Contact Angle Measurements
Contact Angle Measurements
Contact Angle Measurements
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<strong>Contact</strong> <strong>Angle</strong> <strong>Measurements</strong>Purpose• To demonstrate the relationship between the properties andchemistry of a surface:Wettability (ability of a fluid to cover a surface) varies with boththe completeness of the monolayer and its degree of order.Wettability also varies with the polarity of the monolayer surfacefunctional groups.• To determine the surface free energy of the monolayers bymeasuring contact angles as a function of surface tension of a seriesof liquids.TheoryTangent todrop profileθ = <strong>Contact</strong> <strong>Angle</strong>θLiquid dropSubstrateYoung’s Equation:γ sg = γ sl + γ lg cos θγ = surface tension which can be thought of as the energy required tocreate a unit area of an interfaceEnergy minimization:If the energy required to create the solid-liquid (sl) interface is greaterthan that required for creation of a solid-gas (sg) interface, then thecritical angle will be greater than 90°. In other words, the liquid willbead up on the surface to minimize the solid-liquid interfacial area.
<strong>Contact</strong> <strong>Angle</strong> <strong>Measurements</strong> (continued)Determination of surface free energycos θθZisman plot for monolayer of CH 3 (CH 2 ) 21 SH on gold 1Critical surface tension γ cγ lg (mN/m)• For a particular surface, liquids with a surface tension ≤ this value willwet the surface. A fluid with θ ≤ 10° “wets” the surface.• Surface free energy ∝ γ c so the lower γ c , the lower free energy of thesurface.Critical Surface Tensions γ c1Surfaceγ c , mN/mCF 3 (CF 2 ) 10 CO 2 H/Pt 6Teflon 18CH 3 (CH 2 ) 21 SH/Au 19CH 3 (CH2) 17 SiCl 3 /Si 20Hexatriacontane 22CH 3 (CH 2 ) 17 NH 2 /Pt 24polyethylene 31
<strong>Contact</strong> <strong>Angle</strong> <strong>Measurements</strong> (continued)Experimental Setup:SyringeFiber OpticIlluminationSampleCameraComputerExperiments(1) <strong>Contact</strong> angles on cleaned and uncleaned gold surfaces will bemeasured. This will to show the effect of surface contaminationon the wettability.(2) <strong>Contact</strong> angles on an octadecanethiol monolayer will bemeasured as a function of the annealing time. This will allowcorrelation between the degree of organization of the alkylchain and the wettability.(3) The surface free energy of gold, octadecanethiol monolayers,and MUD monolayers will be determined by measuring thecontact angles of different fluids on these surfaces andconstructing Zisman plots.
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