We Know Simple Fluids Can Flow. Turns Out, Some Can Fracture.

When pulled at 100 millimeters per second, a blend of hydrogen and carbon stretches.

当以每秒 100 毫米的速度拉动时，氢和碳的混合物就会拉伸。

At 300 millimeters per second, the fluid breaks.

以每秒 300 毫米的速度，液体就会破裂。

Adapted with permission from Phys.

经物理许可改编。

Rev.

牧师。

Lett.

莱特。

136, 124002 .

136, 124002 .

Copyrighted by the American Physical Society.

美国物理学会版权所有。

Thamires Lima , a research professor in chemical engineering at Drexel University, studies the properties of thick, viscous liquids — think honey or molasses, though in a lab you’re more likely to find polypropylene or crude oil.

德雷克塞尔大学化学工程研究教授泰米尔斯·利马（Thamires Lima）研究粘稠液体的特性——想想蜂蜜或糖蜜，尽管在实验室里你更有可能找到聚丙烯或原油。

Using a method called extensional rheology, Lima stretches liquids between metal plates to find the force that makes them flow.

利马使用一种称为拉伸流变学的方法在金属板之间拉伸液体，以找到使它们流动的力。

A few years ago, she was conducting a test as part of a project in collaboration with the oil and gas company Exxon Mobil when she heard a short, sharp crack.

几年前，她正在与石油天然气公司埃克森美孚合作进行一项测试，当时她听到一声短促的尖锐破裂声。

“I thought it was the machine,” Lima said.

“我以为是机器，”利马说。

But the crack came from the fluid that the machine was pulling: a gooey, black blend of hydrogen and carbon.

但裂缝来自于机器拉动的液体：一种粘稠的、黑色的氢和碳混合物。

Instead of stretching, the fluid had fractured.

液体没有拉伸，而是破裂了。

Fractures are known to occur in certain elastic complex fluids, which can act like solids under certain conditions.

已知某些弹性复杂流体会发生断裂，这些流体在某些条件下可以像固体一样发挥作用。

But Lima was working with a nonelastic simple fluid.

但利马正在研究一种非弹性简单流体。

Even with almost no elasticity, it snapped apart under stress.

即使几乎没有弹性，它也会在压力下破裂。

“Nobody expected that this would be possible in this kind of simple fluid because viscosity usually just rearranges the molecules,” said Arnold Mathijssen , a fluid physicist at the University of Pennsylvania.

宾夕法尼亚大学流体物理学家阿诺德·马泰森 (Arnold Mathijssen) 表示：“没有人想到这种简单的流体会发生这种情况，因为粘度通常只会重新排列分子。”

“You don’t expect it to crack.

“你不会指望它会破裂。

But it does, so I think that’s what’s really surprising.”

但事实确实如此，所以我认为这才是真正令人惊讶的地方。”

Lima stretched the liquid again and again to prove that the unexpected crack wasn’t a one-off.

利马一次又一次地拉伸液体，以证明意外的裂缝不是一次性的。

“Every time that she measured it, the material would break,” said Nicolas J.

“每次她测量时，材料都会破裂，”Nicolas J.

Alvarez , the professor of chemical engineering at Drexel University whose lab led the research.

德雷克塞尔大学化学工程教授阿尔瓦雷斯（Alvarez）的实验室领导了这项研究。

“It makes a loud pop.

“它会发出很大的爆裂声。

I mean, like you just took a rubber band and pulled it and stretched it and it snapped.”

我的意思是，就像你拿起一根橡皮筋，拉动它，然后拉伸它，它就会折断。”

Thamires Lima , a research professor in chemical engineering at Drexel University, was stretching a liquid in an extensional rheometer when she heard a short, sharp crack.

德雷克塞尔大学化学工程研究教授泰米尔斯·利马（Thamires Lima）正在拉伸流变仪中拉伸液体，突然听到一声短促的尖锐破裂声。

Convinced the snap wasn’t a fluke, Lima and Alvarez used high-speed cameras to look at the phenomenon more closely.

