Raindrops are tiny lightning bolts, and they’re corroding cars, study finds
Raindrops are tiny lightning bolts, and they’re corroding cars, study finds
研究发现：雨滴如同微型闪电，正在腐蚀汽车
The standard explanation for why rain causes corrosion is that water carries dissolved salts and acids to a surface, constant drumming of raindrops abrades whatever protective coating is on it, and oxygen does the rest. Nearly all our tools to prevent this—paints, polymer films, or oxide layers—are built around this idea. But we’ve apparently been missing something important about the rain. 关于雨水为何会导致腐蚀，标准的解释是：水将溶解的盐分和酸性物质带到物体表面，雨滴不断的敲击磨损了表面的保护层，剩下的工作则由氧气完成。我们几乎所有用于防腐的手段——油漆、聚合物薄膜或氧化层——都是基于这一理念构建的。但显然，我们忽略了关于雨水的一个重要事实。
A new study led by Zhongyuan Ni, Rüdiger Berger, and Hans-Jürgen Butt at the Max Planck Institute for Polymer Research in Mainz, Germany, has shown that water drops routinely arrive at a surface carrying an electrical charge large enough to punch through an insulating coating. Not scratch it. Not slowly dissolve it. Electrically blow a hole in it, the way a spark jumps a gap. 德国美因茨马克斯·普朗克高分子研究所的倪中元（音译）、Rüdiger Berger 和 Hans-Jürgen Butt 领导的一项新研究表明，水滴在落到物体表面时，通常携带足以击穿绝缘涂层的电荷。不是刮擦，也不是缓慢溶解，而是像火花跳过间隙一样，通过电击在涂层上炸出一个洞。
Charged rain
带电的雨水
The starting point of the study is a phenomenon called “slide electrification,” which has only been properly quantified in the past few years. When a water drop slides across an insulating surface like a leaf, a painted wall, a windowpane, or a plastic panel, it strips charge from that surface and leaves an opposing charge behind. The voltages involved are not trivial. Drops charged this way have been measured at up to 9,000 volts. 这项研究的起点是一种被称为“滑动起电”的现象，该现象直到近几年才被准确量化。当水滴在叶片、漆墙、窗玻璃或塑料板等绝缘表面上滑动时，它会从表面剥离电荷，并留下相反的电荷。其产生的电压不容小觑，经测量，这种方式带电的水滴电压最高可达 9,000 伏。
The question Ni, Berger, Butt, and their colleagues asked was what this charge does to the surface that the drop lands on next. To find out, the team released 35-microliter water drops, about the size of a large raindrop, containing a pinch of salt to mimic rainwater, onto a surface tilted at 50 degrees. The drops slid about four centimeters, picked up a charge, rolled off the edge, and then fell five millimeters onto a copper plate coated with a 60-nanometer film of Teflon, which is one of the most chemically resistant coatings on the market today. 倪中元及其同事提出的问题是：这些电荷会对水滴随后落下的表面产生什么影响？为了找出答案，研究小组将 35 微升（约大雨滴大小）含有少量盐分以模拟雨水的水滴，滴在倾斜 50 度的表面上。水滴滑动约 4 厘米后带上电荷，从边缘滚落，随后下落 5 毫米，撞击到一块涂有 60 纳米特氟龙薄膜的铜板上。特氟龙是目前市面上化学稳定性最好的涂层之一。
The tilted surfaces were mostly chosen to mimic raindrops in the real world. One was a leaf from a Tradescantia spathacea plant growing in one of the researchers’ offices. Another was a PVC foam board from a hardware store. The third one was a sheet of transparent polystyrene sold as window glazing. Only the fourth one, the fluorinated coating on quartz, was more of a lab creation than something people usually see everywhere around them. 这些倾斜表面大多是为了模拟现实世界中的雨滴环境。其中一个是研究人员办公室里种植的紫背万年青叶片；另一个是五金店买的 PVC 泡沫板；第三个是作为窗户玻璃销售的透明聚苯乙烯板。只有第四种——石英上的氟化涂层——更像是实验室产物，而非人们随处可见的材料。
The charges the drops picked up ranged from 0.2 nanocoulombs off the leaf to two nanocoulombs off the fluorinated quartz. A nanocoulomb in something the size of a raindrop works out to be a few thousand volts. After 3,000 drops, roughly equivalent to an afternoon of moderate rain, the copper plate beneath all four surfaces had corroded, despite its Teflon coating. Atomic force microscopy of the impact zones found pits several nanometers deep in places deeper than the entire thickness of the Teflon film, meaning the damage ran clean through the coating and into the metal. Drops that fell directly onto the target without sliding first (and therefore carried no charge) left the surface pristine after the same 3,000 impacts. 水滴携带的电荷量从叶片上的 0.2 纳库仑到氟化石英上的 2 纳库仑不等。对于雨滴大小的物体来说，1 纳库仑的电荷量相当于几千伏的电压。在 3,000 次滴落（相当于一个下午的中雨）后，尽管有特氟龙涂层保护，四种表面下的铜板均出现了腐蚀。原子力显微镜观察撞击区发现，有些地方出现了深达数纳米的凹坑，甚至深过特氟龙薄膜的总厚度，这意味着损伤已经穿透涂层直达金属。而那些没有经过滑动、直接落到目标上（因此不带电）的水滴，在同样的 3,000 次撞击后，表面依然完好无损。
