{"428411":{"#nid":"428411","#data":{"type":"news","title":"Smart Hydrogel Coating Creates \u201cStick-slip\u201d Control of Capillary Action","body":[{"value":"\u003Cp\u003ECoating the inside of glass microtubes with a polymer hydrogel material dramatically alters the way capillary forces draw water into the tiny structures, researchers have found. The discovery could provide a new way to control microfluidic systems, including popular lab-on-a-chip devices.\u003C\/p\u003E\u003Cp\u003ECapillary action draws water and other liquids into confined spaces such as tubes, straws, wicks and paper towels, and the flow rate can be predicted using a simple hydrodynamic analysis. But a chance observation by researchers at the Georgia Institute of Technology will cause a recalculation of those predictions for conditions in which hydrogel films line the tubes carrying water-based liquids.\u003C\/p\u003E\u003Cp\u003E\u201cRather than moving according to conventional expectations, water-based liquids slip to a new location in the tube, get stuck, then slip again \u2013 and the process repeats over and over again,\u201d explained \u003Ca href=\u0022http:\/\/www.me.gatech.edu\/faculty\/fedorov\u0022\u003EAndrei Fedorov\u003C\/a\u003E, a professor in the \u003Ca href=\u0022http:\/\/www.me.gatech.edu\/\u0022\u003EGeorge W. Woodruff School of Mechanical Engineering \u003C\/a\u003Eat Georgia Tech. \u201cInstead of filling the tube with a rate of liquid penetration that slows with time, the water propagates at a nearly constant speed into the hydrogel-coated capillary. This was very different from what we had expected.\u201d\u003C\/p\u003E\u003Cp\u003EThe findings resulted from research sponsored by the Air Force Office of Scientific Research (AFOSR) through the BIONIC center at Georgia Tech, and were reported earlier this month in the journal \u003Cem\u003ESoft Matter\u003C\/em\u003E.\u003C\/p\u003E\u003Cp\u003EWhen the opening of a thin glass tube is exposed to a droplet of water, the liquid begins to flow into the tube, pulled by a combination of surface tension in the liquid and adhesion between the liquid and the walls of the tube. Leading the way is a meniscus, a curved surface of the water at the leading edge of the water column. An ordinary borosilicate glass tube fills by capillary action at a gradually decreasing rate with the speed of meniscus propagation slowing as a square root of time.\u003C\/p\u003E\u003Cp\u003EBut when the inside of a tube is coated with a very thin layer of poly(N-isopropylacrylamide), a so-called \u201csmart\u201d polymer (PNIPAM), everything changes. Water entering a tube coated on the inside with a dry hydrogel film must first wet the film and allow it to swell before it can proceed farther into the tube. The wetting and swelling take place not continuously, but with discrete steps in which the water meniscus first sticks and its motion remains arrested while the polymer layer locally deforms. The meniscus then rapidly slides for a short distance before the process repeats. This \u201cstick-slip\u201d process forces the water to move into the tube in a step-by-step motion.\u003C\/p\u003E\u003Cp\u003EThe flow rate measured by the researchers in the coated tube is three orders of magnitude less than the flow rate in an uncoated tube. A linear equation describes the time dependence of the filling process instead of a classical quadratic equation which describes filling of an uncoated tube.\u003C\/p\u003E\u003Cp\u003E\u201cInstead of filling the capillary in a hundredth of a second, it might take tens of seconds to fill the same capillary,\u201d said Fedorov. \u201cThough there is some swelling of the hydrogel upon contact with water, the change in the tube diameter is negligible due to the small thickness of the hydrogel layer. This is why we were so surprised when we first observed such a dramatic slow-down of the filing process in our experiments.\u201d\u003C\/p\u003E\u003Cp\u003EThe researchers \u2013 who included graduate students James Silva, Drew Loney and Ren Geryak and senior research engineer Peter Kottke \u2013 tried the experiment again using glycerol, a liquid that is not absorbed by the hydrogel. With glycerol, the capillary action proceeded through the hydrogel-coated microtube as with an uncoated tube in agreement with conventional theory. After using high-resolution optical visualization to study the meniscus propagation while the polymer swelled, the researchers realized they could put this previously-unknown behavior to good use.