On Sep 8th, an article titled “A Nanoconfined Water–Ion Coordination Network for Flexible Energy-Dissipation Devices” was published online in Advanced Materials (IF: 29.4). The article was authored by Prof. Yuan Gao, a faculty member at the School of Mechanical Science and Engineering at the Huazhong University of Technology, and was elected in the “Rising Star” series. Prof. Yuan Gao served as the first and corresponding author, and Prof. Baoxing Xu at the University of Virginia and Prof. Weiyi Lu at Michigan State University also served as corresponding authors. State Key Laboratory of Digital Manufacturing Equipment and Technology was the primary affiliation of the research work.
Liquids subjected to nanoconfinement can gain unusual mechanical and thermal properties contrasting to their bulk counterpart. These unique properties can be utilized to develop novel functional nanofluidics devices. For example, the system comprises hydrophobic nanopores, and water can be used in flexible anti-impact devices. Owing to the extreme surface-to-volume ratio of nanopores, the energy dissipation efficiency can reach 100 J/g. However, in the presence of ions, the hydrogen-bonding network will be disturbed by ions, and the mechanical properties of the confined solution vary correspondingly, which influences the performance of the anti-impact system. Moreover, the traditional theory of structure maker/breaker may not be applicable due to the size effect. Understanding the interaction mechanisms between ions and water molecules is critical to the design and development of nanofluidics anti-impact systems.
Using molecular dynamics (MD) simulations, the authors analyzed the LiCl solution confined in a silica nanopore, focusing on the interaction between the water molecules and ions. The results indicated that the ions destroy a portion of hydrogen bonds. However, new water-ion and ion-ion interactions can be established with improved strengths and extended lifetime compared to that of hydrogen bonds (water-water interactions). Therefore, it was fair to consider water-ion and ion-ion interactions as part of an interaction network, based on which the concept of a “water-ion coordination network” was proposed. Next, the research studied the strength and connectivity of the water-ion coordination network and the associated tuning approaches and molecular mechanisms.
Comprehensive analyses proved that water-water, water-ion, and ion-ion interactions have different strengths of interactions, and dynamic analyses were conducted on these interactions. The results showed that the ion-ion interactions are the strongest among the three with the longest lifetime, while the hydrogen bonds are the most fragile. These findings can explain the enhanced cohesion energy with a higher concentration of ions.
The reported mechanisms were utilized in the design of anti-impact devices, where a flexible plastic shell was used as a container of hydrophobic nanopores immersed in LiCl solutions. Owing to the hydrophobic nature of nanopores, the liquid was not able to infiltrate the nanopores spontaneously. Upon an external loading, such as an explosion, the liquid can be forced to infiltrate the nanopores, during which the friction at the solid-liquid interface can dissipate the mechanical energy of the impact. After the liquid completely infiltrated the nanopore, a water-ion coordination network with high cohesion energy was formed inside the nanopore, which accelerated the pressure reduction and improved the performance of the device. More detailed calculations suggested that the pressure reduction depends on solid-liquid energy, environmental temperature, and the radius of nanopores. A scaling law was proposed to predict the pressure reduction of the system by considering all the factors above. The scaling law was validated by both MD simulations and experiments.
These results and findings can provide theoretical guidance and experimental evidence for the design and development of anti-impact devices based on nanofluidics systems, which can be potentially applied in sport wears and safety systems in transportation.
Article link： https://onlinelibrary.wiley.com/doi/full/10.1002/adma.202303759