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Introduction of Micro- /Nano-fluidic Flow Surface Tension 6/1/2015 1 J. L. Lin Assistant Professor Department of Mechanical and Automation Engineering
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Outline 6/1/2015 2 Surface tension concept and origin Surface tension induced pressure, Laplace law, minimal surfaces, meniscus on a fiber Influence of gravity, capillary length, capillary rise Contact angle, Young’s law Spreading parameter Zismann equation Contact angle measurements, contact angle hysteresis Surface roughness, Wenzel and Cassie-Baxter equations Superhydrophobic surfaces Electrowetting, electrically tunable surfaces
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Surface tension
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Liquid Jet 4
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jet speed 10 km/s
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Liquid Jet 6 case explosiveliner
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Liquid Jet
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Surface tension U U/2 a l dx
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Surface tension Liquid T [°C] [mN/m] Acetone 20 23.7 Diethyl ether 20 17.0 Ethanol 20 22.27 Glycerol 20 63 n-Hexane 20 18.4 Isopropanol 20 21.7 Mercury 15 487 Methanol 20 22.6 n-Octane 20 21.8 Water 0 75.64 Water 25 71.97 Water 50 67.91 Water 100 58.85
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Laplace Equation Δp for water drops of different radii Droplet radius 1 mm 0.1 mm 1 μm 10 nm Δp (atm) 0.0014 0.0144 1.436 143.61
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Zero curvature surface z x b
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Capillary length, capillary rise h 2R2R
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Contact angle solid liquid
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Youngs' Equation Contact angle is determined by the interfacial tensions : solid liquid dxdx SL SV LV Equilibrium
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Spreading parameter - total wetting - partial wetting
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Zismann equation cc cos 1 - const ( - c ) (Fox & Zismann (1950))
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Contact angle measurements Camera 1 (control) Camera 2 (measurement) Sample Experimental setup deposition system
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Contact angle hysteresis no stick-slip advancing receding stick-slip - hysteresis
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Wenzel Equation Contact angle is determined by the interfacial tensions : dxdx SL SV LV Equilibrium solid liquid
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Composite surfaces liquid solid A 1 A 2 00 3 m 0 = Cassie & Baxter (1944) Cassie – Baxter equation
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Superhydrophobic surfaces Solvent evaporation induced i-PP gel Porous isotactic polypropylene (i-PP) Fractal alkylketene dimer (AKD) AKD solidified from melt 0 = 109° 0 = 174° fractal 0 = 160° porous flat H.Y. Erbil, A.L. Demirel,Y. Avcy, O. Mert (2003) S. Shibuichi, T. Onda, N. Satoh, K. Tsujii (1996) 5 m 0 = 104° flat
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Superhydrophobic surfaces
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Topography hierarchy in lotus leaves A. Large-scale SEM image of the lotus leaf. Every epidermal cell forms a papilla and has a dense layer of epicuticular waxes superimposed on it. B. Magnified image on a single papilla of A. Micro- and nanostructures on the lotus leaf (Nelumbo nucifera)
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Superhydrophobic surfaces Examples
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Superhydrophobic surfaces Self-cleaning surfaces
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Superhydrophobic surfaces Examples Nanostructured surface of the superhydrophobic wings of cicada (Cicada orni).
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Superhydrophobic surfaces Examples Nanostructured surface of the superhydrophobic legs of the water strider (Gerris remigis).
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Electrowetting conducting liquid V conductive electrode dielectric film d Example: Water droplet on Cytop® surface [º] Equilibrium
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Electrowetting Equation Contact angle is determined by the interfacial tensions : dxdx SL SV LV Equilibrium solid liquid
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Electrowetting Substrate : Si / 60 nm SiO 2 / 20 nm CF 1.55 (CVD) Liquid:1-ethyl-3-methyl-1 H-imidazolium tetrafluoroborate 0 V – 80 V – 0 V
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Electrowetting Substrate : ITO / 250 nm SiN x / 1 m Cytop 0 V – 60 V – 0 V
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Lubrication principle Possible sources of hysteresis and stick-slip –mechanical roughness –compositional inhomogeneity –chemical contamination = 1 = 2 = 3 L F S S F L
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Tunable superhydrophobic surfaces 10 m Rolling ball Sticky droplet superhydrophobic slip boundary hydrophilic no slip liquid solid superhydrophobic hydrophilic
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f 2 >> f 1 cos ~ f strongly nonlinear effect contact angle control contact angle hysteresis control V = 0 V 0 liquid solid 00 liquid solid f1f1 f2f2 conductor isolator low-energy coating Tunable superhydrophobic surfaces
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Rolling ball Sticky droplet Tunable superhydrophobic surfaces
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Electrowetting induced transitions molten salt *, = 62 mN/m * 1-ethyl-3-methyl-1 H-imidazolium tetrafluoroborate 3 m pitch 4 m Tunable superhydrophobic surfaces
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180° 90° cos V 2 [V 2 ] pitch 1.05 m pitch 4 m Tunable superhydrophobic surfaces
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