利马和阿尔瓦雷斯确信这张照片并非偶然，因此使用高速摄像机更仔细地观察了这一现象。

They realized that the break was essentially a “brittle fracture,” the kind you might see when you drop a dish made of glass or porcelain.

他们意识到这种断裂本质上是一种“脆性断裂”，就是当你掉落玻璃或瓷器制成的盘子时可能会看到的那种断裂。

Brittle fractures happen to brittle solids, which have elasticity.

脆性断裂发生在具有弹性的脆性固体上。

Apply some stress to glass or porcelain and it deforms a very tiny bit, and then — if you don’t push it past its breaking point — it springs back to normal once the stress is removed.

对玻璃或瓷器施加一些压力，它会变形一点点，然后——如果你不将其推过其断裂点——一旦压力消除，它就会恢复正常。

However, solids are never perfect.

然而，固体从来都不是完美的。

In most cases, a brittle solid will have a teeny, tiny defect — a crack at the scale of tens of nanometers.

在大多数情况下，脆性固体都会有一个微小的缺陷——几十纳米尺度的裂纹。

Once the solid is stressed past a critical point, it becomes energetically more favorable for the solid to grow the crack than to elastically store the stress.

一旦固体受到的应力超过临界点，固体在能量上就变得更有利于裂纹扩展，而不是弹性存储应力。

At that point, the crack grows catastrophically, rapidly breaking the solid apart.

此时，裂纹会灾难性地增长，迅速将固体分解。

Some complex fluids, called viscoelastic liquids, also have elasticity.

一些复杂的流体，称为粘弹性液体，也具有弹性。

For example, polymer melts — melted versions of the polymers in plastics — are made up of long chains of molecules, which become entangled with one another and increase the material’s elastic component.

例如，聚合物熔体（塑料中聚合物的熔化版本）由长链分子组成，这些分子链相互缠结并增加了材料的弹性成分。

In a 2016 Physical Review Letters paper , Alvarez and colleagues showed that complex fluids like melted polystyrene can fracture in the same way that solids sometimes do.

在 2016 年《物理评论快报》的一篇论文中，阿尔瓦雷斯和同事表明，像熔化的聚苯乙烯这样的复杂流体有时会像固体一样破裂。

“We just thought elasticity was something that was a prerequisite for such solid type of breaking, right?” Alvarez said.

“我们只是认为弹性是这种坚固类型断裂的先决条件，对吗？”阿尔瓦雷斯说道。

As a result, they theorized that elasticity was related to the fracture of liquids as well.

因此，他们推测弹性也与液体的破裂有关。

But the hydrocarbon blend that Lima was working with was a simple fluid.

但利马正在研究的碳氢化合物混合物是一种简单的流体。

Simple fluids don’t store much elastic energy.

简单的流体不会储存太多的弹性能。

And when they are pushed or pulled past their limits, they don’t usually bend or break — they flow.

当它们被推或拉超过极限时，它们通常不会弯曲或断裂 - 它们会流动。

Nobody expected that this would be possible in this kind of simple fluid because viscosity usually just rearranges the molecules.

没有人预料到这在这种简单的流体中是可能的，因为粘度通常只是重新排列分子。

Arnold Mathijssen, a fluid physicist at the University of Pennsylvania

Arnold Mathijssen，宾夕法尼亚大学流体物理学家

So perhaps the old theory about what makes a liquid fracture is wrong.

因此，关于液体破裂原因的旧理论也许是错误的。

“If there is no elasticity in a problem, then how can you think about initiation or growth of a crack?” said Brato Chakrabarti , a physicist who works on fluid mechanics at the International Center for Theoretical Sciences in Bengaluru, India.

“如果问题没有弹性，那么你如何考虑裂纹的产生或扩展？”在印度班加罗尔国际理论科学中心从事流体力学工作的物理学家布拉托·查克拉巴蒂 (Brato Chakrabarti) 说道。

The cracking of the hydrocarbon blend made the researchers look back at the papers of Daniel D.

碳氢化合物混合物的裂解让研究人员回顾了 Daniel D.

Joseph, a mechanical engineer at the University of Minnesota.

