{"id":3806,"date":"2019-09-10T09:23:32","date_gmt":"2019-09-10T07:23:32","guid":{"rendered":"http:\/\/junq.info\/?p=3806"},"modified":"2019-09-10T09:28:03","modified_gmt":"2019-09-10T07:28:03","slug":"when-water-dances-the-mystery-of-the-bouncing-water-droplets","status":"publish","type":"post","link":"https:\/\/junq.info\/?p=3806","title":{"rendered":"When water dances: The mystery of the bouncing water droplets"},"content":{"rendered":"\n

Curious things happen around us all the\ntime \u2013 and sometimes we are so familiar with them that we do not even notice\nthem anymore. <\/p>\n\n\n\n

If you read the title you might now think that\nthis article was about the Leidenfrost effect [1], that is, this little funny\ndance water droplets perform on a hot surface such as a frying pan. It is not,\nthough. The Leidenfrost effect occurs when a material \u2013 usually a liquid \u2013 meets\na surface far above its boiling temperature. A thin layer of the droplet\u2019s\nsurface will then evaporate rapidly, causing a protective gas coating to appear\nthat effectively insulates the droplet and lets it last longer on the hot\nsurface. Similar effects can also be seen with liquid nitrogen on a material at\nroom temperature. These droplets appear to travel around due to ejected gasses.\nBut does a similar phenomenon also occur without the necessity of a hot\nsurface?<\/p>\n\n\n\n

\"\"<\/figure><\/div>\n\n\n\n

There is in fact a location where such an\neffect occurs regularly without us usually noticing: The bathroom. Under\ncertain conditions water droplets can be seen moving on a surface of water as\nif they had hydrophobic properties. The easiest way to see them is in the\nshower, when the shower floor is already covered in a thin layer of water. If new\nwater droplets now impact on this surface at certain angles and speeds, they\ncan be seen rushing around for a while before disappearing. It turns out that\nin recent years a few scientific publications were dedicated to investigating\nthis effect more closely. [2,3] With a high-speed camera, the bouncing effect\ncan be visualized rather easily, as shown in Fig. 1: The droplet appears to\ncause a dent in the water surface and then bounce off without merging with the\nrest of the liquid. Of course, the first idea that comes into mind now is the\nLeidenfrost effect, where a similar behavior can be seen caused by a layer of\nvapor. However, here no high temperatures are involved and thus the generation\nof water vapor is negligible.<\/p>\n\n\n\n

\"\"
Figure 2: A schematic depiction of the resistance time phenomenon. On impact, a thin layer of gas (air) is compressed on the surface, causing a protection from immediate coalescence. However, eventually, the air escapes and the lower periphery of the droplet merges with the rest of the liquid. The surface tension can then rapidly squeeze the edges of the droplet together, causing the upper half of the droplet to be cut off from the rest. It can then repeat the bouncing process if the conditions are right. Reproduced from [4]. <\/figcaption><\/figure><\/div>\n\n\n\n

The first intuition of an air coating to\nprotect the water droplet is still standing, though, and thus the scientists\ntried to model the behavior. It turns out that there is indeed a protective\ncoating of air, which can get compressed when the droplet approaches the surface\nof the liquid underneath. The air simply cannot escape quickly enough and\ntherefore protects the droplet on impact and pushes away from the water surface.\nThis phenomenon causes what is called the residence time of a droplet, that is,\nthe time a droplet can sit on top of a pool of the same liquid before\ncoalescing (see Fig. 2). The theory was confirmed by lowering the ambient air\npressure around the experiment, which caused the residence time to decrease.\n[4] However, one would expect that this thin layer of gas should not withstand\na heavy impact of a droplet coming from e.g. the shower head with a lot of\nspeed and thus kinetic energy.<\/p>\n\n\n\n

