Almost a year ago Leonie Mueck posted as question of the month \u201cWhy does coffee form ring-shaped\u00a0stains?\u201c. The answer was: Coffee is a colloidal suspension in water. This means that a coffee drop on\u00a0a hydrophilic surface is quite flat: It exhibits a large contact area with the surface and therefore\u00a0retains a low contact angle at its edges. The drops edge on the surface is called contact line. For cases\u00a0like coffee on glass this contact line is pinned, meaning the drop\u2019s area is constant. In such a pinned\u00a0drop capillary flow to the contact line occurs, causing (coffee) particle deposition near the contact\u00a0line upon evaporation of the solvent. Consequently the dried drop leaves a ring-shaped stain. This is\u00a0generally referred to as the \u2018coffee ring effect\u2019.<\/p>\n
Recently a group of Belgian chemists set out to unravel why many bacterial colonies do not leave such\u00a0a ring-shaped stain upon drying.[1] This question is important for battling bacteria but also to\u00a0improve any industrial application of colloidal solutions that are supposed to dry uniformly.<\/p>\n
The existence of drops that dry differently implies that there are effects counteracting the \u2018coffee\u00a0ring effect\u2019. These so called \u2018Marangoni effects\u2019 all result out of gradients of surface tension in the\u00a0droplet. Thermal cooling may be the most plausible of these effects: Evaporation causes cooling of\u00a0the drop\u2019s edges which in turn results in recirculating flows due to the temperature gradient\u00a0generated. The next likely candidate to cause Marangoni effects are surfactants, since they change\u00a0surface tension. In fact the surfactant sodium dodecyl sulfate (SDS) generates \u2018Marangoni eddies\u2019\u00a0which result in an absolutely uniform particle deposition.[2]<\/p>\n
To answer the question: Some bacteria do not leave coffee ring stains because they produce non-ionic surfactants that generate strong swirling of the bacteria inside the drying drop. Due to this\u00a0distribution forces the bacteria form an uniform deposit upon drying.[1] The authors argue that\u00a0uniform deposition on surfaces is advantageous to bacteria since wide-spread bacteria have better\u00a0access to nutrients. With this new hypothesis, however, the question is not yet fully answered: We\u00a0do now understand the actual mechanism, but we do not understand the actual advantage. Why do\u00a0some kind of bacteria are content with leaving coffee rings stains and others are not? Is there really a\u00a0selective advantage?<\/p>\n
Felix Spenkuch<\/p>\n
Read more:<\/p>\n
[1] W. Sempels, R. De Dier, H. Mizuno, J. Hofkens and J. Vermant, Nature Comm. 2013, 4, 1757, doi:10.1038\/ncomms2746 Last accessed 18.05.2013.<\/p>\n
[2] T. Still, P. J. Yunker, A. G. Yodh, Langmuir 2012, 28, 4984\u20134988.<\/p>\n","protected":false},"excerpt":{"rendered":"
Almost a year ago Leonie Mueck posted as question of the month \u201cWhy does coffee form ring-shaped\u00a0stains?\u201c. The answer was: Coffee is a colloidal suspension in water. This means that a coffee drop on\u00a0a hydrophilic surface is quite flat: It exhibits a large contact area with the surface and therefore\u00a0retains a low contact angle at… Read More »Why do not all kinds of bacteria leave coffee ring stains?<\/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],"tags":[],"aioseo_notices":[],"_links":{"self":[{"href":"http:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/posts\/1811"}],"collection":[{"href":"http:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/posts"}],"about":[{"href":"http:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/types\/post"}],"author":[{"embeddable":true,"href":"http:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/users\/12"}],"replies":[{"embeddable":true,"href":"http:\/\/junq.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=1811"}],"version-history":[{"count":7,"href":"http:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/posts\/1811\/revisions"}],"predecessor-version":[{"id":1837,"href":"http:\/\/junq.info\/index.php?rest_route=\/wp\/v2\/posts\/1811\/revisions\/1837"}],"wp:attachment":[{"href":"http:\/\/junq.info\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=1811"}],"wp:term":[{"taxonomy":"category","embeddable":true,"href":"http:\/\/junq.info\/index.php?rest_route=%2Fwp%2Fv2%2Fcategories&post=1811"},{"taxonomy":"post_tag","embeddable":true,"href":"http:\/\/junq.info\/index.php?rest_route=%2Fwp%2Fv2%2Ftags&post=1811"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}