Spontaneous Liquid Outflow Driven by Gas-Liquid Interaction in Hydrophobic Nano-Channels
Liquid motion in nano-environment has immense importance in various engineering applications. Recently, a novel nanofluidics-based energy absorption system, namely liquid nanofoam (LN) has been developed. The main working mechanism of the LN is by dissipating mechanical energy as solid-liquid interfacial energy. It has been demonstrated that the energy absorption capacity of LN is determined by the solid-liquid interaction in the confined nano-environment. As the energy dissipation mechanism is based on the pressure-aided nanoscale liquid motion without damaging the nano-channels, the LN holds great promise for the development of reusable energy absorbers, which is particularly important for repetitive head impacts in sports and battlefield. The reusability of LN is determined by the degree of spontaneous liquid outflow (DSLO) from the hydrophobic nano-channels when the external pressure is removed.
Previous studies have suggested that the DSLO is related to the solid-liquid interaction in the hydrophobic nano-channels. We hypothesize that upon unloading, the gas-liquid interaction instead of solid-liquid interaction in the nano-channel drives the liquid outflow. To test this hypothesis, we have set the excessive solid-liquid interfacial tension of four different LN systems as a constant. One type of nanoporous material with the same porous structure and surface properties has been immersed into four different aqueous electrolyte solutions with the same surface tension. This unique experimental setup provides us the opportunity to investigate the effect of gas-phase on the liquid outflow by decoupling the gas-liquid interaction from the solid-liquid interaction.
The same excessive solid-liquid interfacial tension of all LN samples is validated by the liquid infiltration tests. In all LN systems, when the external pressure reaches the same threshold value, the liquids are forced into the nano-channels as indicated by a clear pressure plateau. At the end of the loading process, the nano-channels are filled with infiltrated liquid molecules and air molecules initially stored in the nano-channels. Upon unloading, partial liquid outflow has been observed and quantified by the pressure plateau width of a second consecutive loading-unloading cycle. The DSLO of the LN is defined as the ratio between the pressure plateau widths of the 1st and 2nd cycles.
Air bubbles are observed in the LN samples at the completion of the first cycle, indicating that gas escapes from the nano-channels to the bulk liquid phase. As the gas solubility is much enhanced in the nano-channel, a gas concentration gradient between the nano and bulk liquid phases exists and determines the gas diffusion rate. Larger concentration gradient promotes gas diffusion from the nano-channels to the bulk phase. Consequently, less DSLO of the LN system. The different DSLO of the four LN systems suggests that the gas concentration gradient is highly sensitive to the ion species which is the only variable of the designed LN systems. In addition, different from the bulk phase scenario, the anions have a more profound effect than cations.
In summary, the gas-liquid interaction in the nano-channels determines the DSLO. Lower bulk gas solubility and smaller oversolubility factor promote DSLO and recoverability of the LN systems. This fundamental understanding on the mechanism of liquid outflow combining with the pressure-induced liquid flow mechanism enable us to functionalize the nanofluidics-based system to be developed into reusable energy absorbers with high efficiency.
Spontaneous Liquid Outflow Driven by Gas-Liquid Interaction in Hydrophobic Nano-Channels
Category
Poster Presentation
Description
Session: 16-01-01 National Science Foundation Posters - On Demand
ASME Paper Number: IMECE2020-24790
Session Start Time: ,
Presenting Author: Lijiang Xu
Presenting Author Bio: Lijiang Xu has been working as a lead researcher in structure engineering at Michigan State University for over four years. Her research is focused on understanding the working mechanism of a high-performance energy absorption system, Liquid Nanofoam (LN) system, which dissipates kinetic energy through pressure-induced liquid flow in hydrophobic nanoporous material. By designing a unique experiment, she identified the gas effect on liquid outflow in nano-channels, which can be used to improve the system reusability desired for multi-impact mitigation in auto-collision and natural disasters. Furthermore, the pressure effect has also been determined through the gas diffusion from hydrophobic nano-channels to bulk liquid, which has long been ignored in the bulk liquid diffusion process. Her educational background in Structure Engineering, Mechanics and Materials has given her a broad base in experimental design, material characterization, and computer-aided design using FEM and CFD software. Relevant examples of computer-aided design are as follows, analyzing the failure behavior of composite sandwich panels under the low-velocity impact, optimizing the geometry of cruciform specimens, and modeling the equilibrium state of water using smoothed boundary method.
Authors: Lijiang Xu Michigan State University
Mingzhe Li Michigan State University
Weiyi Lu Michigan State University