超疏水表面的动态接触角测定

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attension APPLICATION NOTE 9 This application note illustrates how the Attension Theta OpticalTensiometer can be used for measuring advancing and recedingcontact angles on superhydrophobic surfaces. Introduction By a common definition， a surface is superhydrophobic if thecontact angle of water is larger than 150° and water dropletsreadily slide off the surface if the surface is tilted slightly.[1] Super-hydrophobic surfaces are desired in applications includingself-cleaning and non-wetting coatings and fabrics， [2] as well asanti-fogging， [3] anti-icing， [4] and drag-reducing [5] coatings. For creating a superhydrophobic surface， two factors arerequired. First， the surface must have suitable roughness at micro-and nanoscales. Second， the surface must have a hydrophobicsurface chemistry. [6] A famous model surface for superhydropho-bicity from nature， the lotus leaf， has 10-micron papillae in combi-nation with a nanostructure created by hydrophobic wax crystals.This combination results in a surface with water contact angles ofabout 160°， and enables droplets to roll off at a tilt angle smallerthan 5°.[7] The facile movement of the droplets indicates that thecontact angle hysteresis， i.e.， the difference between the advanc-ing and receding contact angle is small. Only if the contact angle hysteresis is negligible， the wettingproperties of a surface can be characterized by a static contactangle， which is measured by placing a droplet to the surface andoptically determining the contact angle. Generally for roughsurfaces， dynamic， i.e.， advancing and receding contact anglesneed to be measured， since a static contact angle can take anyvalue between the advancing and receding ones. In principle，thedynamic contact angles can be defined either by changing thedroplet volume or by tilting the droplet or by using a Wilhelmyplate method with the force tensiometry. This application notedescribes the dynamic contact angle measurement by changingthe droplet volume on superhydrophobic surfaces， as illustratedin Figure 1. Figure 1. The principle of dynamic contact angle measurement. Microfluidics is a growing field of science which studies the behav-ior of fluidics in microscale. The application areas for microfluidicsrange from analytical and diagnostic microchips to microfluidicfuel cells. Water-based liquids can be manipulated in thesedevices by integrating superhydrophobic areas or superhydro-phobic tracks on them.Mertaniemi et al. have demonstrated suchtracks for enabling fast and simple transport of water droplets inmicrofluidic devices. [8] The superhydrophobic tracks were prepared in metal plates bymilling or laser cutting and in silicon by ion etching. The metalsurfaces were coated using a combination of silver microstructureand a fluorinated thiol surfactant， and the silicon wafers werecoated with a fluoropolymer in CF， plasma. Figure 2 shows a waterdroplet in a superhydrophobic track. Wetting properties of a superhydrophobic copper surface werecharacterized using the Attension Theta optical tensiometer. Themeasured data on five different spots on the surface is shown inFigure 3. Figure 2. A Water droplet in a superhydrophobic track. First， a 1-pl droplet was applied on the surface， and the needlewas lowered behind the drop so that the tip was about at themidway of the droplet height. The volume of the droplet wasslowly increased to 2 pl， at a drop rate of 0.05 pl/s. In order tominimize hysteretic effects， the addition of water was stopped for30 s before starting the contact angle measurement. To measure the advancing contact angle， the volume of thedroplet was increased from 2ul to 10 ul at 0.05 ul/s， recordingimages at 0.6 frames per second. Next， the droplet volume wasincreased to 15 ul and decreased back to 11 ul. After this， thedroplet volume was decreased slowly to 10 ul. In order tominimize hysteretic effects， the removal of water was stopped for30 s before starting the receding contact angle measurement. The receding contact angle was measured by decreasing thevolume at a rate of 0.05 pl/s， starting at a drop volume of 10 pl.Images were recorded at 0.6 frames per second until the drop lostcontact with the surface. The water contact angles of different materials used for preparingsuperhydrophobic tracks are shown in Table 1. The similarity ofcontact angle values of the zinc and copper surfaces wasexpected， since both surfaces were coated using the samemethod. However， the larger variance of receding contact anglesobserved for the zinc surface suggests that the coating had somedefects. The very small contact angle hysteresis of the siliconsurface makes it the optimal choice for applications whereextremely high mobility of water droplets on a surface is desired. Copper 166±2° 164±2° Zinc 168±2° 166±4° Silicon 170±2° 170±2° Table 1. Dynamic contact angles of a 6 pl water droplet on super-hydrophobic surfaces. Superhydrophobic surfaces， i.e.， surfaces with a contact anglelarger than 150°and a small contact angle hysteresis， are desiredfor their special wetting properties. Water droplets applied to asuperhydrophobic surface easily slide off， if the surface is tiltedslightly. For obtaining a superhydrophobic surface， a suitableroughness in combination of a hydrophobic surface chemistry isrequired. Wetting properties of superhydrophobic surfaces canbe characterized by measuring dynamic contact angles using anoptical tensiometer. ( References ) ( [1]Dorrer， C. & Ruhe，J. Some thoughts on superhydrophobic wetting. Soft Matter 5， 51 - 61， doi ： 10.1039/B8119 4 5G (2009). ) ( [2]Zimmermann， J.， Reifler， F . A.，F o rtunato， G . ， Gerhardt， L . -C. & Seeger， S. A Simple， One-Step Approach to Durable and RobustSuperhydrophobic Textiles. Advanced Functional Materials 18， 3662-3669， d oi： 1 0.1002/adfm.200800755(2008). ) ( [3] Gao， X. et al . The Dr y -Style Antifogging Properties of Mosquito Compound Eyes and Artificial Analogues Prepared by Soft Lithography. Advanced Materials 19， 2213- 2 217， doi：10.1002/adma.200601946 (2007). ) ( [4] Cao， L.， Jones ， A . K.， Sikka， V. K. ， Wu， J. & Gao， D. Anti-Icing Superhydrophobic Coatings. L a ngmuir 25， 12444-12 4 48， doi：10.1021/la902882b(2009). ) ( [5] Shirtcliffe ， N . J.， McHale， G.， Newton， M. I . & Zhang， Y. Superhydrophobic Copper Tubes with Possible Flow Enhance-ment and Drag Reduction. ACS Applied Materials & Interfaces 1，1316-1323， d oi：10.1021/am9001937(2009). ) ( [6] Gao， L. & McCarthy， T . J. The"Lotus E ffect"Explained： T w o Reasons Why Two Length Scales of Topography Are Important. Langmuir 22， 2966-2967， doi：10.1021/la0532149(2006). ) ( [7] Barthlott， W. & Neinhuis， C. Purity of the sacred lotus， orescape from c o ntamination in b i ological surfaces. P lanta 202， 1 -8，doi：10.1007/s004250050096(1997). ) ( [8] Mertaniemi， H . et al . Superhydrophobic Tracks for Low-Fr i ction， Guided Transport of W ater D roplets. A dv Mater 23， 2911-2914， doi ： 10.1002/adma.201100461(2011). ) Contact information ( At te ns i on Bio l i n Sci ent if ic Tie tajant i e 2 F I N - 0 21 3 0 Es po o， F i n lan d ) ( T E L + 3 58 9 5497 3300FA X +35 8 9 5 49 7 3 33 3 i nf o@ a tte ns i o n. c omWww.a t t en s io n .co m ) Attension products and services are provided to customers all over theworld through Biolin Scientific in co-operation with a highly competentnetwork of Distribution Partners. For a list of relevant contact details，please visit www.attension.com ATTENSION AN