Dynamic behaviors of approximately ellipsoidal microbubbles photothermally generated by a graphene oxide-microheater

OPEN SUBJECT AREAS: FLUID DYNAMICS OPTICAL MANIPULATION AND TWEEZERS Received 16 April 2014 Accepted 29 July 2014 Published 15 August 2014 Dynamic behaviors of approximately ellipsoidal microbubbles photothermally generated by a graphene oxide-microheater Xiaobo Xing1*, Jiapeng Zheng2*, Fengjia Li1, Chao Sun1, Xiang Cai3, Debin Zhu1, Liang Lei4, Ting Wu3, Bin Zhou2, Julian Evans2 & Ziyi Chen2 1 Education Ministry’s Key Laboratory of Laser Life Science & Institute of Laser Life Science, College of Biophotonics, South China Normal University, Guangzhou, Guangdong, 510631, China, 2Centre for Optical and Electromagnetic Research, South China Academy of Advanced Optoelectronics, South China Normal University, 510006 Guangzhou, China, 3Department of Light chemical engineering, Guangdong Polytechnic, Foshan, 528041, China, 4School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, China. Correspondence and requests for materials should be addressed to X.X. (xingxiaobo@ scnu.edu.cn) or D.Z. () * These authors Thermal microbubbles generally grow directly from the heater and are spherical to minimize surface tension. We demonstrate a novel type of microbubble indirectly generated from a graphene oxide-microheater. Graphene oxide’s photothermal properties allowed for efficient generation of a thermal gradient field on the microscale. A series of approximately ellipsoidal microbubbles were generated on the smooth microwire based on heterogeneous nucleation. Other dynamic behaviors induced by the microheater such as constant growth, directional transport and coalescence were also investigated experimentally and theoretically. The results are not only helpful for understanding the bubble dynamics but also useful for developing novel photothermal bubble-based devices. contributed equally to this work. M icroscale bubbles have found many applications in medical imaging1, biomedical analysis2, drug delivery3, and microfluidic elements4–7. Various methods have been adopted to generate microbubbles, including ultrasonic cavitation8, laser induced cavitation9,10, microfluidic flow-focusing11, and liquid boiling12. Photothermal heaters produced by laser light combined together with photothermal materials can carry on localized heating to initiate the phase transition of the liquid13–18. Previous reports have shown that the microbubbles can be successfully produced by using highly focused laser beams or optical fiber to illuminate photothermal substrates13–15, liquids16, and nanoparticles17,18. Compared with other systems8–11, photothermal system allows for accurately controlling the forming location and the status of microbubbles. The microheater has a dominant influence on the growth and movement of microbubbles. For example, it has demonstrated that the growth rate of microbubbles can be tuned by the power of the microheater15,16. The microbubbles are attracted towards the microheater by thermocapillary forces and Marangoni forces14,19. So far, the reported photothermal microbubbles were generated directly on the microheater13–18. Whether the microbubbles can be emerged away from the microheater? It is an interesting topic, which has not been detailedly explored. Our previous study has been demonstrated a novel microscale photothermal heater based on the combination of a microwire and graphene oxide (GO)20. GO nanosheets (GONs) in N,N-Dimethylformamide (DMF) solvent were successfully trapped and deposited on the surface of the microwire with enhanced evanescent fields21–24 by injecting near-infrared light into the microwire. The GO-deposition exhibited strong photothermal properties25–29, serving as a line-shaped heater with a length of several hundred micrometers. When the superheat limit of liquid was reached, a lot of spherical microbubbles with diameters from tens to hundreds of micrometers were generated directly on the GO-microheater immersed in DMF, which is a common phenomenon in a traditional photothermal system13–18. The present work is devoted to novel microbubbles with approximately ellipsoidal shapes that produced on the smooth microwire rather than on the microheater. Similar to liquid drops on thin wires30,31, each microbubble exhibited a symmetrical conformation with respect to the microwire axis. Bubble behaviors induced by the microheater including constant growth, directional transport, and coalescence were investigated experimentally and theoretically, showing the similar behaviors to the reported photothermal SCIENTIFIC REPORTS | 4 : 6086 | DOI: 10.1038/srep06086 1 www.nature.com/scientificreports microbubble14,19. The existence of the approximately ellipsoidal microbubbles will gain new insight into the mechanism of bubble formation and bubble dynamics at the microscale. Results Figure 1a shows schematic illustration of experimental setup to obtain GO-microheater. An inverted optical microscope with a charge-coupled device (CCD) camera was used for real-time monitoring. A liquid cell was mounted on an x-y manual translation stage of the microscope. A 2.2-mm diameter, 2.0-mm long microwire was fabricated by drawing a single mode optical fiber (SMF-28, Corning Inc.) using two stepper motors through the flame-heated technique. The microwire was immersed in the GONs-DMF suspension and fixed by two microadjusters. Figure 1b shows a scanning electron microscopy (SEM) image of 2.2-mm-diameter wire with good surface smoothness. An amplified spontaneous emission broadband light source (ASE, ,20 mW, 1527–1566 nm) was excited by an erbium-doped fiber amplifier (EDFA, 1546–1562 nm), whose output power was 40 mW at 1527–1566 nm. Light from EDFA was coupled into the microwire, and output spectra were recorded by an optical spectrum analyzer (OSA, Yokogawa AQ 6370C) with a resolution of 0.02 nm. Transmission electron microscopy (TEM, Fig. 1c) and SEM (see supplementary information, Fig. S1) images show that GONs have scale-like shape with micrometer long wrinkles. In the experiment, the concentration of GONs-DMF suspension was optimized at 0.05 mg/ml. Since a higher concentration could affect imaging due to absorption, and a lower concentration would limit the quantity of GO-deposition. Mode characteristics of the microwire were analyzed in the supplementary information, demonstrating a 2.2-mm-diameter wire submerged in DMF produced a tight field confinement and strong evanescent field. Figure 1d shows a 2D field profile of the HE11 mode at wavelength of 1550 nm. Effective diameter of mode field (Deff) and power ratio outside the microwire (g) of a 2.2-mm-diameter wire were 12.23–13.76 mm and 86.59–88.32% at wavelength of 1527– 1566 nm (see supplementary information, Fig. S1). When the light propagated along the microwire, the evanescent field outside the microwire was absorbed by GONs, producing a thermal gradient based on effective optical-to-thermal energy conversion of GONs20. And then a weak convective flow was gen (...truncated)