脈衝雷射誘發雙氣泡間薄液層之不穩定性; Instability of thin liquid layers between two pulsed laser induced cavitation bubbles

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题名:	脈衝雷射誘發雙氣泡間薄液層之不穩定性;Instability of thin liquid layers between two pulsed laser induced cavitation bubbles
作者:	陳彥宏;Yen-Hung Chen
贡献者:	物理研究所
关键词:	氣泡;雷射;Bubble;Cavitation;Laser
日期:	2009-04-15
上传时间:	2009-09-22 10:59:40 (UTC+8)
出版者:	國立中央大學圖書館
摘要:	我們利用光學顯微鏡下的高速攝影技術來研究兩個鄰近的脈衝雷射所誘發的微米氣泡之間透過薄液層的交互作用。利用雙雷射光束精準地以不同雷射能量、發射時間、聚焦位置所産生的氣泡來研究被氣泡局限的薄液層在不同擠壓條件下的不穩定性。在對稱擠壓的情況下，被局限的薄液層可以潤滑快速靠近的氣泡表面而避免氣泡碰觸所造成的接合。一旦以不同雷射能量或發射時間來産生氣泡去破壞擠壓條件的對稱性，局限在氣泡間的潤滑液層會變成不穩定的狀態。在能量不對稱的情況，爆發性的沸騰效應導致氣泡團簇的形成進而使不穩定的潤滑液層破裂。受到氣泡團簇干擾所引起的潤滑液層不穩定性可藉由研究在氣泡間的潤滑液層以雷射産生的氣泡干擾後所得的結果而了解。在時間不對稱的情況下，氣泡團簇在受到壓力梯度而展開並且進入氣泡內的潤滑液層周圍形成。隨著時間不對稱的加強，氣泡間的薄液層不能再被局限住。進而我們研究一個極端的情況，一個在不受約束的氣泡表面邊的薄液層如何受到一個膨脹氣泡推擠的影響下造成形變。在不受約束的氣泡表面旁作非均向性膨脹的氣泡形成一個被液體薄層覆蓋的突起。隨著氣泡之間的距離減少，薄液層的破壞使得被推擠的液層形變從尖剌狀的突起轉變成冠狀的水花。此時，從氣泡間的接合處流入的液體噴射流發散且造成薄液層破壞。另外，我們發現不受約束的氣泡表面上的曲率會影響液層的形變。流入高曲率氣泡表面的入射水流會因此被集中，而此集中的入射水流會在另一個膨脹氣泡表面上造成反噴射流的形成。 The interaction of two nearby pulsed laser induced expanding bubbles mediated with a thin liquid layer in a confined liquid sheet is investigated experimentally using high-speed photography under the optical microscope. The precise manipulation of dual laser beams for generating bubbles at the different laser energies, onset times, and bubble positions enables the study of the instability of the trapped liquid layer under the different condition of compression. The thin liquid layer trapped by the two symmetrically growing bubbles lubricates the nearby bubble surfaces and prevents the bubble coalescence. As the symmetry is broken by generating two bubbles at the different laser energies or the onset times, the lubrication layer becomes unstable. In the energy-asymmetric case, the explosive boiling leads to the bubble cluster formation which destabilizes the lubrication layer. The breakup of the thin lubrication layer perturbed by the boiling induced bubbles can be understood by studying the thin lubrication layer perturbed by a pulsed laser generated defect bubble, where the single defect bubble causes the bubble cluster formation. In the time-asymmetric case, the bubble cluster appears near the lubrication layer which spreads into the bubble under the pressure gradient. The thin liquid layer cannot be trapped anymore as the time-asymmetry is enhanced. The deformation of the thin liquid layer near a free bubble surface repelled by an expanding bubble is studied in the highly time-asymmetric case. The protrusion of the anisotropically expanding bubble toward the free bubble surface is shelled by a thin liquid layer. As decreasing the inter-bubble distance, the deformation structure of the repelled thin liquid shell transits from the single spike to the crown-shaped splash, where the liquid jets flowing through the junctions between the two bubbles become divergent and make the thin liquid layer break. The curvature of the free bubble surface affects the deformation structure, where flow through the highly curved bubble surface is focused. Therefore, it causes the sharp transition of the deformation structure. The localized flow through the highly curved free surface leads to the counter-jet formation on the other expanding bubble.
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