A long-pursued goal, which is also a grand challenge, in nanoscience and nanotechnology is to create nanoscale devices, machines and motors that can do useful work. However, loyal to the scaling law, combustion would be impossible at nanoscale be- cause the heat loss would profoundly dominate the chemical reactions. Thus, in addition to the solid propellants, a preliminary study was also conducted to understand the feasibility of combustion inside nanobubbles.
First, an experimental study was conducted to confirm the occurrence of combustion inside nanobubbles. These nanobubbles were produced from short-time (< 2000 μs) water electrolysis by applying high-frequency alternating sign square voltage pulses (1-500 kHz), which resulted in H2 and O2 gas production above the same electrode. A 10 nm thick Pt thermal sensor (based on resistance thermometry) was fabricated underneath the combustion electrodes to measure the temperature changes. Signifi- cant bubble production was seen up to 30 kHz but after that the bubble production decreased drastically, although the amount of faradaic current measured remained unchanged. The temperature changes measured were also found to increase above this threshold frequency of 30 kHz.
Next, non-reactive molecular dynamic simulations were performed to determine how does the surface tension of water surrounding the electrodes is affected by the presence of dissolved external gases, which would in turn help to predict the pressures inside nanobubbles. Knowing the bubble pressure is a perquisite towards understanding the combustion process. The surface tension of water was found to decrease with an increase in the supersaturation ratio (or an increase in the external gas concentration), thus, the internal pressure inside a nanobubble is much smaller than what would have been predicted using the planar-interface surface tension value of water. Once the pressure behavior as a function of external gas supersaturation was understood, then as a next step, reactive molecular dynamic simulations were performed to study the effects of surface-assisted dissociation of H2 and O2 gases and initial system pressure on the ignition and reaction kinetics of the H2/O2 system. Significant amount of hydrogen peroxide (H2O2), 6-140 times water (H2O), was observed in the combustion products, which was attributed to the low temperature (~300 K) and high pressure (2- 80 atm) conditions at which the chemical reactions were being taken place. Moreover, the rate at which heat was being lost from the combustion chamber (nanobubble) was also compared to the rate at which heat was being released from the chemical reactions and only a slight rise in the reaction temperature was observed (~68 K), signifying that at such small-scales the heat losses dominate.
Dr. Shourya Jain - Ph.D., School of Aeronautics and Astronautics, Purdue University
Dr. Li Qiao - Associate Professor, School of Aeronautics and Astronautics, Purdue University
Dr. Aamer Mahmood for his help and advice with the fabrication techniques involved in the design of the micro-thermal sensor needed for the experimental work involving combustion in nanobubbles
NSF - National Science Fondation
S Jain, L. Qiao, “Understanding combustion of H2/O2 gases inside nanobubbles generated by water electrolysis using reactive molecular dynamic simulations”, Journal of Physical Chemistry A, 122, 24, (2018)
S Jain, L. Qiao, “MD simulations of the surface tension of oxygen-supersaturated water”, AIP Advances 7, 045001 (2017)
S Jain, A. Mahmood, L. Qiao, “Quantifying heat produced during spontaneous combustion of H2/O2 nanobubbles”, 2016 IEEE Sensor proceedings, Orlando, FL, Oct 30 - Nov 3, 2016
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