液滴撞在固體上是個常見現象並且有廣泛的應用，譬如噴墨打印，塗層，工業冷卻以及飛機上的積冰。當液滴撞在乾燥的表面上會出现很多的結果：它可能完全反彈，或平緩的展開，或猛烈地飛濺出許多微小的液滴。影響撞擊過程包括撞擊速度，液體和固體的性質以及周圍的空氣。最近的研究發現降低周圍氣體壓強可以抑制液體飛濺。這表明空氣對於飛濺機制起了關鍵作用。因此，了解在液固撞擊過程中空氣的重要性會給這一基本現象帶來新的發展，並可能改善如控制飛濺和表面塗層等一些實踐過程。 / 採用高速攝影和顯微成像技術，我們做了液滴撞擊光滑且乾燥固體表面的實驗，研究了被困在裡面的氣體。我們觀察了在液體接觸襯底之前一層氣膜被壓縮並形成的過程，發現其壓強比大氣壓值要高得多。壓縮程度取決於撞擊速度，我們通過減速的液體與壓縮空氣的平衡關係給出了解釋。在液體與襯底接觸後，空氣膜的邊緣在豎直方向上擴展，壓強在幾十微秒內迅速降低。這個厚的邊沿在表面張力，慣性力和粘性阻力的複雜相互作用下最終收縮成氣泡。 / 當液滴以很高的速度撞擊平滑固體表面上，液體發生飛濺。人們對這個常見的現象缺乏基本的了解。採用高速攝影技術，我們觀察到是被困在展開液體前沿下的超薄氣膜觸發了飛濺現象。因為這層薄膜比空氣分子的平均自由程更薄，具有與聲速相當的非常高速來轉移動量，並產生一個比常規計算強十倍的應力。如此"強風"在小尺度上開起了開爾文-亥姆霍茲不穩定性並且有效地產生飛濺。我們的模型定量地與實驗驗證相符，並給出了一個對於液滴在光滑表面上飛濺的基本解釋。 / The impacts of liquid drops onto solid substrates are ubiquitous and appear in a variety of applications, such as ink-jet printing, spray coating, industrial cooling processes and ice accumulation on aircraft. When a liquid drop hits a dry substrate, there are many possible outcomes: it can rebound completely, spread smoothly, or splash dramatically ejecting many tiny daughter droplets. Clearly, the hitting dynamics may be influenced by different factors, including the velocity of impact, liquid and surface properties, as well as the surrounding air. Recent study reveals that by lowering the ambient gas pressure, liquid splash could be eliminated. It indicates that the role of air is a significant key to the mechanism of splash creation. Therefore, understanding the behavior of air during liquid-solid impacts will bring new advances to this fundamental phenomenon, and may benefit practical processes such as splash control and surface coating. / Using high-speed photography coupled with optical interference, we experimentally study the air entrapment during a liquid drop impacting a smooth and dry solid substrate. We observe the formation of a compressed air film before the liquid touches the substrate, with internal pressure considerably higher than the atmospheric value. The degree of compression highly depends on the impact velocity, as explained by balancing the liquid deceleration with the large pressure of the compressed air. After contact, the air film expands vertically at the edge, reducing its pressure within a few tens of microseconds and producing a thick rim on the perimeter. This thick-rimmed air film subsequently contracts into an air bubble, governed by the complex interaction between surface tension, inertia and viscous drag. Such a process is universally observed for impacts above a few centimeters high. / When a fast-moving drop impacts onto a smooth substrate, splashing will be produced at the edge of the expanding liquid sheet. This ubiquitous phenomenon lacks a fundamental understanding. With high speed photography, we illustrate that an ultra-thin air film trapped at the expanding liquid front triggers splashing. Because this film is thinner than the mean free path of air molecules, the interior air flow transfers momentum with an unusually high velocity comparable to the speed of sound, and generates a stress ten times stronger than the conventional calculation. Such a "strong wind" initiates Kelvin-Helmholtz instabilities at small length scales and effectively produces splashing. Our model agrees quantitatively with experimental verifications, and brings a fundamental understanding to the ubiquitous phenomenon of drop splashing on smooth substrates. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Detailed summary in vernacular field only. / Liu, Yuan = 液體固體碰撞過程中空氣重要性的研究 / 劉嫄. / Thesis (Ph.D.) Chinese University of Hong Kong, 2014. / Includes bibliographical references (leaves 78-80). / Abstracts also in Chinese. / Liu, Yuan = Ye ti gu ti peng zhuang guo cheng zhong kong qi zhong yao xing de yan jiu / Liu Yuan.
| Identifer | oai:union.ndltd.org:cuhk.edu.hk/oai:cuhk-dr:cuhk_1077720 |
| Date | January 2014 |
| Contributors | Liu, Yuan , active 2014 (author.), Wong, King Young (thesis advisor.), Chinese University of Hong Kong Graduate School. Division of Physics, (degree granting institution.) |
| Source Sets | The Chinese University of Hong Kong |
| Language | English, Chinese |
| Detected Language | English |
| Type | Text, bibliography, text |
| Format | electronic resource, electronic resource, remote, 1 online resource (xviii, 80 leaves) : illustrations (some color), computer, online resource |
| Rights | Use of this resource is governed by the terms and conditions of the Creative Commons “Attribution-NonCommercial-NoDerivatives 4.0 International” License (http://creativecommons.org/licenses/by-nc-nd/4.0/) |
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