"Temple on the second" Science: "liquid bridge" is what bridge?

When it comes to the bridge, we should first think of the traffic bridge, such as bridge, stone arch bridges, reinforced concrete cable-stayed bridge, and so on, these bridges are hard material into solid structures.

The famous "Zhaozhou bridge"

　　So if you see a bridge is made of water or liquid made out of?

The answer is certain! --Wash your hands when you are in time, refers to the liquid column is " liquid bridge".

Liquid bridge--"liquid bridge"

　　What is a liquid bridge?

Liquid bridge is not your typical transport bridge, but in solids of liquid column. Is called liquid bridge is because "bridge" connecting means between the two places, liquid bridge is connected to a liquid between two solid surfaces. So, can responsibly tell everyone to stand in the liquid on the bridge to see the scenery, it is not possible.

　　How liquid bridge formed?

Gas-liquid Interfacial surface tension exists between, making surface is like a thin sheet of liquid elastic membrane. Is such a "virtual" films, makes the surface of liquid bridge is maintained, but not "collapse". Due to the surface tension is very weak, so the liquid bridge formed under normal gravity environment size is very small (usually only a few millimeters), beyond this size, liquid bridge will not be able to balance the gravitational collapse.

Liquid bridge size is small, but do not underestimate its role well! We have such an experience: on the beach, dry sand with a hand, let go was loose sand; but if mixed in the sand in the water, you can make various shapes of sand.

Dry sand

Water-sand (source see watermark)

This is because after dry loose sand with water, the water in the liquid bridge formed between fine particles of sand, allowing Sanders to come together. We write with a brush, dip it in the ink forms a flick, but also because the pen between the liquid bridge. Careful observation, liquid bridge of life there are a lot of examples.

Sand "liquid bridge" role of the sand

　　Mysterious space experiment device

You might think these small examples in life is not surprising, but in fact, liquid bridges are "space micro-gravity fluid mechanics research" an important subject.

Do you know? In the forthcoming launch of "temple on the second", and carrying a mysterious scientific experiment –" liquid bridge of thermocapillary convection experiment."

The test device consists of national microgravity laboratory, Institute of mechanics, CAs is responsible for the development, devoted to the liquid in the temple on the second bridge of thermocapillary convection experiment.

Temple on the second of the "liquid bridge of thermocapillary convection experiment"

The liquid bridge of thermocapillary convection experiment box for 13kg, size is smaller than a typical desktop PC. Red completed electrical interface power supply and communications. Scientists on the ground experiments on the actions and instructions, test box after receiving the instruction, Interior agencies will coordinate with each other to complete a series of experimental operations.

So here's the thing:

　　Fluid in the space bridge and on the ground, what is the difference between liquid bridge place?

　　Feature 1: can build large size liquid bridge

In microgravity, gravity almost vanished, objects in a "floating" State. Droplet can only be formed on the ground, to the space station to form a big ball of liquid. This is because gravity disappeared, surface tensions to show Kamui.

Then, using the micro-gravity environment of space, you can build a large liquid bridge, something which would be impossible on the ground.

At present, on the international space station have made a liquid bridge 50mm. Want to keep liquid bridge stability is a prerequisite, in theory, need to keep liquid bridge height and diameter of less than a certain value. Otherwise, the liquid bridge too thin, Raleigh instability will happen, even in space will be "broken bridge".

　　Feature 2: can produce thermal capillary flow

When the liquid temperature at each end of the bridge is not the same when one end hot-cold, under the action of surface tension in liquids, thermocapillary flows. Thermcapillary flows is a major form of natural convection under microgravity environment of space.

Surface tension will change with temperature, low temperature high surface tension, low temperature high surface tension, surface tension is not even a factor in micro-gravity driven flows. Due to the surface tension and capillary forces, so the temperature effect of the surface tension-driven flows, also known as thermal capillary flow, otherwise known as Marangoni convection.

Surface tension-driven mechanism of thermocapillary convection of liquid bridge

Micro-gravity environment of convection and surface in natural convection is very different: in the ground, buoyant convection is the main form of natural convection, after the fluid expands, it will float and fluid cooling reduced, will sink, natural convection is formed.

Flowing from the kettle into general circulation is buoyant convection mechanisms.

Create buoyancy convection due to gravity, so in the microgravity environment of space, buoyancy convection disappeared, thermocapillary flows come to dominate natural convection.

Buoyancy convection under normal gravity environment

Due to the buoyancy effect of cover on the ground, thermocapillary effect once overlooked. Scientists used to think that "as long as there is no gravity, convection will disappear." So, is ideal for convection-free environment in space, if in this environment to produce high purity Crystal will get high purity single crystal.

Sketch of crystal growth

Growth of high-purity (99.999%) tantalum Crystal

However, the reality is always cruel. Many scientists on the international space station and sounding rockets (microgravity) crystal growth experiments were carried out. What was the result? Similar to the ground, there are defects.

Crystal growth of the pair with defects

Finally, the scientists found that in the micro-gravity environment, natural convection is not going away. Although buoyant convection disappeared, but little-known on the ground of thermocapillary convection works, became a stumbling block, which is about slang used to say "press the calabash gourd".

