Sarah Vitak: This is Scientific American’s 60 Second Science. I’m Sarah Vitak.
Sarah Vitak：这里是《科学美国人》的 60 秒科学。我是Sarah Vitak。
If you are kitchen-savvy, you probably know “the water test” method for getting stainless steel pans to the right temperature.
如果你擅长烹饪，你可能知道用于测试不锈钢平底锅是否加热到合适温度的“水滴法”。
[CLIP: Sizzling sounds]
[剪辑：咝咝作响的声音]
All you have to do is splash a few drops of water in a pan as it is being heated. When the pan reaches the right temperature, instead of evaporating, the drops form into beads and skate all over the pan.
你所需要做的就是在加热平底锅时往里面洒几滴水，达到合适的温度后水不会继续蒸发，而是会形成小水珠在锅面快速滑动。
You have to do this test in an empty pan. And that is because, as the not-kitchen-savvy among us may have learned the hard way, mixing oil and water in a hot pan is a recipe for flying burning hot liquids. The phenomenon that happens when beads of water skate around in your empty pan has been understood for a long time. Here is Dr. Kripa Varanasi from the mechanical engineering department at M.I.T. explaining it:
测试必须在无油的平底锅中进行，某些厨房小白或许有过向热油锅里加水时的惨痛经历，这会让水迅速沸腾带出滚烫的油星四处飞溅。人们很早就观察到了水珠跑锅的现象，但一直没弄清原理。麻省理工学院(MIT)机械工程系的克丽帕·瓦拉纳西(Kripa Varanasi)博士如此解释道：
Kripa Varanasi: They’re levitating on their vapor layer with the vapor cushion that’s forming because the liquid is evaporating. And so they can move around with very little friction: that’s called the Leidenfrost state. And then, if you’re at a lower temperature, you could start boiling the drop, but the drop is sticking to the surface.
Kripa Varanasi：水在蒸发时会形成隔热的蒸汽层，水珠悬浮在蒸汽层上，摩擦力极小，所以水珠能四处滑动：这一现象被称作莱顿弗罗斯效应(Leidenfrost Phenomenon)。当平底锅温度较低时，这些水珠就会开始沸腾，但此时它们会直接附着在锅面上。
Vitak: But recently Dr.Varanasi and his graduate student Victor Leon set out to look at hot oil and water interactions a bit more closely.
Vitak： 但最近，瓦拉纳西博士和他的研究生维克多·利昂(Victor Leon)开始更仔细地研究热油与水之间的相互作用。
Varanasi: What we found here that’s very interesting is: When you have a thin oil layer, you have the drop on it, you still can get both those states. But in between, there is another state, and it can cause it to race at very high speeds on the surface.
Varanasi：我们发现了非常有趣的现象：如果表面上有一层薄薄的油层，滴上水珠也能得到上述两种现象。但在这两者之间，还存在着另一种状态，能让水珠以极高的速度在表面滚动。
Vitak: The experiments were so simple, you could run them in your kitchen. They just used a hot plate with either a metal plate or a silicon wafer on top, then the oil (they used silicon oil), then a syringe to precisely place a water drop. The one thing they did a little more precisely than you would in the kitchen is that they applied the oil by spin coating, a process that creates a very thin and very uniform surface. Their oil layer was about 10 microns, or about a hair’s width thick. They used high-speed imaging equipment to capture the result.
Vitak： 实验相当简单，在你家厨房里就能做。研究人员只用到了一块热的面板，可以选择金属板或是表面覆盖硅片的平板，涂上油(他们使用的是硅油)，然后用注射器精确滴下水珠。相比你在厨房里的实验，实验室里更精准的地方只在于涂油时他们用到了旋涂法，这样能够涂出极薄且非常均匀的油层，厚度大约为10微米，只有头发丝直径的一半。最后，研究人员使用高速成像设备对实验结果进行成像。
And what they saw in these videos was that when the droplets were placed on the hot oil, they would propel themselves in random directions at very high speeds.
视频中他们看到：水珠落到热油上时，会以极高的速度散向四面八方。
Several things about this were very surprising. First, that the droplet propels itself. It was clear pretty quickly how this was happening.
这里有好几个点令人惊讶：首先，水珠是自发滚动出去的。这一点很快就得到了解释。
Varanasi: An easy way to think about this is it's called momentum transfer. So if you take a balloon and blow air in it, and you release it, the balloon just flies everywhere, right?
Varanasi：简单来说，这是动量传递的结果。如果在给气球充气之后松手，气球会没头没脑朝各个方向飞出去，对吧?
Vitak: Similar to the balloon, what was happening on the hotplate was that the oil was forming a thin layer around the drop. The oil cloak acts as the balloon. The water inside boils and turns into vapor. Eventually, this pocket of steam bursts through the oil, and much like letting go of the balloon this propels the drop.
