BUBBLE PINCH-OFF AND BREAKUP DUE TO INSTABILITY IN MICRO- JETTING S. Xiong 1 , T. Tandiono 2 , C. D. Ohl 3 and A. Q. Liu 1* 1 School of Electrical and Electronic Engineering, Nanyang Technological University, SINGAPORE 639798 2 Institute of High Performance Computing, A*STAR, SINGAPORE 138632 3 School of Physical & Mathematical Sciences, Nanyang Technological University, SINGAPORE 637371 ABSTRACT This paper presents the dynamics of microbubbles that are pinched-off and breakup in micro-jetting. A microjet is generated in two bubble interaction when a laser induced bubble is impacting a static gas bubble in a microchannel. Gas layers with a height of hundreds of nanometer are pinched off from the accelerated gas bubble wall during the jetting process. They are further modified by surface tension and break into small bubbles due to the fluidic instability. KEYWORDS: Microbubble, Jet, Instability INTRODUCTION Jetting is widely used in biomedical applications, such as drug delivery, cell lysis and microsurgery [1-3]. Complex behaviours are observed during high speed jetting [4]. For instance, during interpenetration process between two phases, a strong shear will destabilize the liquid jet, which further fragments into droplets. Jets originate from many different sources, such as liquid burst from nozzle, impact of a liquid container, and bubble collapse near a boundary [5-7]. Recently, jet generated by bubble-bubble interaction attracts attention due to its rich phenomena and its similar dynamics as jet close to an biomaterial. The mutual interaction between two laser induced bubbles associated with jetting and fragmentation have been addressed in a 10-µm high liquid gap [8]. Microjets have also been observed from the interaction of tandem microbubbles, which are generated by laser in a 25-µm liquid layer [9]. Cells placed on the axis of the jet are deformed, and highly localized membrane poration is demonstrated. In this paper, the jet is generated when a static gas bubble is impacted by a laser-induced bubble in a microchannel with height of 4 µm. During the jetting process, a part of the gas bubble is pinched off from the main body, and break into small bubbles. This novel phenomenon is caused by the confined boundary and instability in the microchannel. WORKING PRINCIPLE Figure 1 shows the schematic illustration of jets created by focusing a laser pulse inside a liquid filled microchannel. Optical breakdown leads to a bubble formation by absorption of the high laser energy. Bubble expansion is accompanied with the generation of pressure wave, which travels for a distance D and reflects on the surface of a gas bubble. Liquid is focused by the curved surface and forms a jet penetrating towards the centre of gas bubble. The jetting strength is depending on the laser energy (E) and the distance from the laser spot to the gas bubble surface (D). The displacement of the gas bubble surface is different in the vertical direction due to the high shear stress near the channel wall. Figure 2 shows the velocity profile of the impulsive flow in the microchannel with the height of 4 µm. Due Figure 2: Velocity profile of impulsive flow in a microchannel. D h Laser bubble Gas bubble u x Pulsed laser Figure 1: Schematic illumination of the jet formation in a microchannel. 978-0-9798064-6-9/µTAS 2013/$20©13CBMS-0001 71 17th International Conference on Miniaturized Systems for Chemistry and Life Sciences 27-31 October 2013, Freiburg, Germany