• Energy Research

Water electrolysis with a fast change of polarity generates a high concentration of bulk nanobubbles containing H 2 and O 2 gases. When this concentration reaches a critical value, a microbubble pops up, which is terminated quickly in an explosion process. In this paper, we provide experimental information on the phenomenon concentrating on the dynamics of exploding microbubble observed from the top and from the side. An initial bubble with a size of 150 μ m expands to a maximum size of 1200 μ m for 150 μ s and then shrinks in the cavitation process. The sound produced by the event is coming from two sources separated in time: exploding bubble and cavitating bubble. The observed dynamics supports expansion of the bubble with steam but not with H 2 and O 2 mixture. A qualitative model of this puzzling phenomenon proposed earlier is refined. It is demonstrated that the pressure and temperature in the initial bubble can be evaluated using only the energy conservation law for which the driving energy is the energy of the combusted gas. The temperature in the bubble reaches 200 ∘ C that shows that the process cannot be ignited by standard combustion, but the surface-assisted spontaneous combustion agrees well with the observations and theoretical estimates. The pressure in the microbubble varies with the size of the merging nanobubbles and is evaluated as 10–20 bar. Large pressure difference between the bubble and liquid drives the bubble expansion, and is the source of the sound produced by the process. Exploding microbubbles are a promising principle to drive fast and strong micropumps for microfluidic and other applications.

Water electrolysis with a fast change of polarity generates a high concentration of bulk nanobubbles containing H 2 and O 2 gases. When this concentration reaches a critical value, a microbubble pops up, which is terminated quickly in an explosion process. In this paper, we provide experimental information on the phenomenon concentrating on the dynamics of exploding microbubble observed from the top and from the side. An initial bubble with a size of 150 μ m expands to a maximum size of 1200 μ m for 150 μ s and then shrinks in the cavitation process. The sound produced by the event is coming from two sources separated in time: exploding bubble and cavitating bubble. The observed dynamics supports expansion of the bubble with steam but not with H 2 and O 2 mixture. A qualitative model of this puzzling phenomenon proposed earlier is refined. It is demonstrated that the pressure and temperature in the initial bubble can be evaluated using only the energy conservation law for which the driving energy is the energy of the combusted gas. The temperature in the bubble reaches 200 ∘ C that shows that the process cannot be ignited by standard combustion, but the surface-assisted spontaneous combustion agrees well with the observations and theoretical estimates. The pressure in the microbubble varies with the size of the merging nanobubbles and is evaluated as 10–20 bar. Large pressure difference between the bubble and liquid drives the bubble expansion, and is the source of the sound produced by the process. Exploding microbubbles are a promising principle to drive fast and strong micropumps for microfluidic and other applications.

Isotropic etching of silicon in HF-based solutions exhibits some level of anisotropy. We study this anisotropy in detail by etching silicon via circular mask openings for wafers of different orientations. The in-plane shape of the cavities is analyzed with high precision as a function of the etching time and opening size. Fourier expansion of the cavity shape is used to analyze different anisotropy components in relation to the crystal symmetry. It is found that the anisotropy pattern is in agreement with the crystal symmetry with a precision better than 0.4%. The relative anisotropy does not depend on the etching time and increases with the reduction of the opening size. For radii of mask holes a > 4 µm all the Fourier coefficients demonstrate a universal behavior increasing in absolute value linearly with a-1/3. The smaller holes exhibit saturation of these coefficients. The maximal anisotropy is about 9% for both (1 0 0) and (1 1 0) wafers but only 1.5% for (1 1 1).

Isotropic etching of silicon in HF-based solutions exhibits some level of anisotropy. We study this anisotropy in detail by etching silicon via circular mask openings for wafers of different orientations. The in-plane shape of the cavities is analyzed with high precision as a function of the etching time and opening size. Fourier expansion of the cavity shape is used to analyze different anisotropy components in relation to the crystal symmetry. It is found that the anisotropy pattern is in agreement with the crystal symmetry with a precision better than 0.4%. The relative anisotropy does not depend on the etching time and increases with the reduction of the opening size. For radii of mask holes a > 4 µm all the Fourier coefficients demonstrate a universal behavior increasing in absolute value linearly with a-1/3. The smaller holes exhibit saturation of these coefficients. The maximal anisotropy is about 9% for both (1 0 0) and (1 1 0) wafers but only 1.5% for (1 1 1).

The spontaneous combustion of hydrogen–oxygen mixture observed in nanobubbles at room temperature is a puzzling phenomenon that has no explanation in the standard combustion theory. We suggest that the hydrogen atoms needed to ignite the reaction could be generated on charged sites at the gas–liquid interface. Equations of chemical kinetics augmented by the surface dissociation of hydrogen molecules are solved, keeping the dissociation probability as a parameter. It is predicted that in contrast with the standard combustion, the surface-assisted process can proceed at room temperature, resulting not only in water, but also in a perceptible amount of hydrogen peroxide in the final state. The combustion time for the nanobubbles with a size of about 100 nm is in the range of 1–100 ns, depending on the dissociation probability.

The spontaneous combustion of hydrogen–oxygen mixture observed in nanobubbles at room temperature is a puzzling phenomenon that has no explanation in the standard combustion theory. We suggest that the hydrogen atoms needed to ignite the reaction could be generated on charged sites at the gas–liquid interface. Equations of chemical kinetics augmented by the surface dissociation of hydrogen molecules are solved, keeping the dissociation probability as a parameter. It is predicted that in contrast with the standard combustion, the surface-assisted process can proceed at room temperature, resulting not only in water, but also in a perceptible amount of hydrogen peroxide in the final state. The combustion time for the nanobubbles with a size of about 100 nm is in the range of 1–100 ns, depending on the dissociation probability.

Isotropic etching of silicon in HF-based solutions is expected to be controlled by the diffusion of fluoride to the silicon surface. In order to gain quantitative understanding of the process, we studied etching of silicon in HF/HNO3/H2O via circular mask openings and compared the results with the theoretical expectations. The cavity edges due to etching under the mask were analyzed with a high precision by processing the optical microscope images. Dependence on the etching time and opening size was investigated. A small anisotropy was observed in perfect agreement with the crystal orientation symmetry. A weak effect of free convection induced by the gravitation was resolved. Importance of careful temperature control is stressed. The observed time dependence agrees perfectly well with the theoretical prediction. It was verified with 1% precision. Dependence on the opening size predicted theoretically is not fully supported by the experiment. There is a small (4%) but clearly observable deviation from the theory. It is demonstrated that both time and opening size dependencies can be predicted with 1% precision if one introduces an effective diffusion “constant��? that changes slightly with the opening size.