Imaging Nanobubbles in Water with Scanning Transmission Electron Microscopy

Abstract

We present a technique based on scanning transmission electron microscopy (STEM) that is capable of probing nanobubble dynamics with nanometer spatial resolution. A vacuum-tight vessel holds a sub-micrometer layer of water between two electron-transparent dielectric membranes. Electrical current pulses passing through a platinum wire on one of the membranes inject sufficient heat locally to initiate single bubble formation. In the absence of power input, all bubbles are observed to be unstable against collapse, but the STEM beam alone can cause a shrinking bubble to grow.

Opportunities and challenges in liquid cell electron microscopy

“…In each design, the electrode materials and geometry can be customized for the particular applications (14,16,18,(20)(21)(22). A heating element (23) or cooling capability (24) can be integrated, and the silicon nitride surfaces can be patterned or chemically modified to enable reactions with species in solution (25)(26)(27)(28). Careful assembly procedures and cleaning are necessary (29).…”

“…Heating water nucleates bubbles, and their nucleation, growth, and stability can be measured and compared with thermodynamic models (23). Localized heating is achieved via Joule strips made of Pt lines, and the temperature gradients can in principle be modeled.…”

Individual Liquids and Liquid Solutions Under Confinement

“…Whereas there is a general agreement that the key processes governing gas phase nucleation occur on nano-and mesoscopic scales, the majority of experimental studies have been limited to macroscopic observations due to numerous challenges associated with direct experimental probing of the initial stages of cavitation at the nanoscale [26].…”

Nanobubbles on Electron Transparent Electrodes

“…The graphene is subsequently transferred to a silicon chip with gold contacts and a 20 nm thick SiN x electron transparent window [3]. To create the fluid cell, a small droplet of liquid is placed between the chip and a second silicon chip without electrodes, which is maneuvered such that the SiN x windows are aligned [4]. Applying epoxy around the edges of the cell creates a vacuum-tight seal, which keeps the graphene wet and the electron-transparent windows aligned when the cell is placed in the microscope's high vacuum.…”

“…Previously, platinum wires have been used to Joule-heat water and produce nanobubbles for observation with in situ STEM [4]. While a Pt wire effectively creates individual nanobubbles, it most likely obscures the corresponding nucleation sites in the STEM projection image.…”