The Wilhelmy balance. How can we measure surface tension? Surface tension, contact angles and wettability. Measuring surface tension.

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1 ow can we measure surface tension? Surface tension, contact angles and wettability Measuring surface tension The Wilhelmy balance F Some methods: Wilhelmy plate (next slide apillary rise De Nouy ring Pendant drop shape Stallagmometer KSV Sigma 70 Scientific Papers of the Bureau of Standards, Vol. 21, p (1926 The De Nouy ring method is similar, but uses a metal ring instead. (Image from face-kyowa.co.jp

2 Surface tension components Surface tension is caused by intermolecular forces (or the lack of such over the interface, and it can be dividied into contributions from different forces: Determination of sv from contact angles According to van ss and Good* the following is true for a contact angle: (1 cos 2 ( + = + + l s l s l s l = LW + AB Dispersive contributions LW = Lifshitz/van der Waals AB l = 2 + Polar contributions AB = Acid/Base + = Acceptor-contribution (Lewis-acid = Donor-contribution (Lewis-base Three liquids with different properties result in this system of equations: ( ( ( (1 + cos = l1 1 s l1 s l1 s l1 (1 + cos = l 2 2 s l 2 s l 2 s l 2 (1 + cos = l3 3 s l3 s l3 s l3 = 0 for the nonpolar liquid, gives s LW directly. Measured Sought: s LW, s+, s - Three unknown, three eq s...! * There are many models suggesting how s can be estimated: van ss, Adv. olloid Interface Sci., 28, 35 (1987; Good, Fundamentals of Adhesion New York:Plenum press (1991. ontact angle measurements ontact angle methods Solid (s Liquid (l sl sv sl < 0 > 90 Low energy surface Vapour (v sv Balance the horizontal forces: = + cos Young s equation: sv sl > 0 0 < < 90 cos = sv sl 0 igh energy surface ontact angles can be used to investigate surface properties! A E B D A Static, or Sessile Drop B Wilhelmy Plate aptive Bubble D apillary Rise E Tilted-drop ontact angle measurements need to be analyzed with care since factors like operator error surface roughness surface heterogeneity contaminated fluids sample geometry can influence the result.

3 Video goniometer ontact angle hysteresis Used for The result of a contact angle measurement depends on the direction of the three-phase line over the surface: a r Sessile drop aptive bubble Tilted drop ontact angle hysteresis: = a > r Advancing angle ysteresis is caused by chemical or topographical inhomogeneities on the surface. a > r Receding angle KSV AM200 Drop shape analysis Drop volume analysis ysteresis emerges as the three-phase line is pinned in metastable states with local energy minima. These appear since there are many points along the surface where the geometric contact angle is equivalent to Young s contact angle. r a Alternative derivation of Young s equation onsider the free energy change dg during an infinitesimal expansion dσ of the droplet: dσ cos dg = dσ + dσ cos dσ ls sv sv dσ sl In equilibrium, when no further expansion (or contraction occurs, dg = 0: dσ + dσ cos dσ = 0 ls sv cos = Diaz, Physical Review E 94, (2016, DI: /PhysRevE

4 Approaches for calculation of macroscopic contact angles Line tension For small droplets the contact angle is also determined by the tension of the line where the three distinct phases coexist, τ s. Surface tension: excess free energy per unit surface of an interface. Line tension: excess free energy per unit length of a three-phase contact line. The line tension makes the droplet shrink and its effect on the contact angle can be accounted for: sv sl τ s cos = R sl τ s R sv Diaz, Physical Review E 94, (2016, DI: /PhysRevE ydrophilicity / hydrophobicity Water can be used to assess the polar contribution to the surface energy: Polar surface, wetted by water, hydrophilic leophobicity 0 Non-polar surfaces, not wetted by water hydrophobic Polyethylene (PE 2 3 Long-chain alkanes ( 3 ( 2 n 3 can be used to assess the dispersive (van der Waals contribution to the surface energy. il F F leophobic surface, not wetted by long-chain alkanes il rganized monolayer ow does a liquid droplet behave on a surface? Wettability vs. hydrophobicity Wettability (a surface property The propensity of a liquid to spread over a surface. Poor wetting ( non-wetting Good wetting ( wetting ydrophilicity / ydrophobicity (bulk properties Note! ydrophilic and A material s capacity to absorb (imbibe water. hydrophobic are frequenly used to describe wettability - also in this course! ydrophobic ydrophilic

5 Zisman-plot, critical surface tension Spreading of hexanol on water Determination of sv and sl is difficult for solid surfaces! Initially At equilibrium Zisman: the surface energy is equal to the surface tension of the liquid with greatest tension, which completely wets the solid. cos = 1 β ( s c Pick from a homologous series of liquids, and plot cos vs. to obtain a line whose intersection with cos = 1 gives the critical surface tension c. β is system dependent, typically m/mn c hexanol water = 25 mn/m = 73 mn/m water/hexanol = 7 mn/m Shexanol / vatten = vatten hexanol vatten / hexanol = = 41 > 0 ; spreading! exanol is sparsely soluble in water, and will spread over the water surface and change the interfacial tension, so that at equilibrium, water+hexanol is 29 mn/m, and Shexanol / vatten = vatten+ hexanol hexanol vatten / hexanol = = 3 < 0 ; no spreading! (A Top view of events starting from the spill of oil on water surface followed by the addition of herder (150 mg/m 2 altering the interfacial forces and thus retracting the oil slick. (B A positive value of spreading coefficient S refers to the spreading of crude oil onto seawater surface. ( Application of herder at the edge of oil slick lowers the air-seawater interface tension, causing a retraction of the oil slick with S < 0. Water flowing uphill...? Wettability gradients! Gupta et al., Science Advances, 1(5, e (2015 haudhury, Science, 256, 1539 (1992

6 Superhydrophobicity The Lotus effect A Water on die cast polypropylene (PP, contact angle 104. (B Water on superhydrophobic PP, contact angle 160. [20 mg/ml PP dissoed in 60% p-xylen/40% MEK at 100, whereupon the soent was removed at 70 in a vacuum oven.] ( SEM image of the superhydrophobic surface. Erbil et al., Science 299, 1377 (2003 A superhydrophobic surface leaning of the leaf: Particle adhesion to the water droplet is stronger than to the leaf, and particles thus roll off the leaf with the water. heng, Appl. Phys. Lett (2005 Mincor TX TT textile coating (BASF Wall paint (Sto AG The Silic T-shirt (Kickstarter PNAS 108(19, (2011 DI: /pnas

7 ontact angles on structured surfaces Young Wentzel assie(-baxter AS Appl. Mater. Interfaces 3, 2630 (2011 = cos R cos Genzer, Biofouling, 22, 339 (2006 * = cos = 1+ φ [ cos + 1] Surface roughness... Increases wetting if < 90 Decreases wetting if < 90 Real area R = Projected area s sv sl = 1+ φs + 1 φ s = Surface fraction in contact with the liquid. Effective angle > if pores are air-filled. Superhydrophobic surfaces Effective surface composition Yoshimitsu, Langmuir 18, 5818 (2002 onsider composite surfaces, or mixed monolayers assie s law: cos n = i= 1 f cos i is the contact angle for a surface of component i, and f i is the fraction of component i in the composite. i i n i= 1 f = 1 i Polystyrene particles on a smooth surface Shiu, hem. Mater., 16, 561 (2004. B Smooth Si wafer With microstructure D With micro- and nanostructure Example, a binary composite: cos1 cos cos2 Liu, Adv. Mater., 21, 665 (2009 cos = f cos + f cos f1 + f2 =