Even more amazing is that when the temperature exceeds the critical condition, this thermal capillary flow will enter a State of oscillatory flow, can be represented as temperature oscillations. Think of crystal growth process of fits and starts and stripes in not just strange.

Through the comparison of space and on the ground, people do not know, even on the ground of thermocapillary flow will cause this kind of micro-structure with defects.

It can be said that thermocapillary flow turbulence is a sabotage of crystal growth bear children.

As to why the liquid bridge oscillations can occur, science still holds two different views, however.

Liquid bridge international space station experiments

　　Research team: years of efforts, ushered in the first space experiment

Chinese scientists have been looking forward to experiment in the microgravity environment of space, unravel the mysteries of thermocapillary convection.

After years of efforts, "thermocapillary convection of liquid bridge" project finally got a piece of "temple on the second" the "tickets", which is also China's first liquid bridge under microgravity condition of thermocapillary convection experiment.

The project's Chief Designer, researcher at the Institute of mechanics, Chinese Academy of Sciences, Kang Qi said: "in order to produce high quality semiconductor materials, scientifically controlled Silicon during crystal growth buoyant convection and thermal capillary convection effects, and unique micro-gravity environment of space will give scientists an in-depth analysis of thermocapillary convection process. ”

Thermocapillary convection of liquid bridge research project Director, designer, researcher at the Institute of mechanics, Chinese Academy of Sciences, Kang Qi

As part of the space program, study researchers have many matching tests were carried out on the ground.

They use Sapphire to build a liquid bridge device in the lab, by particle image velocimetry and advanced equipment such as infrared thermal imager observation and analysis of flow of liquid bridges, got a lot of field experimental data.

Designer Duan Li, Deputy Director of the project researcher said, "due to fewer opportunities, high cost of space experiments and thermocapillary convection phenomena and the relationship between the factors was not can be accurately described by the simple formula, you need to make the ground test, the smooth progress of space experiments providing scientific and reasonable reference ranges. ”

Designers, mechanics, CAs research Duan Li, Deputy Director of the project researcher

After 3 years of engineering development, thermocapillary convection box through the test of layer upon layer of ground tests. The device would take the temple launched two spacecraft slowly, when scientists can use space world interactive way to carry out a series of experiments.

Looking forward to our independent research and development of this fruitful scientific results space science experiment, let us glimpse the magic of thermocapillary convection in a deeper mystery.

(Editors: Dou Yuan UN833)
2016-09-14 17:38:28
Xinhua

“天宫二号”科普：“液桥”是座什么桥？

　　说到桥，大家首先想到的应该是交通道路的桥梁结构，比如独木桥、石拱桥、钢筋混凝土桥、斜拉桥等等，这些桥都是质地坚硬的固体结构材料搭建而成。

著名的“赵州桥”

　　那么你是否见过一种桥是由水或者液体做成的呢？

　　答案其实是一定的！——当你在洗手的时候，指间的小液柱就是“液桥”。

液体组成的桥——“液桥”

　　什么是液桥？

　　液桥其实并不是我们常见的交通运输的桥梁，而是在固体间的小液柱。之所以被称为液桥，是因为“桥”字有连接两地的含义，液桥就是连接着两个固体表面之间的一段液体。所以，可以很负责任地告诉大家，想站在液桥上看风景，那是不可以的。

　　液桥如何形成？

　　气液界面之间存在着表面张力，使得液体表面好比有一层很薄的弹性薄膜一样。正是这样一层“虚拟”的薄膜，使得液桥的表面形貌得以维持，而不会“垮塌”。由于表面张力很弱，所以在正常重力环境下形成的液桥尺寸很小（通常只有几毫米），超过这个尺寸，液桥将无法平衡其重力而垮塌。

　　液桥的尺寸虽小，可是却不能小看它的作用哦！我们一定有这样的体会：在沙滩上，如果用手捏一把干燥的细砂，松手还是散沙；但是如果在沙子中掺一点水，就可以捏出各种形状的沙团。

干燥的细沙

掺水的沙团（图片来源见水印）

　　这是因为干燥的散沙加入水后，水在细微的沙子颗粒之间形成了液桥，使得散沙能聚集起来。我们用写字的毛笔，蘸了墨水后能形成一个笔锋，也是因为在笔毫间形成了液桥。仔细观察，生活中液桥的例子还有很多。

　　沙粒间“液桥”对沙粒的作用

　　神秘的空间实验装置

　　也许你觉得这些生活中的小例子不足为奇，但其实，液桥是“太空微重力流体力学研究”的一个重要课题。

　　你知道吗？在即将发射的“天宫二号”上，就搭载着这样一个神秘的科学实验装置——“液桥热毛细对流实验箱”。

　　该实验装置由中国科学院力学研究所国家微重力重点实验室负责研制，在天宫二号上专门用来开展液桥热毛细对流实验。

天宫二号上的“液桥热毛细对流实验箱”

　　这套液桥热毛细对流实验箱重13kg，大小比普通台式电脑还要小。红色电接口完成装置的供电和通信。科学家在地面安排实验动作并上注指令，实验箱收到指令后，内部机构将相互配合完成一系列的实验操作。