约瑟夫是明尼苏达大学的机械工程师。

In 1995 and 1998, Joseph suggested that any liquid, regardless of how elastic it is, could fracture under a sufficient amount of tearing stress .

1995年和1998年，约瑟夫提出任何液体，无论其弹性如何，在足够大的撕裂应力下都可能破裂。

Alvarez wonders if the breaking point of a liquid is related not to a property like elasticity, but to something more fundamental to the liquid’s structure.

阿尔瓦雷斯想知道液体的断裂点是否与弹性等属性无关，而是与液体结构的更基本的东西有关。

“Maybe, just maybe, the thing that causes [certain] fluids to break …

“也许，只是也许，导致[某些]液体破裂的原因......

[is] somehow related to this cohesive energy that holds the molecules together,” he said.

[是]在某种程度上与将分子结合在一起的内聚能有关，”他说。

Simple fluids do have a way of relieving stress, no breaking required: They form intermolecular voids (bubbles) in a process called cavitation.

简单流体确实有一种无需破裂即可缓解压力的方法：它们在称为空化的过程中形成分子间空隙（气泡）。

If the blades of a propeller spin rapidly in a simple fluid, for example, the fluid on one side of the blade can slosh much faster than the fluid on the other, leading to a drop in pressure on that side.

例如，如果螺旋桨的叶片在简单流体中快速旋转，则叶片一侧的流体会比另一侧的流体晃动得快得多，从而导致该侧的压力下降。

This drop can cause the liquid to cavitate.

该液滴会导致液体产生空化。

Engineers work to avoid this, because once those bubbles collapse, they generate shock waves that can damage propellers and pumps.

工程师们努力避免这种情况，因为一旦这些气泡破裂，它们就会产生冲击波，从而损坏螺旋桨和泵。

In his papers in the ’90s, Joseph predicted that cavitation would allow simple fluids to fracture.

约瑟夫在 90 年代的论文中预测，空化现象会使简单流体破裂。

“If you think about what holds a fluid together, it’s cohesiveness, or the intermolecular interactions between the molecules,” Alvarez said.

阿尔瓦雷斯说：“如果你考虑是什么将流体保持在一起，那就是内聚力，或者分子之间的分子间相互作用。”

If you pull those molecules apart, you can create a bubble.

如果将这些分子拉开，就可以产生气泡。

Usually, viscous liquids stay cohesive when bubbles form, by changing shape around them.

通常，当气泡形成时，粘性液体会通过改变气泡周围的形状来保持粘性。

But if enough bubbles form in quick succession, they could theoretically crack a liquid like a pane of glass.

但如果快速连续形成足够多的气泡，理论上它们可以像玻璃一样使液体破裂。

At Drexel, the researchers found that once a crack nucleates inside a simple fluid, it propagates extremely fast, precisely because the fluid is not elastic.

在德雷克塞尔大学，研究人员发现，一旦裂纹在简单流体内成核，它就会以极快的速度传播，这正是因为流体没有弹性。

“If you can get that nucleation event of the crack to begin, because there is no elasticity in the material, that crack can propagate as fast as physics will allow it,” Alvarez said.

阿尔瓦雷斯说：“如果你能让裂纹的成核事件开始，因为材料没有弹性，那么裂纹就会以物理学允许的速度传播。”

In previous work on complex fluids, the Drexel researchers found that cracks in melted polystyrene propagate at approximately 0.07 meters per second.

在之前对复杂流体的研究中，德雷克塞尔大学的研究人员发现，熔化的聚苯乙烯中的裂纹以每秒约 0.07 米的速度扩展。

In their new study, Lima and colleagues showed that cracks propagate far more rapidly in the simple liquids they studied, reaching velocities of approximately 500 to 1,500 meters per second.

在他们的新研究中，利马和同事表明，裂缝在他们研究的简单液体中传播得更快，达到每秒约 500 至 1,500 米的速度。

“That has something to do with the way that the material is able to dissipate energy,” Alvarez said.

