科学家跟踪观察沸水纳米气泡由生到灭过程

可应用于打印机和癌症治疗

Traces of nanobubbles determine nano-boiling

       Using a microscope and some extreme “snapshot” photography with shutter speeds only a few nanoseconds long, researchers from the National Institute of Standards and Technology and Cornell University have uncovered the traces of ephemeral “nanobubbles” formed in boiling water on a microheater. Their observations suggest an added complexity to the everyday phenomenon of boiling, and may affect technologies as diverse as inkjet printers and some proposed cancer therapies.

      You might think that the science of boiling had been worked out some time ago, but it still has some mysteries, particularly at the nanometer scale. As water and other fluids change from their liquid state to a vapor, bubbles of the vapor form. The bubbles usually form at “nucleation sites,” which can be small surface irregularities on the container or tiny suspended particles in the fluid. The exact onset of boiling depends on the presence and nature of these sites.

      To observe the process, the NIST/Cornell team used a unique ultrafast laser strobe microscopy technique with an effective shutter speed of eight nanoseconds to photograph bubbles growing on a microheater surface about 15 micrometers wide. At this scale, a voltage pulse of only five microseconds superheats the water to nearly 300 °C, creating a microbubble tens of microns in diameter. When the pulse ends, the microbubble collapses as the water cools.

      What the team found was that if a second voltage pulse follows closely enough, the second microbubble forms earlier during the pulse and at a lower temperature apparently, as conjectured by the team, because nanobubbles formed by the collapse of the first bubble become new nucleation sites for the growth of later bubbles. The nanobubbles themselves are too small to observe, but by changing the timing between voltage pulses and observing how long it takes the second microbubble to form, the researchers were able to estimate the lifetime of the nanobubbles—roughly 100 microseconds.

      These experiments are believed to be the first evidence that nanoscale bubbles can form on hydrophilic surfaces (previous evidence of nanobubbles was found only for hydrophobic surfaces like oilcloth) and the method for measuring nanobubble lifetimes may improve models for optimal heat transfer design in nanostructures. The work has immediate implications for inkjet printing, in which a metal film is heated with a voltage pulse to create a bubble that is used to eject a droplet of ink through a nozzle. If inkjet printing is pushed to higher speeds (repetition rates above about 10 kilohertz), the work suggests, nanobubbles on the heater surface between pulses will make it difficult or impossible to control bubble formation properly.

      The findings also may impact proposed thermal cancer therapies in which nanoscale objects are designed to accumulate in tumors and are subsequently heated remotely by infrared radiation or alternating magnetic fields. Each particle acts as a nanoscale heater, with nanobubbles being created if the applied radiation is sufficient. The bubbles may have a therapeutic effect through additional heat delivered and mechanical stresses they may impart to the surrounding tissue.

以下为译文：

气泡痕迹，确定纳米沸点 

      使用显微镜和一些极端的“快照”摄影的快门速度只有几纳秒的时间长，从国家标准和技术研究所和美国康奈尔大学的研究人员已经发现了短暂的沸水中形成一个微换热器“气泡”的痕迹。 他们的意见，建议增加的复杂性，沸腾的日常现象，可能会影响不同的喷墨打印机和一些建议癌症治疗技术。

      你可能会认为，沸腾的科学已经制定了前一段时间，但它仍然有一些奥秘，特别是在纳米尺度。 由于水和其他流体的变化从他们的液体状态，蒸气，蒸气形式气泡。 气泡通常会形成“核点”，可以在小容器或流体中的微小悬浮颗粒表面的不规则。 沸腾的确切发病取决于这些网站的存在和性质。

      观察的过程中，八纳秒有效的快门速度成长为一个大约宽15微米的微加热器表面的照片气泡的NIST /康奈尔大学的研究小组用一种独特的超快激光频闪显微技术。 在这个规模，一个只有五微秒的脉冲电压，过热温度的水近300°C间，建立在微泡直径几十微米。 当脉冲结束后，气泡崩溃的水冷却。

      该研究小组发现，如果第二个电压脉冲紧随不够，第二个微气泡形式在脉搏，在较低温度下很显然，作为推测的球队，因为气泡形成的第一个泡沫崩溃成为新的核点后来泡沫的增长。 气泡自己太小观察，但研究人员通过改变之间的电压脉冲和观察需要多久的第二个微泡形式的时间，能够估计的气泡，大约100微秒的寿命。

      这些实验被认为是第一个证据，纳米气泡可以形成亲水性的表面（气泡以前的证据被发现，只有像油布疏水表面）和气泡寿命的测量方法，可以提高在纳米结构的热传递最佳设计车型。 这项工作有直接影响喷墨打印，在一个金属膜加热电压脉冲创建一个用来弹出通过一个喷嘴墨滴泡沫。 如果喷墨打印推到更高的速度（约10千赫以上的重复率）的工作表明，脉冲之间的加热器表面的气泡将使其难以或无法控制泡沫形成正确。

      调查结果还可能影响拟议热癌症纳米级物体的目的是积聚在肿瘤，随后由红外辐射或交替磁场加热远程治疗。 每个粒子作为纳米级加热器，正在创建的气泡如果应用的辐射是足够的。气泡可能通过额外提供热和机械应力，他们可以传授给周围组织有治疗效果。

来源：http://www.physorg.com/news94493776.html