| Date | 9th, Jul 2020 |
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For nearly a century, the liquid surface structure at nanometer scale resolution remains a mystery due to the lack of an effective visualization tool to directly observe the structure and dynamics of the liquid-vapor interface.
“Under the electron microscope, the dynamics of the liquid-vapor interface at nanometer scale look like ocean waves hitting a bank,” said the first author, Amy Ren, “It is amazing.”
The liquid-vapor interface was generated by electron beam irradiated sodium chloride crystal. Image credit: Gang Ren via Yuotube
Recently, a team of scientists from Lawrence Berkeley National Laboratory (LBNL), and students from University of California at Santa Barba and Brown University devised a new method for observation of the dynamic interface of liquid sodium under the electron microscope.
The paper, published 25 May in Scientific Reports, entitled “Real-time observation of dynamic structure of liquid-vapor interface at nanometer resolution in electron irradiated sodium chloride crystals”, explores a simple, cost-effective method for a liquid-cell that enables real-time, nanometer-resolution to encapsulate the liquid sodium inside a small chamber for high magnification imaging under the TEM column vacuum.
“The method is robust and allows us to observe the liquid dynamics for over one hour” said the second author, David Lu. The key is that sealing materials must be strong enough to withstand its vacuum pressure while also having enough transparency to allow beam penetration for imaging.
The liquid sodium was generated by an electron beam irradiating NaCl crystals containing a tiny account of calcium silicate. The liquid was sandwiched between two Formvar plastic films and sealed by vacuum grease. “The calcium silicate was important for the encapsulation of the free-flowing liquid at timescales of over an hour of observation without leakage”, said David.
“Other than common phenomena, such as Brownian motion of nanoparticles in liquid and gas, the merging of nanoparticles, most interestingly, we first-time observed the detailed structure of the liquid-vapor interface”, said Amy, “the interface itself is comprised of a layer of nanometer-scale fibers, like wheatgrass swinging in the wind”.
The fibers were reported as extending from the bulk liquid side towards the vapor side, and transfers nanoparticles from the bulk liquid to a layer of clusters of nanoparticles in the vapor side. “The nearby nanoparticles were suddenly sucked towards the nanofibers, like nearby stars were stretched out just before it gets sucked into a black hole in outer space”, said Amy.
Although the phenomena were observed based on liquid sodium, the similarities of the phenomena to computer predictions suggest the real-time observed dynamic structure validates long-debated theoretical models, disfavoring the zero-width capillary-wave model, predicted by J. Willard Gibbs a century ago, and favoring the nonzero-width bilayer capillary-wave models predicted decades ago. These images allow scientists to further understand thermodynamic principles and fluid behaviors at the nanoscale and benefit microfluidic developments in the future.
Written by Gang Ren, PhD, rengroup.lbl.gov
