Evolution of Bulk Nanobubbles under Different Freezing Conditions.

The freezing process of aqueous solutions plays a crucial role in various applications including cryopreservation, glaciers, and frozen materials. However, less research has focused on the influence of nanoscale gas bubble formation or collapse in water during freezing, which may significantly impact the formation of ice crystals. Herein, we report for the first time that the freezing process can produce nanobubbles in aqueous solutions, and their size and number concentration could be changed by different cooling rates, i.e., the size would decrease as the cooling rate increased, and the maximum number concentration was found at the -80 °C system. Furthermore, increasing the dissolved gas content in the solution enhanced the production of nanobubbles, whereas for preexisting nanobubbles, the freezing resulted in a decrease in their number concentration, which was negatively correlated with the cooling rate. Our results indicated that a moderate cooling rate of -80 °C favored nanobubble generation, whereas a higher cooling rate was preferable for maintaining preexisting nanobubbles. Conversely, a lower cooling rate could be employed to eliminate preexisting nanobubbles. This study explored the evolution and stability of nanobubbles during the freezing process, providing valuable insights into the application or elimination of nanobubbles.

Controllable generation of interfacial gas structures on the graphite surface by substrate hydrophobicity and gas oversaturation in water.

Spherical nanobubbles and flat micropancakes are two typical states of gas aggregation on solid-liquid surfaces. Micropancakes, which are quasi-two-dimensional gaseous structures, are often produced accompanied by surface nanobubbles. Compared with surface nanobubbles, the intrinsic properties of micropancakes are barely understood due to the challenge of the highly efficient preparation and characterization of such structures. The hydrophobicity of the substrate and gas saturation of solvents are two crucial factors for the nucleation and stability of interfacial gas domains. Herein, we investigated the synergistic effect of the surface hydrophobicity and gas saturation on the generation of interfacial gas structures. Different surface hydrophobicities were achieved by the aging process of highly oriented pyrolytic graphite (HOPG). The results indicated that higher surface hydrophobicity and gas oversaturation could create surface nanobubbles and micropancakes with higher efficiency. Strong surface hydrophobicity could promote nanobubble nucleation and higher gas saturation would induce bigger nanobubbles. Degassed experiments could remove most of these structures and prove that they are actually gaseous domains. Finally, we draw a region diagram to describe the formation conditions of nanobubbles, micropancakes based on observations. These results would be very helpful for further understanding the formation of interfacial gas structures on the hydrophobic surface under different gas saturation.

Interfacial Micropancakes: Gas or Contaminations?

Micropancake, a flat domain with micrometer-scale lateral size and a few nanometer thickness, is usually accompanied by the generation of interfacial nanobubbles at the liquid/solid surfaces. Unlike the nanobubbles, micropancakes are difficult to be produced efficiently, impeding further investigations of their mysterious properties. Very recently, An et al. even argued that the previously observed micropancakes were most likely the contaminate, not the gas layers. Herein, to reveal the nature of micropancakes with solid evidence, we presented the in situ characterization of micropancakes at a highly oriented pyrolytic graphite (HOPG) surface produced by the ethanol-water exchange or gas-supersaturated water. By washing with deeply degassed water (DW), the dissolution of those micropancakes was clearly observed, indicating that they may very well be composed of gas. In addition, the analysis of the force measurements showed the intrinsic differences between those gaseous micropancakes and the insoluble organic films. The data and results supported the interpretation that the real existence of gas micropancakes at liquid/solid surfaces.

Phonon Spectrometric Evaluation of the Solute-Solvent Interface in Solutions of Glycine and Its N-Methylated Derivatives.

From the perspective of O:H-O bond cooperativity, we analyzed the solute capability of transiting the O:H-O bond from the mode of ordinary water to the hydration state and its consequence on the solution viscosity and surface stress. Phonon spectrometric results suggest that glycine and its N-methyl derivatives strongly affect the surrounding solvent molecules through H ↔ H repulsion and dipolar polarization. The H ↔ H interproton repulsion disrupts the surface stress, and the polarization enhances the solution viscosity.