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 1^st^ and 2^nd^ 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

Division / Sponsors: 
Aerospace, Bioengineering, Applied Mechanics, Design Engineering Division, Advanced Energy Systems, Manufacturing Engineering Division, Noise Control and Acoustics, Nondestructive Evaluation, Diagnosis, and Prognosis (NDPD), Nuclear Engineering Division, Process Industries Division, Fluids Engineering, Heat Transfer, Materials, Microelectromechanical Systems, Safety Engineering and Risk Analysis, Computers & Information in Engineering


Paper Proceedings: https://asmedigitalcollection.asme.org/IMECE/IMECE2020/volume/84478

IMECE2020 - Virtual Conference

Supporting the need among researchers worldwide for additional online and multimedia content, ASME now offers the ASME Conference Video Collection. This platform will collect and provide long-term access to conference presentation videos, encompassing all technology areas within ASME's technical conference and events program. About 30,000-40,000 engineers across all mechanical engineering and related disciplines participate in ASME conferences each year. This includes authors of approximately 6,000 to 8,000 technical papers published by ASME, the majority of whom also submit video presentations for our virtual conferences. The video collection will be available as an add-on to existing product packages or as a separate subscription.