• Journal of the Korean Society of Visualization
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• v.21 no.2
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• pp.91-101
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• 2023

In this study, we investigated how the performance of a Venturi changes when a hemispherical bump is applied to the divergent part of the Venturi tube and what causes the performance difference. The Venturi-tunnel experiment was conducted in the Reynolds number range of 0.2 × 10⁵ - 1.2 × 10⁵ and cavitation number range of 0.9 - 10. The bump was found to reduce the pressure loss coefficient and increase the discharge coefficient by shortening the cavitation length. The decrease in the cavitation length by the bump was explained by the strengthening of the re-entrant jet. The wake generated from the hemispherical bump seems to increase the adverse pressure gradient on the Venturi surface, thereby strengthening the re-entrant jet.

• Transactions of the Korean Society of Mechanical Engineers A
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• v.28 no.8 s.227
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• pp.1087-1092
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• 2004

Adhesion and friction forces influence adversely on performance and durability of MEMS. It has been reported that the adhesion and friction forces can be reduced with the introduction of micro surface bumps into the contacting interfaces. In this study experiments were conducted to investigate comparatively the effect of hemispherical and torus micro bumps on the adhesion and friction forces. It is confirmed that micro bumps reduce the adhesion and friction forces, and their effect is more pronounced with the bumps of smaller outer boundary radius. Moreover, the results shows that the torus bumps exhibit more rapid decrease of the adhesion and friction forces with the decrease in the outer boundary radius of bump than the hemispherical bumps. When the magnitude of adhesion force is same, the torus bumps generate smaller friction force than the hemispherical bumps. The usage of hemispherical and torus bumps to reduce the adhesion and friction forces in MEMS is discussed.

• Tribology and Lubricants
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• v.32 no.3
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• pp.75-81
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• 2016

Future electronics such as electronic paper and foldable cellphone are required to be flexible and transparent and should have a high performance. In order to fabricate the flexible electronics using flexibility transfer process, techniques for transferring various devices from rigid substrate onto flexible substrate by elastomeric stamp, have been developed. Adhesion between the elastomeric stamp and various devices is crucial for successful transfer process. The adhesion can be controlled by the thickness of the stamp, separation velocity, contact load, and stamp surface treatment. In this study, we fabricated the bump shape stamp consisting of a UV-curable polymer and investigated the effects of curing condition, separation velocity, and contact load on the adhesion characteristics of bumps. The bumps with hemispherical shape were fabricated using a dispensing process, which is one of the ink-jet printing techniques. Curing conditions of the bumps were controlled by the amount of UV irradiation energy. The adhesion characteristics of bumps are evaluated by adhesion test. The results show that the pull-off forces of bumps were increased and decreased as UV irradiation energy increased. For UV irradiation energies of 300 and 500 mJ/cm², the pull-off forces were increased as the separation velocity increased. The pull-off forces also increased with the increase of contact load. In the case of UV irradiation energy above 600 mJ/cm², however, the pull-off forces were not changed. Therefore, we believe that the bump shape stamp can be applied to roll-based transfer process and selective transfer process as an elastomeric stamp.