Physical cleaning based on underwater ultrasound is widely used in industry and its cleaning efficiency is known, from recent studies, to be augmented by promoting the mechanical activity of cavitation bubbles. As a side effect, ultrasound cleaning may give rise to material damage from violent collapse of cavitation bubbles. Traditionally, degassed water is favored as cleaning solution in order to reduce the probability of having cavitation (and the resulting erosion); however, there is no chance to promote cleaning efficiency with this approach. In this review, we introduce our recent effort toward the development of an erosion-free ultrasonic cleaning technique using aerated water. Under dissolved gas supersaturation in aerated water, bubbles can be created easily with low-intensity ultrasound and the resulting bubble dynamics are expected to be mild enough to avoid erosive collapse. To demonstrate this conjecture, we run a series of ultrasound cleaning tests with glass samples on which submicron SiO2 particles are spin-coated; we use a transparent cleaning bath for visualization of acoustic phenomena by a high-speed camera. Dissolved oxygen (DO) supersaturation in water (aerated by oxygen microbubbles) and ultrasound frequency (at 28 kHz or 200 kHz) are varied as parameters, while the input power to drive the ultrasound transducer is fixed and the ultrasound amplitude at the pressure antinode is set at Pe=1.0 atm (root-mean-square value) for the case of degassed water. Particle removal efficiency (PRE) is defined based on image analysis of light scattering from the residual particles (i.e., the so-called Haze method). We see that there exists an optimal DO supersaturation to maximize the PRE. We also see that the PRE is higher in the case of lower frequency (28 kHz), for its cavitation inception threshold is reduced and the number of activated bubbles is thus increased.
We carried out molecular dynamics simulations of water infiltration into a slit pore for the understanding of the rinsing process of semiconductors, whose minimum length scale seemingly reached the continuum limit. The possibility of water infiltration strongly depended on the wettability, i.e., the sign of the cosine of contact angle due to the large Laplace pressure. The time needed for the infiltration was small due to the high meniscus velocity, and the gas molecules remaining in the slit did not have remarkable effects on this process. Macroscopic relations gave reasonable estimates for these static and dynamic processes even at the nanoscale.
PVA brushes are widely used for cleaning semiconductor device surfaces. PVA brushes are a soft porous polymer which can be absorb much amount of water. In this review, we briefly introduce the proposed cleaning mechanism. Then some results of frictional analysis of PVA brushes are described. In the frictional analysis, viscoelastic behavior of the brush, surface wettability of a plate, and brush deformation during scrubbing were important. In addition, we introduce a visualization result of real contact area between PVA brush and contact surface.
Chemical solvents used for industrial washing process have a negative impact on the environment and the health of their users, and they are high-cost cleaning. In order to reduce the amount of chemical solvent used in the washing process, the non-chemical washing technology with microbubble is expected. We focus on the microbubble generator with a Venturi tube. This generator can generate the bubbles of hundred-micrometer diameter in high void fraction. And it is maintenance-free because of its simple structure. In this paper, authors introduce capability of oil washing with microbubbles generated using a Venturi tube and photoresist removal with Ozone microbubbles, which is friendly for the environment and no harm to the product.
A steam injector (SI) works as a passive jet pump and has high heat transfer performance due to direct contact condensation between steam flow and subcooled water jet. The objective of the present study is to clarify heat transfer and flow characteristics of an ultra-micro steam injector (UMSI) with throat diameter of 1.0 mm. To this end, internal flow was visualized by a high-speed video camera. Pressure and temperature were measured with the visualization simultaneously. The main parameters were inlet steam pressure, inlet water flow rate and inlet water temperature. As a result, the formation of the water jet was confirmed. In that case, it was verified that discharged pressure became higher than the inlet pressure. The UMSI was successfully operated by achieving simultaneous maintenance of water jet and boosting pressure. In addition, water cooling capacity and steam heating power were evaluated. The operable region was suggested to exist in the large water cooling capacity condition.
In order to clarify mechanism of ultrafine bubble generation by high-speed stirring, mixing process of two-phase flow in the vessel was calculated using lattice Boltzmann method. The process of elongated gas pillars splitting to generate a many small bubbles was confirmed. We proposed the interface length as a new index to quantitatively evaluate the formation of small bubbles. It was found the interface length was the longest when the initial gas-liquid volumetric ratio was 1: 1, and this result qualitatively agreed with the experimental results.
Relative to the vacuum method, there are drawbacks to the underwater arc discharge method, including weak discharge intensity, low discharge sustainability, and low production volumes of CNTs. Likewise, there are also disadvantages to the arc discharge method in foam as the method requires additional distillation processes to eliminate the beer foam after the production process in spite of its advantages. This study, therefore, investigates an alternative method by replacing the conventional beer foam, used for the discharge location, with foam made from a surfactant (SDS). Experimental results indicated that CNTs can be produced feasibly and efficiently in the same way as in the conventional beer foam method. Further studies were performed to determine whether CNT synthesis and dispersion processes are carried out simultaneously when the CNTs, produced by the arc discharge in SDS foam, are dispersed in the SDS.