A spark
火花
The scientists managed to catch the mechanism in the act with high-speed cameras. In the videos, researchers saw that a neutral drop approaching the copper plate keeps a smooth, round bottom right up until contact. A charged drop does something very different. As it nears the surface, its underside stretches into a Taylor cone—a sharp shape a liquid takes when electrostatic force overwhelms its own surface tension. The Taylor cone was a sign that the electric field between a drop and metal had gotten strong enough to deform water. 科学家们利用高速摄像机捕捉到了这一过程。视频显示，中性水滴在接近铜板时，底部始终保持平滑圆润，直到接触瞬间。而带电水滴的表现则截然不同：当它接近表面时，底部会拉伸成一个“泰勒锥”（Taylor cone）——这是当静电力超过液体自身表面张力时，液体呈现的一种尖锐形状。泰勒锥的出现表明，水滴与金属之间的电场强度已足以使水发生形变。
The field’s strength should keep climbing as the gap closes. To quantify this effect, the team modeled the drop as a conducting sphere hovering over a conducting wall, then calculated how the field strength scales with distance. A drop carrying two nanocoulombs, they found, reaches 60 kilovolts per millimeter, the breakdown threshold of Teflon—the electric field strength at which an insulator stops insulating. This happens while the drop is still roughly 10 micrometers away from the surface. For a polystyrene coating, which gives up at 19 kilovolts per millimeter, breakdown happens 50 micrometers out. 随着间隙缩小，电场强度会持续攀升。为了量化这一效应，研究小组将水滴建模为悬浮在导电壁上的导电球，并计算了电场强度随距离的变化。他们发现，携带 2 纳库仑电荷的水滴，其电场强度可达每毫米 60 千伏，这正是特氟龙的击穿阈值（即绝缘体失去绝缘能力的电场强度）。这一现象发生在水滴距离表面约 10 微米时。对于击穿阈值为每毫米 19 千伏的聚苯乙烯涂层，击穿则发生在距离表面 50 微米处。
Because the coating stops being an insulator when its breakdown threshold is reached, the charge rips through it in a miniature dielectric breakdown, the same failure mode that kills capacitors and transformer insulation. The charge measurements scientists conducted confirm the transfer is nearly total—a drop arriving with two nanocoulombs dumped 1.8 of them into the copper on impact and bounced away with 0.016 nanocoulombs, less than 1 percent of what it started with. 由于涂层在达到击穿阈值后不再是绝缘体，电荷会通过微型介电击穿瞬间穿透涂层，这与导致电容器和变压器绝缘失效的模式相同。科学家的电荷测量证实，电荷转移几乎是完全的——一个携带 2 纳库仑电荷的水滴在撞击时向铜板释放了 1.8 纳库仑，反弹离开时仅剩 0.016 纳库仑，不到初始电荷的 1%。
Because the amount of charge scales with how far a drop slides and the coating’s ability to resist scales with its thickness, the effect has a limit. Twelve-micrometer polystyrene films got punched through while 130-micrometer films survived. Most of the paint coatings we use are just a few micrometers thick, and the researchers note that nanocoulomb-scale charges can break through most of these. 由于电荷量与水滴滑动的距离成正比，而涂层的抵抗能力与厚度成正比，因此这种效应存在极限。12 微米的聚苯乙烯薄膜被击穿了，而 130 微米的薄膜则幸免于难。我们使用的大多数油漆涂层仅有几微米厚，研究人员指出，纳库仑级的电荷足以击穿其中大部分涂层。
The damage
损伤
Scientists found that once the coating has been breached, the exposed metal sits in a salty drop with a fresh electrical potential across it. This leaves the ordinary electrochemistry that causes corrosion free to work on the surface it was supposed to be locked out of. The researchers identified the corrosion products on copper by Raman spectroscopy and X-ray diffraction. They found cuprous oxide and basic cupric chloride, the pale green compound familiar from weathered copper roofs. 科学家发现，一旦涂层被破坏，暴露的金属就会浸泡在含有电势差的盐水滴中。这使得导致腐蚀的常规电化学反应能够在其本应被隔绝的表面上自由进行。研究人员通过拉曼光谱和 X 射线衍射鉴定了铜表面的腐蚀产物，发现了氧化亚铜和碱式氯化铜——即我们在风化的铜屋顶上常见的淡绿色化合物。
Elemental mapping of the damaged zones showed oxygen and chlorine flooding in and fluorine and carbon from the Teflon flooding out—exactly what should happen when a coating has been disrupted. The polymer itself gets chemically rearranged, too. Polystyrene films hit by charged drops developed rough patches that glowed green under a laser, which plain polystyrene does not do. 对受损区域的元素映射显示，氧和氯涌入，而特氟龙中的氟和碳流失——这正是涂层被破坏后应有的表现。聚合物本身也发生了化学重排。被带电水滴撞击后的聚苯乙烯薄膜出现了粗糙斑块，在激光照射下会发出绿光，而普通的聚苯乙烯则不会。