“这与材料耗散能量的方式有关，”阿尔瓦雷斯说。

According to one hypothesis, in a complex fluid, energy is absorbed by the long chains of molecules as they break.

根据一种假设，在复杂的流体中，长链分子在断裂时会吸收能量。

But in a simple fluid, “there’s really nothing to slow that crack down,” he said.

但在简单的流体中，“确实没有什么可以减缓这种打击，”他说。

This seems to affect the shape of the crack, which in complex fluids looks like the horn of a trumpet and in simple fluids looks like a crack moving through glass, the researchers found.

研究人员发现，这似乎会影响裂缝的形状，在复杂的流体中，裂缝看起来像喇叭，在简单的流体中，裂缝看起来像穿过玻璃的裂缝。

Surprisingly, despite their different ways of cracking, both the complex fluids and the simple fluids that researchers tested tended to fracture at the same critical measure of stress: 2 megapascals.

令人惊讶的是，尽管复杂流体和简单流体的破裂方式不同，但研究人员测试的复杂流体和简单流体都倾向于在相同的临界应力下破裂：2兆帕。

The researchers varied the temperature of the hydrocarbon blend — a simple fluid — to change its viscosity and found that only the least viscous liquid they tested failed to fracture.

研究人员通过改变碳氢化合物混合物（一种简单流体）的温度来改变其粘度，结果发现，只有他们测试的粘度最低的液体无法破裂。

The team observed that the critical stress level at which liquids fracture is proportional to their viscosity times the strain rate (how quickly they are being pulled or stretched apart and how the diameter of the liquid is changing).

研究小组观察到，液体破裂的临界应力水平与其粘度乘以应变率（液体被拉或拉伸的速度以及液体直径的变化方式）成正比。

The machine had a limit — albeit a high one — to how quickly it could move: 500 millimeters per second.

这台机器的移动速度有一个限制（尽管很高）：每秒 500 毫米。

“There are very few instruments comparable to ours,” Lima said.

“很少有仪器可以与我们的仪器相媲美，”利马说。

Lima thinks that potentially, if they had a machine that could pull on the liquids faster, they could fracture less viscous liquids like honey or even water.

利马认为，如果他们有一台可以更快地拉动液体的机器，他们就可以破碎蜂蜜甚至水等粘性较小的液体。

In the future, Lima wants to use a more transparent liquid so she can capture the crack as it forms.

未来，利马希望使用更透明的液体，这样她就可以捕捉到裂缝形成的情况。

She would also like to try freezing the surface of the liquid as soon as it snaps and to probe it using a high-resolution microscope that scans surfaces at a nanometer scale.

她还想尝试在液体破裂后立即冻结其表面，并使用以纳米级扫描表面的高分辨率显微镜对其进行探测。

Alvarez is keen to explore simple fluids in the context of spinning materials into fibers — which can have applications in engineering and medicine.

阿尔瓦雷斯热衷于探索将材料纺成纤维的简单流体——这可以在工程和医学领域得到应用。

Fractures in fluids could also have implications for inkjet printing, brain injury protection, and soft robotics.

流体破裂也可能对喷墨打印、脑损伤保护和软机器人技术产生影响。

But Alvarez is most excited to learn what it means for a simple fluid to fracture in the first place.

但阿尔瓦雷斯最兴奋的是首先了解简单流体破裂意味着什么。

“[It’s] different than what we’ve been thinking about in the literature for a very long time,” he said.

“[这]与我们长期以来在文献中所思考的不同，”他说。

Get highlights of the most important news delivered to your email inbox

获取发送到您的电子邮件收件箱的最重要新闻的摘要

Quanta Magazine moderates comments to facilitate an informed, substantive, civil conversation.

《Quanta》杂志对评论进行审核，以促进知情、实质性、文明的对话。

Abusive, profane, self-promotional, misleading, incoherent or off-topic comments will be rejected.

辱骂、亵渎、自我推销、误导、语无伦次或偏离主题的评论将被拒绝。

Moderators are staffed during regular business hours (New York time) and can only accept comments written in English.

版主在正常工作时间（纽约时间）有工作人员，并且只能接受英文评论。