strict quantitative terms. “The reason this is so important,” Wang says, “is because nucleation pretty much happens in everything, in a lot of physical processes, whether it’s natural or in engineered materials and systems. Because of that, I think understanding this more fundamentally is a really powerful opportunity.”

The process they used, called phase-enhanced environmental scanning electron microscopy (p-ESEM), makes it possible to peer through the electronic fog caused by a cloud of electrons scattering from moving gas molecules over the surface being imaged.

Conventional ESEM “can image a very wide sample of material, which is very unique compared to a typical electron microscope, but the resolution is poor” because of this electron scattering, which generates random noise, Zhang says.

Further Reading: Massachusetts Institute of Technology