Dongsik Kim

Enhancement of airborne shock wave by laser-induced breakdown of liquid column in laser shock cleaning

In laser shock cleaning (LSC), the shock wave is generated by laser-induced breakdown of the ambient gas. The shock wave intensity has thus been a factor limiting the performance of the LSC process. In this work, a novel method of amplifying a laser-induced plasma–generated shock wave by the breakdown of a liquid column is proposed and analyzed. When the laser beam is focused on a microscale liquid column, a shock wave having a significantly amplified intensity compared to that generated by air breakdown alone can be generated in air. Therefore, substantially amplified cleaning force can be obtained. The dynamics of a shock wave induced by a Q-switched Nd:YAG laser was analyzed by laser flash shadowgraphy. The peak pressure of the laser-induced shock wave was approximately two times greater than that of air breakdown at the same laser fluence. The proposed method of shock wave generation is expected to be useful in various applications of laser shock processing, including surface cleaning.

Wetting characteristics of CaO−SiO2−Al2O3 ternary slag on refractory oxides, Al2O3, SiO2 and TiO2

The wettabilities of the metallurgical slag, CaO−55.2 wt.%SiO2−15 wt.%Al2O3 on refractory oxides, Al2O3, SiO2 and TiO2 were investigated at temperatures of 1350, 1400 and 1470°C. Contact angle measurements were performed using a combination of the sessile drop method and the X-ray fluoroscopic technique. The contact angle was obtained from the numerical solution of the Young-Laplace equation. The steady contact angles obtained were 31°, 24°, and 15° for SiO2, Al2O3 and TiO2 at 1470°C, respectively. The spreading rate of the liquid slag was characterized by the formation of cognate interface between the liquid slag and solid oxide, and further enhanced by the formation of a diffuse layer around the slag drop at higher temperatures.

The Effects of Complex Task Presentation Strategies and Test Formats on Learning Outcomes, Cognitive Load, and Instructional Efficiency

The purpose of this study is to investigate the effects of complex task presentation strategies and test format on learning outcomes (retention and transfer), perceived cognitive load, and instructional efficiency. 155 middle school students were participated in the study, and were randomly assigned to two complex task presentation strategies(presenting task element interactivity simultaneously and presenting isolated element gradually) and each of the groups were divided into different test formats(restudy, completion task, and conventional problem solving). The results are as follow: for the transfer test, the completion task group showed significantly higher outcomes than the conventional problem solving and restudy groups, and the presenting isolated element gradually group showed significantly higher outcomes than the presenting element interactivity simultaneously group. Furthermore, the conventional problem solving group showed significantly lower cognitive load than the completion task and restudy groups. Lastly, the groups which engaged in the completion task showed significantly higher instructional efficiency than the conventional problem solving and restudy groups. These findings of the study partially support the testing effect, demonstrated by the group with completion task. Based on these findings, a conclusion could be made that the scaffolded test such as completion task would work as an effective stepping stone between the learning phase and the final test phase.