An explanation can be found using a simple\nspeaker membrane: When the droplets are put in contact with an oscillation surface,\nlike water on an oscillating speaker, the bouncing is facilitated, and the\ndroplets can remain intact for much longer. Moreover, the droplets now travel\naround just like they do in a shower! High-speed camera footage can show the\nreason for this change in behavior: The surface of the water pool, excited into\nperiodic up- and down-movement patterns, gently catches the droplet if the\nsurface is moving downwards in the moment of impact and therefore prevents the\nimpact from destroying the protective gas layer. It is just like gently catching\na water balloon with your hand by grabbing it in motion and then slowing it\ndown. Additionally, the continuous movement of the surface seems to stabilize\nthe gas layer and therefore massively increases the residence time, all while allowing\nthe droplet to travel from minimum to minimum, thus creating the \u201cwalking\nwater\u201d effect. [6] In a shower, the impact of many, many droplets cause the\nsurface of the water pool on the ground to oscillate in a similar manner,\ncreating landing spots for some droplets that then move around the surface. The\nphenomenon can thus be explained by the residence time of a droplet together\nwith an oscillating surface.<\/p>\n\n\n\n

Finally, one can reproduce a similar\nbehavior in space, where microgravity does not pull the droplets down. An air\nbubble inside of a water bubble can thus act like an isolated system where\ndroplets can form and move\u2026 excited by the sound of a cello! If you got\ncurious, please check out the beautiful footage in Ref. [6] where much of the\ninspiration of this article came from.<\/p>\n\n\n\n

As stated initially, the most curious\nthings happen around us and we simply have to notice them.<\/p>\n\n\n\n

— Kai Litzius<\/p>\n\n\n\n

<\/p>\n\n\n\n

References:<\/p>\n\n\n\n

[1] https:\/\/www.engineersedge.com\/physics\/leidenfrost_effect_13089.htm<\/a><\/p>\n\n\n\n

[2] Y. Couder et al., From Bouncing to\nFloating: Noncoalescence of Drops on a Fluid Bath, Phys. Rev. Lett. 94, 177801\n(2005).<\/p>\n\n\n\n

[3] J. Mol\u00e1cek & J. W. M. Bush, Drops bouncing on a vibrating bath, J. Fluid Mech. 727, 582-611 (2013).<\/p>\n\n\n\n

[4] I. Klyuzhin et al., Persisting Water\nDroplets on Water Surfaces, J. Phys. Chem. B 114, 14020-14027 (2010).<\/p>\n\n\n\n

[5] https:\/\/upload.wikimedia.org\/wikipedia\/commons\/1\/1d\/Bouncing_droplets.gif<\/a><\/p>\n\n\n\n

[6] https:\/\/www.youtube.com\/watch?v=KJDEsAy9RyM<\/a> (Water bubble in space at time index 8:18).
\n <\/p>\n","protected":false},"excerpt":{"rendered":"

Curious things happen around us all the time \u2013 and sometimes we are so familiar with them that we do not even notice them anymore. If you read the title you might now think that this article was about the Leidenfrost effect [1], that is, this little funny dance water droplets perform on a hot… Read More »When water dances: The mystery of the bouncing water droplets<\/span><\/a><\/p>\n","protected":false},"author":12,"featured_media":0,"comment_status":"open","ping_status":"open","sticky":true,"template":"","format":"standard","meta":{"neve_meta_sidebar":"","neve_meta_container":"","neve_meta_enable_content_width":"","neve_meta_content_width":0,"neve_meta_title_alignment":"","neve_meta_author_avatar":"","neve_post_elements_order":"","neve_meta_disable_header":"","neve_meta_disable_footer":"","neve_meta_disable_title":"","footnotes":""},"categories":[6,84],"tags":[85],"aioseo_notices":[],"_links":{"self":[{"href":"https:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/posts\/3806"}],"collection":[{"href":"https:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"https:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"https:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/users\/12"}],"replies":[{"embeddable":true,"href":"https:\/\/junq.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=3806"}],"version-history":[{"count":3,"href":"https:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/posts\/3806\/revisions"}],"predecessor-version":[{"id":3811,"href":"https:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/posts\/3806\/revisions\/3811"}],"wp:attachment":[{"href":"https:\/\/junq.info\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=3806"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"https:\/\/junq.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=3806"},{"taxonomy":"post_tag","embeddable":true,"href":"https:\/\/junq.info\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=3806"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}