　　那么问题来了：

　　太空中的液桥和地面上的液桥有什么不同的地方呢？

　　特点1：可以建立大尺寸液桥

　　在微重力环境下，重力几近消失，物体处于“漂浮”的状态。地面上只能形成的小液滴，到了空间站便可以形成大的液球。这是因为重力消失后，表面张力大显神威。

　　于是，利用太空的微重力环境，可以建立起很大尺寸的液桥，而这在地面上将是不可能的事情。

　　目前，国际空间站上已经做出了直径50mm的液桥。想让液桥保持稳定是有前提的，理论上讲，需要保证液桥的高度和直径的比例小于一定值。否则，液桥过于细长，将会发生Raleigh不稳定，即使是在太空还是会“断桥”。

　　特点2：可以产生热毛细流动

　　当液桥两端的温度不一样的时候，一端热一端冷，在液体表面张力的作用下，会产生热毛细流动。热毛细流动是空间微重力环境下的主要自然对流形式。

　　表面张力会随着温度变化，温度高的地方表面张力低，温度低的地方表面张力高，表面张力不均匀成为驱动微重力流动的因素。由于表面张力又称毛细力，所以这种表面张力温度效应驱动流动，又称为热毛细流动，或者叫做Marangoni对流。

表面张力驱动的液桥热毛细对流机理

　　微重力环境下的对流和地面自然对流有很大的不同：在地面上，浮力对流是自然对流的主要形式，流体受热膨胀后，就会往上浮；而流体冷却缩小后，就会下沉，自然对流就形成了。

　　从开水壶里的流动到大气环流都是浮力对流机制。

　　产生浮力对流的根本原因是重力作用，所以在空间微重力环境下，浮力对流消失，热毛细流动开始主导自然对流。

正常重力环境下的浮力对流

　　由于地面浮力效应的掩盖，热毛细效应曾经一度被忽视。科学家们曾经以为，“只要没有重力，对流就会消失”。那么，在太空中将是理想的无对流环境，如果在这种环境中制造高纯度晶体，将会得到高纯度的单晶。

晶体生长示意图

生长出的高纯（99.999%）单晶钽

　　但是，现实永远是残酷的。很多科学家专门在国际空间站和探空火箭上（微重力环境）开展了晶体生长实验。结果呢？与地面类似，还是有条纹缺陷。

晶体生长得到的条带缺陷

　　最后，科学家发现，在微重力环境下，自然对流并没有消失。虽然浮力对流消失了，但是在地面上名不见经传的热毛细对流却起作用了，变成了拦路虎，这大约就是俗话常说的“按了葫芦起了瓢”吧。

　　更不可思议的是，当温差超过临界条件时，这种热毛细流动还会进入一种振荡流的状态，可以表现为温度的振荡。想想晶体在生长的过程中，忽冷忽热的，不出现条纹才怪。

　　通过太空和地面的对比实验，人们才知道，即使在地面热毛细流动也会导致这种条带缺陷微观结构。

　　可以说，热毛细流动震荡就是晶体生长中一个专搞破坏的熊孩子。

　　至于液桥为什么会出现振荡，目前科学界依然持有两种不同的观点，在此不过多展开。

　　国际空间站液桥实验

　　研究团队：多年努力，迎来第一次太空实验

　　中国科学家一直以来期待着能在空间微重力环境进行实验，揭开热毛细对流的神秘面纱。

　　经过多年努力，“液桥热毛细对流”研究项目终于拿到了一张“天宫二号”的“船票”，这也是中国第一次在微重力条件下开展液桥热毛细对流实验。

　　该项目的主任设计师、中国科学院力学研究所康琦研究员表示：“为生产出高质量的半导体材料，就要科学控制单晶硅在晶体生长过程中浮力对流和热毛细对流的影响，而太空特有的微重力环境将使科学家深入剖析热毛细对流的真实过程。”

液桥热毛细对流研究项目主任设计师、中国科学院力学研究所康琦研究员

　　为配合此次空间项目的开展，研究人员已经在地面开展许多匹配试验研究。

　　他们在实验室利用蓝宝石搭建了一套液桥装置，通过粒子图像测速和红外热像仪等先进设备观测和分析液桥流动，得到了大量的地面实验数据。

　　项目副主任设计师段俐研究员表示，“由于空间实验机会少、成本较高，而且热毛细对流现象与其影响因素之间的关系并不是用简单公式就可以准确描述的，所以需要进行地面实验，为空间实验的顺利进行提供科学合理的参考范围。”

项目副主任设计师、中国科学院力学研究段俐研究员

　　历经3年多的工程研制，热毛细对流箱通过了层层的地面试验的考验。这套装置将搭乘天宫二号飞船冉冉升空，届时科学家可以用天地互动的方式开展一系列的空间实验。

　　期待我们国家自主研发的这套空间科学实验装置带来丰硕的科学成果，让我们窥探神奇的热毛细对流更深层次的奥秘吧。

(责任编辑：窦远行 UN833)
2016-09-14 17:38:28
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