\u003C\/p\u003E\u003Cp\u003EWater absorption by the hydrogels occurs only when the materials remain below a specific transition temperature. When heated above that temperature, the materials no longer absorb water, eliminating the \u201cstick-slip\u201d phenomenon in the microtubes and allowing them to behave like ordinary tubes.\u003C\/p\u003E\u003Cp\u003EThis ability to turn the stick-slip behavior on and off with temperature could provide a new way to control the flow of water-based liquid in microfluidic devices, including labs-on-a-chip. The transition temperature can be controlled by varying the chemical composition of the hydrogel.\u003C\/p\u003E\u003Cp\u003E\u201cBy locally heating or cooling the polymer inside a microfluidic chamber, you can either speed up the filling process or slow it down,\u201d Fedorov said. \u201cThe time it takes for the liquid to travel the same distance can be varied up to three orders of magnitude. That would allow precise control of fluid flow on demand using external stimuli to change polymer film behavior.\u201d\u003C\/p\u003E\u003Cp\u003EThe heating or cooling could be done locally with lasers, tiny heaters, or thermoelectric devices placed at specific locations in the microfluidic devices.\u003C\/p\u003E\u003Cp\u003EThat could allow precise timing of reactions in microfluidic devices by controlling the rate of reactant delivery and product removal, or allow a sequence of fast and slow reactions to occur. Another important application could be controlled drug release in which the desired rate of molecule delivery could be dynamically tuned over time to achieve the optimal therapeutic outcome.\u003C\/p\u003E\u003Cp\u003EIn future work, Fedorov and his team hope to learn more about the physics of the hydrogel-modified capillaries and study capillary flow using partially-transparent microtubes. They also want to explore other \u201csmart\u201d polymers which change the flow rate in response to different stimuli, including the changing pH of the liquid, exposure to electromagnetic radiation, or the induction of mechanical stress \u2013 all of which can change the properties of a particular hydrogel designed to be responsive to those triggers.\u003C\/p\u003E\u003Cp\u003E\u201cThese experimental and theoretical results provide a new conceptual framework for liquid motion confined by soft, dynamically evolving polymer interfaces in which the system creates an energy barrier to further motion through elasto-capillary deformation, and then lowers the barrier through diffusive softening,\u201d the paper\u2019s authors wrote. \u201cThis insight has implications for optimal design of microfluidic and lab-on-a-chip devices based on stimuli-responsive smart polymers.\u201d\u003C\/p\u003E\u003Cp\u003EIn addition to those already mentioned, the research team included Professor Vladimir Tsukruk from the Georgia Tech School of Materials Science and Engineering and Rajesh Naik, Biotechnology Lead and Tech Advisor of the Nanostructured and Biological Materials Branch of the Air Force Research Laboratory (AFRL).\u003C\/p\u003E\u003Cp\u003E\u003Cem\u003EThis research was supported by the Air Force Office of Scientific Research BIONIC Center through awards FA9550-09-1-0162 and FA9550-14-1-0269, AFOSR award FA-9550-14-1-0015, and by Georgia Tech\u2019s Renewable Bioproducts Institute Fellowship. The content is solely the responsibility of the authors and does not necessarily represent the official views of the sponsors.\u003C\/em\u003E\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003ECITATION\u003C\/strong\u003E: J.E. Silva, et al., \u201cStick-Slip Water Penetration into Capillaries Coated with Swelling Hydrogel,\u201d (Soft Matter, 11, pp. 5933-5939, 2015).