Vitak：热板上的油会包住水珠形成薄薄的油层，就像气球一样，而里面的水受热变成蒸汽，最终冲破油层，就像松开手的气球一样，四处跑动。
The second thing that confused them was that the drops went much faster than those levitating leidenfrost water drops. Ten to 100 times faster, in fact. That was odd because the drop was still in contact with the oily surface. You might expect to slow it down compared to a levitating drop.
第二件令他们困惑的事是，这里水珠运动的速度远远超过了悬浮的莱顿弗罗斯特水珠，实际上快了10到100倍。这很奇怪，因为水珠和油层是相互接触的，它的速度应该比悬浮的水珠低才对。
Vitak: That is Leon who ran the experiments. He means that when he did the back calculations based on the speed that he saw the water drop was moving, the only way to make the calculation work out would be for the drop to be on an infinitely deep pool of oil. But in reality the oil was only 10 microns thick, which should cause the drop to stick--a lot.From a physics perspective the amount of friction the oil surface should have is based on its depth. Think of it like pouring honey out of a bottle. The thin film is like pouring through the small opening in the cap -- the honey moves very slowly. If you remove the cap entirely - the honey has less friction and pours more quickly. If you do the calculation for oil 10 microns thick the drop should move way slower.
Victor Leon：“我们相当困惑，因为水珠的速度看上去就像是在无限的液体池上运动。”展开实验的利昂说道，基于所观察到的水珠移动速度进行反向计算时，他发现唯一能让计算进行下去的情况是水珠运动在无限深的油池上。但实际上，油层只有10微米厚，理应会让液滴黏住，牢牢黏住。从物理学角度来看，油层表面摩擦力的大小取决于它的深度。可以想象一下从瓶子里往外倒蜂蜜，薄油膜就好比通过瓶盖上一个小开口往外倒，蜂蜜流动的速度会非常慢;但如果你完全取下盖子，蜂蜜受到的摩擦力就会变小，倾倒速度也会更快。按照10微米的厚度来算，水珠的移速应该更慢才对。
Leon: I mean, that was one of the moments where I was like, No way like, This is crazy.
Victor Leon：所以，那会儿我觉得完全不可能，这太疯狂了。
Vitak: But what was even more surprising was the explanation. And that came when they looked at the super slow motion videos.
Vitak：但是，更让人吃惊的是这一现象的解释，也就是他们查看超慢动作视频时发现的情况。
Leon: And we actually saw the surface of the droplet moving at this frequency that suggested that it's vibrating due to boiling events like bubbling.
Leon：我们看到的真实情况是，水珠表面在以某种频率移动，而这表明它由于像冒泡这样的沸腾事件而在振动。
Vitak: So as the water inside the drop boils bubbles form. The bubbles trapped inside the oil film cause the drop to go from perfectly circular to changing rapidly between asymmetric shapes. It looks like a sort of chaotic vibrating. And it means that the bottom of the drop is changing between different wavy shapes and no longer fully in contact with the oil the way it would be if it stayed as a perfectly circular drop. So the friction is much lower. And the drop can go really fast.
Vitak： 油膜内部的水不断沸腾产生气泡，它们困在油膜中让水珠从完美的球形快速变成各种不对称的形状，看上去像是某种混乱的振动。也就是说，水珠底部在不同波浪形状之间变化，不再像完美球形时那样与板上的油层紧密接触，因此相应的摩擦力要小得多，运动的速度也快得多。
Varanasi: These are things that you see in very complex physical systems. So we are like, quite both first, quite stumped in the beginning to then later on, delighted by figuring this out.
Varanasi：这些现象往往出现在非常复杂的物理系统中。所以我们一开始完全摸不着头脑，困惑不已，后来又为终于弄明白了其中的原理而非常兴奋。
Vitak: But they don’t have it all figured out yet. There is one question in particular they want to look at next:
Vitak：但是，他们并没有完全解开所有谜团。接下来，他们特别想研究这样一个问题：
Varanasi: But why does the vapor eject from one side, right? Or how does it choose that? That's still a mystery for us. And we are figuring that out. Because once we know that we can then direct the motion of the drops to the way we want them.
Varanasi：蒸汽为什么会从一侧喷出?又或者说，它如何选择从哪一侧喷出?对我们来说，这仍然是个谜，我们正努力解决，因为一旦解开，我们就能依自己所愿控制液滴运动。
Vitak: If they can harness this power these forces could be used for many different applications. From handling tiny amounts of liquid for biotech applications, to cleaning films that build up in water treatment, to moving liquids in microgravity on other planets, to even carrying molecules to make tiny robotic delivery systems. And it all started as a flash of insight in the frying pan.
Vitak：如果他们能够将此利用起来，这种力就可能有多种不同的应用：从生物技术中的微量液体处理，到水处理系统中的清洁薄膜，再到其他行星微重力环境下的液体移动，甚至是携带分子的微型机器人递送系统。所有一切都始于在平底锅上一闪而过的水珠。
Thanks for listening. For Scientific American’s 60 Second Science, I’m Sarah Vitak.
