USING FINITE-ELEMENT ANALYSIS TO CALCULATE THE SHAPES OF GEOMETRICALLY CONFINED DROPS OF LIQUID ON PATTERNED, SELF-ASSEMBLED MONOLAYERS - ANEW METHOD TO ESTIMATE EXCESS INTERFACIAL FREE-ENERGIES GAMMA(SV)-GAMMA(SL)

Drops of normal alkanes (CH3(CH2)nCH3, n = 7-14) were confined to spec ific, geometrically defined areas of a surface by patterning the surfa ce with wetting and nonwetting regions. Each region of the surface com prised a self-assembled monolayer (SAM) of an alkanethiolate on gold, with wettable regions formed from CH3NHCO(CH2)15SH and nonwetting regi ons formed from CF3(CF2)9(CH2)2SH. The asymmetric shapes formed by the geometrically confined drops of liquid were calculated using a finite element analysis that minimized the excess surface free energy and gr avitational potential energy of the drop. A calculation started with t he area and shape of the wettable region of the surface, the volume of the drop, the liquid-vapor free energy, gamma(lv), and an assumed val ue of the difference between the solid-vapor and solid-liquid interfac ial free energies, gamma(sv) - gamma(sl). The value of gamma(sv) - gam ma(sl) was adjusted iteratively to fit calculated drop shapes to exper imental drop shapes. This combination of experiment and numerical anal ysis forms the basis of a new method to estimate values of gamma(sv) - gamma(sl) (and thus, using these values, to calculate the contact ang les, theta, that would be observed for unbounded drops) for liquids on organic surfaces. Estimates of theta in the range 15-degrees < theta < 30-degrees were confirmed using a conventional optical telescope and goniometer; estimates in the range 3-degrees < theta < 15-degrees wer e beyond the resolution of the telescope and goniometer.