\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EResearch News\u003C\/strong\u003E\u003Cbr \/\u003E\u003Cstrong\u003EGeorgia Institute of Technology\u003C\/strong\u003E\u003Cbr \/\u003E\u003Cstrong\u003E177 North Avenue\u003C\/strong\u003E\u003Cbr \/\u003E\u003Cstrong\u003EAtlanta, Georgia 30332-0181\u003C\/strong\u003E\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EMedia Relations Contact\u003C\/strong\u003E: John Toon (\u003Ca href=\u0022mailto:jtoon@gatech.edu\u0022\u003Ejtoon@gatech.edu\u003C\/a\u003E) or (404-894-6986)\u003C\/p\u003E\u003Cp\u003E\u003Cstrong\u003EWriter\u003C\/strong\u003E: John Toon\u003C\/p\u003E","summary":null,"format":"limited_html"}],"field_subtitle":"","field_summary":[{"value":"\u003Cp\u003ECoating the inside of glass microtubes with a polymer hydrogel material dramatically alters the way capillary forces draw water into the tiny structures, researchers have found. The discovery could provide a new way to control microfluidic systems, including popular lab-on-a-chip devices.\u003C\/p\u003E","format":"limited_html"}],"field_summary_sentence":[{"value":"Capillary action inside glass tubes coated with a hydrogel behaves in unexpected ways."}],"uid":"27303","created_gmt":"2015-07-25 10:59:58","changed_gmt":"2016-10-08 03:19:15","author":"John Toon","boilerplate_text":"","field_publication":"","field_article_url":"","dateline":{"date":"2015-07-27T00:00:00-04:00","iso_date":"2015-07-27T00:00:00-04:00","tz":"America\/New_York"},"extras":[],"hg_media":{"428381":{"id":"428381","type":"image","title":"Capillary action in coated tube","body":null,"created":"1449254358","gmt_created":"2015-12-04 18:39:18","changed":"1475895167","gmt_changed":"2016-10-08 02:52:47","alt":"Capillary action in coated tube","file":{"fid":"202819","name":"capillary-action1791.jpg","image_path":"\/sites\/default\/files\/images\/capillary-action1791_0.jpg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/capillary-action1791_0.jpg","mime":"image\/jpeg","size":1673866,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/capillary-action1791_0.jpg?itok=8GFy8Rfh"}},"428391":{"id":"428391","type":"image","title":"Studying capillary action in coated microtubes","body":null,"created":"1449254358","gmt_created":"2015-12-04 18:39:18","changed":"1475895167","gmt_changed":"2016-10-08 02:52:47","alt":"Studying capillary action in coated microtubes","file":{"fid":"202820","name":"capillary-action35.jpg","image_path":"\/sites\/default\/files\/images\/capillary-action35_0.jpg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/capillary-action35_0.jpg","mime":"image\/jpeg","size":1434489,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/capillary-action35_0.jpg?itok=BFaRDSAG"}},"428401":{"id":"428401","type":"image","title":"Studying capillary action in coated microtubes2","body":null,"created":"1449254358","gmt_created":"2015-12-04 18:39:18","changed":"1475895167","gmt_changed":"2016-10-08 02:52:47","alt":"Studying capillary action in coated microtubes2","file":{"fid":"202821","name":"capillary-action60.jpg","image_path":"\/sites\/default\/files\/images\/capillary-action60_0.jpg","image_full_path":"http:\/\/hg.gatech.edu\/\/sites\/default\/files\/images\/capillary-action60_0.jpg","mime":"image\/jpeg","size":1325301,"path_740":"http:\/\/hg.gatech.edu\/sites\/default\/files\/styles\/740xx_scale\/public\/images\/capillary-action60_0.jpg?itok=3H8pkNAu"}}},"media_ids":["428381","428391","428401"],"groups":[{"id":"1188","name":"Research Horizons"}],"categories":[{"id":"141","name":"Chemistry and Chemical Engineering"},{"id":"145","name":"Engineering"},{"id":"146","name":"Life Sciences and Biology"},{"id":"135","name":"Research"},{"id":"150","name":"Physics and Physical Sciences"}],"keywords":[{"id":"2781","name":"Andrei Fedorov"},{"id":"136721","name":"capillary action"},{"id":"3356","name":"hydrogel"},{"id":"7343","name":"lab-on-a-chip"},{"id":"12427","name":"microfluidics"},{"id":"1492","name":"Polymer"}],"core_research_areas":[{"id":"39441","name":"Bioengineering and Bioscience"},{"id":"39471","name":"Materials"},{"id":"39491","name":"Renewable Bioproducts"}],"news_room_topics":[{"id":"71881","name":"Science and Technology"}],"event_categories":[],"invited_audience":[],"affiliations":[],"classification":[],"areas_of_expertise":[],"news_and_recent_appearances":[],"phone":[],"contact":[{"value":"\u003Cp\u003EJohn Toon\u003C\/p\u003E\u003Cp\u003EResearch News\u003C\/p\u003E\u003Cp\u003E\u003Ca href=\u0022mailto:jtoon@gatech.edu\u0022\u003Ejtoon@gatech.edu\u003C\/a\u003E\u003C\/p\u003E\u003Cp\u003E(404) 894-6986\u003C\/p\u003E","format":"limited_html"}],"email":["jtoon@gatech.edu"],"slides":[],"orientation":[],"userdata":""}}}