Surface Tension

表面张力

The cohesive forces between liquid molecules are responsible for the phenomenon known as surface tension. The molecules at the surface do not have other like molecules on all sides of them and consequently they cohere more strongly to those directly associated with them on the surface. This forms a surface "film" which makes it more difficult to move an object through the surface than to move it when it is completely submersed.

液体分子之间的内聚力是表面张力现象的原因。表面的分子在周围并不都与其他同类分子接触，因此它们更强烈地与表面附近的分子结合。这形成了一个表面“膜”，使得在表面移动物体比在完全浸没时更困难。

Surface tension is typically measured in dynes/cm, the force in dynes required to break a film of length 1 cm. Equivalently, it can be stated as surface energy in ergs per square centimeter. Water at 20°C has a surface tension of 72.8 dynes/cm compared to 22.3 for ethyl alcohol and 465 for mercury.

Decrease in water surface tension with heating

加热时水的表面张力降低

表面张力通常以dynes/cm为单位测量，即在1厘米长的薄膜上所需打破的力，单位为dynes。等效地，它也可以表示为每平方厘米的表面能，单位为ergs。20°C时，水的表面张力为72.8 dynes/cm，相比之下，乙醇为22.3 dynes/cm，汞为465 dynes/cm。

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Cohesion and Surface Tension

内聚力与表面张力

The cohesive forces between molecules down into a liquid are shared with all neighboring atoms. Those on the surface have no neighboring atoms above, and exhibit stronger attractive forces upon their nearest neighbors on the surface. This enhancement of the intermolecular attractive forces at the surface is called surface tension.

液体中分子间的内聚力被所有邻近原子共享。表面的分子没有上方的邻近原子，因此对表面附近的邻近分子表现出更强的吸引力。这种表面处分子间吸引力的增强现象称为表面张力。

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Fluid concepts

索引与流体概念

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Surface Tension of Water

水的表面张力

The surface tension of water is 72 dynes/cm at 25°C . It would take a force of 72 dynes to break a surface film of water 1 cm long. The surface tension of water decreases significantly with temperature as shown in the graph. The surface tension arises from the polar nature of the water molecule.
Hot water is a better cleaning agent because the lower surface tension makes it a better "wetting agent" to get into pores and fissures rather than bridging them with surface tension. Soaps and detergents further lower the surface tension.
热水是一种更好的清洁剂，因为较低的表面张力使其成为更好的“润湿剂”，能够进入孔隙和裂缝而不是用表面张力来堵塞它们。肥皂和洗涤剂进一步降低了表面张力。

Water

水

水的表面张力在25°C时为72达因/厘米。要打破1厘米长的水表面膜，需要72达因的力。如图所示，水的表面张力随温度显著下降。水的表面张力源于水分子的极性。

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索引与流体概念

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Cohesion and Adhesion

凝聚力与附着力

Molecules in the liquid state experience strong intermolecular attractive forces. When those forces are between like molecules, they are referred to as cohesive forces. For example, the molecules of a water droplet are held together by cohesive forces, and the especially strong cohesive forces at the surface constitute surface tension.

液态分子间体验强的分子间吸引力。当这些力作用于同种分子之间时，称为内聚力。例如，水滴中的分子由内聚力结合在一起，而表面特别强的内聚力则构成了表面张力。

When the attractive forces are between unlike molecules, they are said to be adhesive forces. The adhesive forces between water molecules and the walls of a glass tube are stronger than the cohesive forces lead to an upward turning meniscus at the walls of the vessel and contribute to capillary action.

当吸引力作用在不同种类的分子之间时，这些力被称为粘附力。水分子与玻璃管壁之间的粘附力大于内聚力，导致管壁处形成向上的弯月面，从而促进毛细现象。

The attractive forces between molecules in a liquid can be viewed as residual electrostatic forces and are sometimes called van der Waals forces or van der Waals bonds.

液体中分子之间的吸引力可以视为残余的静电作用力，有时也称为范德华力或范德华键。

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Surface Tension Examples

表面张力示例

Walking on water 在水面上行走 Small insects such as the water strider can walk on water because their weight is not enough to penetrate the surface. 小昆虫如水蜘蛛能够走在水面上，因为它们的重量不足以穿透水面。	Floating a needle 浮起一根针 If carefully placed on the surface, a small needle can be made to float on the surface of water even though it is several times as dense as water. If the surface is agitated to break up the surface tension, then needle will quickly sink. 如果小心地放置在水面，一根小针可以使浮在水面上，尽管它比水 dense 多倍。如果表面被搅动以破坏表面张力，那么针会迅速下沉。
Don't touch the tent! 别碰帐篷！ Common tent materials are somewhat rainproof in that the surface tension of water will bridge the pores in the finely woven material. But if you touch the tent material with your finger, you break the surface tension and the rain will drip through. 常见的帐篷材料在一定程度上具有防雨性能，因为水的表面张力可以桥接细密编织材料中的孔隙。但如果你用手指触碰帐篷材料，就会破坏表面张力，雨水就会从孔隙中滴落。	Soaps and detergents 肥皂和洗涤剂 help the cleaning of clothes by lowering the surface tension of the water so that it more readily soaks into pores and soiled areas. 帮助清洁衣物，通过降低水的表面张力，使其更易于渗入毛孔和污渍区域。
Clinical test for jaundice 黄疸临床测试 Normal urine has a surface tension of about 66 dynes/cm but if bile is present (a test for jaundice), it drops to about 55. In the Hay test, powdered sulfur is sprinkled on the urine surface. It will float on normal urine, but sink if the S.T. is lowered by the bile. 正常尿液的表面张力约为66达因/厘米，但如果存在胆汁（用于检测黄疸），则会降至约55。在哈伊测试中，硫粉被撒在尿液表面。它会在正常尿液上漂浮，但如果表面张力因胆汁而降低，则会下沉。	Washing with cold water 用冷水洗 The major reason for using hot water for washing is that its surface tension is lower and it is a better wetting agent. But if the detergent lowers the surface tension, the heating may be unneccessary. 使用热水洗涤的主要原因是其表面张力较低，是一种更好的润湿剂。但若洗涤剂能降低表面张力，加热可能就不再必要。
Surface tension disinfectants 表面张力消毒剂 Disinfectants are usually solutions of low surface tension. This allow them to spread out on the cell walls of bacteria and disrupt them. One such disinfectant, S.T.37, has a name which points to its low surface tension compared to the 72 dynes/cm for water. 消毒剂通常是表面张力较低的溶液。这使它们能够扩散到细菌的细胞壁并破坏细胞壁。其中一种消毒剂，S.T.37，其名称暗示其表面张力比水的72 dynes/cm要低。	Can you think of another? 你还能想到其他什么吗？

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Fluid concepts

Surface tension

索引流体概念表面张力

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Surface Tension and the Water Strider

表面张力与水蜘蛛

This interesting insect can freely run along the surface of a quiet pond. It's tiny mass and the geometry of its legs allow it to be supported by the high surface tension of water.

这种有趣的昆虫可以自由地在平静池塘的表面奔跑。它的质量及其腿部的几何形状使其能够被水的高表面张力支撑。

Water

水

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Fluid concepts

索引与流体概念

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