谢谢收听。这里是《科学美国人》的 60 秒科学，我是 Sarah Vitak。
Sarah Vitak: This is Scientific American’s 60 Second Science. I’m Sarah Vitak.
If you are kitchen-savvy, you probably know “the water test” method for getting stainless steel pans to the right temperature.
[CLIP: Sizzling sounds]
All you have to do is splash a few drops of water in a pan as it is being heated. When the pan reaches the right temperature, instead of evaporating, the drops form into beads and skate all over the pan.
You have to do this test in an empty pan. And that is because, as the not-kitchen-savvy among us may have learned the hard way, mixing oil and water in a hot pan is a recipe for flying burning hot liquids. The phenomenon that happens when beads of water skate around in your empty pan has been understood for a long time. Here is Dr. Kripa Varanasi from the mechanical engineering department at M.I.T. explaining it:
Kripa Varanasi: They’re levitating on their vapor layer with the vapor cushion that’s forming because the liquid is evaporating. And so they can move around with very little friction: that’s called the Leidenfrost state. And then, if you’re at a lower temperature, you could start boiling the drop, but the drop is sticking to the surface.
Vitak: But recently Dr.Varanasi and his graduate student Victor Leon set out to look at hot oil and water interactions a bit more closely.
Varanasi: What we found here that’s very interesting is: When you have a thin oil layer, you have the drop on it, you still can get both those states. But in between, there is another state, and it can cause it to race at very high speeds on the surface.
Vitak: The experiments were so simple, you could run them in your kitchen. They just used a hot plate with either a metal plate or a silicon wafer on top, then the oil (they used silicon oil), then a syringe to precisely place a water drop. The one thing they did a little more precisely than you would in the kitchen is that they applied the oil by spin coating, a process that creates a very thin and very uniform surface. Their oil layer was about 10 microns, or about a hair’s width thick. They used high-speed imaging equipment to capture the result.
And what they saw in these videos was that when the droplets were placed on the hot oil, they would propel themselves in random directions at very high speeds.
Several things about this were very surprising. First, that the droplet propels itself. It was clear pretty quickly how this was happening.
Varanasi: An easy way to think about this is it's called momentum transfer. So if you take a balloon and blow air in it, and you release it, the balloon just flies everywhere, right?
Vitak: Similar to the balloon, what was happening on the hotplate was that the oil was forming a thin layer around the drop. The oil cloak acts as the balloon. The water inside boils and turns into vapor. Eventually, this pocket of steam bursts through the oil, and much like letting go of the balloon this propels the drop.
The second thing that confused them was that the drops went much faster than those levitating leidenfrost water drops. Ten to 100 times faster, in fact. That was odd because the drop was still in contact with the oily surface. You might expect to slow it down compared to a levitating drop.
Vitak: That is Leon who ran the experiments. He means that when he did the back calculations based on the speed that he saw the water drop was moving, the only way to make the calculation work out would be for the drop to be on an infinitely deep pool of oil. But in reality the oil was only 10 microns thick, which should cause the drop to stick--a lot.From a physics perspective the amount of friction the oil surface should have is based on its depth. Think of it like pouring honey out of a bottle. The thin film is like pouring through the small opening in the cap -- the honey moves very slowly. If you remove the cap entirely - the honey has less friction and pours more quickly. If you do the calculation for oil 10 microns thick the drop should move way slower.
Leon: I mean, that was one of the moments where I was like, No way like, This is crazy.
Vitak: But what was even more surprising was the explanation. And that came when they looked at the super slow motion videos.
Leon: And we actually saw the surface of the droplet moving at this frequency that suggested that it's vibrating due to boiling events like bubbling.
Vitak: So as the water inside the drop boils bubbles form. The bubbles trapped inside the oil film cause the drop to go from perfectly circular to changing rapidly between asymmetric shapes. It looks like a sort of chaotic vibrating. And it means that the bottom of the drop is changing between different wavy shapes and no longer fully in contact with the oil the way it would be if it stayed as a perfectly circular drop. So the friction is much lower. And the drop can go really fast.
Varanasi: These are things that you see in very complex physical systems. So we are like, quite both first, quite stumped in the beginning to then later on, delighted by figuring this out.
Vitak: But they don’t have it all figured out yet. There is one question in particular they want to look at next:
Varanasi: But why does the vapor eject from one side, right? Or how does it choose that? That's still a mystery for us. And we are figuring that out. Because once we know that we can then direct the motion of the drops to the way we want them.
Vitak: If they can harness this power these forces could be used for many different applications. From handling tiny amounts of liquid for biotech applications, to cleaning films that build up in water treatment, to moving liquids in microgravity on other planets, to even carrying molecules to make tiny robotic delivery systems. And it all started as a flash of insight in the frying pan.
Thanks for listening. For Scientific American’s 60 Second Science, I’m Sarah Vitak.
