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Langmuir 2004, 20, 2726-2729
Analysis of Pseudopartial and Partial Wetting of Various Substrates by Lead Keith D. Humfeld,† Stephen Garoff,*,† and Paul Wynblatt‡ Department of Physics, Center for Complex Fluids Engineering, and Department of Materials Science and Engineering, Carnegie Mellon University, Pittsburgh, Pennsylvania 15213 Received September 19, 2003. In Final Form: January 20, 2004 Lead drops exhibit partial wetting on some substrates and pseudopartial wetting on others. In pseudopartial wetting, a film is in equilibrium with a capillary body with a nonzero contact angle. Using a free energy formulation appropriate for the experiments, we show the conditions under which minimization of the system energy is accurately achieved by minimizing the energy of the film alone. Using a set of simple surface energy isotherms, we explain the various wetting behaviors of lead. We contrast isotherms for autophobing systems and the metallic systems considered here.
I. Introduction Pseudopartial wetting, where the bulk phase of a fluid has a nonzero contact angle on top of a film of the same fluid, has been of interest in the scientific literature.1-6 Theoretical studies of pseudopartial wetting have established how the contact angle and the film thickness, and thus the wetting state (complete, partial, or pseudopartial), relate to the thickness dependence of the free energy per unit area of the film. Experimental studies directly observing equilibrium pseudopartial wetting are nearly absent from the literature. Here, we examine experiments where the same singlecomponent material, lead, wets six different surfaces, some exhibiting partial wetting and some pseudopartial wetting.7-12 As an example of a pseudopartial wetting, Figure 1 shows lead drops on Cu(111) with a growing thin film. At later times, the film covers the substrate and equilibrium pseudopartial wetting is achieved. First, we provide a brief overview of the experiments to provide context for our modeling. In the modeling section of this paper, we will explain the observed pseudopartial wetting in terms of the effects of an adsorbed layer on the solid-vapor surface tension. We will identify the Gibbs * Corresponding author. E-mail:
[email protected]. † Department of Physics, Center for Complex Fluids Engineering. ‡ Department of Materials Science and Engineering. (1) Brochard-Wyart, F.; di Meglio, J.-M.; Que´re´, D.; de Gennes, P.-G. Langmuir 1991, 7, 335-338. (2) Churaev, N. Rev. Phys. Appl. 1988, 23, 975-987. (3) Hirasaki, G. Thermodynamics of Thin Films and Three-Phase Contact Regions. In Interfacial Phenomena in Petroleum Recovery; Morrow, N., Ed.; Marcel Dekker: New York, 1991. (4) Hirasaki, G. Shape of Meniscus/Film Transition Region. In Interfacial Phenomena in Petroleum Recovery; Morrow, N., Ed.; Marcel Dekker: New York, 1991. (5) Sharma, A. Langmuir 1993, 9, 3580-3586. (6) Solomentsev, Y.; White, L. R. J. Colloid Interface Sci. 1999, 218, 122-136. (7) Wang, Z.; Wynblatt, P.; Chatain, D. Interface Sci. 1999, 7, 173180. (8) Rao, G.; Zhang, D. B.; Wynblatt, P. Acta Metall. Mater. 1993, 41, 3331-3340. (9) Shi, Z.; Lowekamp, J. B.; Wynblatt, P. Metall. Mater. Trans. 2002, 33A, 1003. (10) Moon, J.; Lowenkamp, J.; Wynblatt, P.; Garoff, S.; Suter, R. Surf. Sci. 2001, 488, 73-82. (11) Gangopadhyay, U.; Wynblatt, P. Metall. Mater. Trans. 1994, 25A, 607-615. (12) Wang, Z. Wetting and Energies of Metal/SiC and Metal/Graphite Interfaces. Ph.D. Thesis, Carnegie Mellon University, Pittsburgh, PA, 1998.
Figure 1. Lead film around a lead droplet on the Cu(111) substrate (ref 10). Halos in the photo are the growing film.
free energy of a system of a drop on top of a film and minimize it. For the micron scale drops investigated, we find that minimizing the energy of the entire system is accurately accomplished by minimizing the surface tension of the film alone. We will predict the wetting state for six simple solid-vapor surface tension isotherms. Comparing these results to the experiments, we determine which shape could describe each experimental system. In the discussion section, we will compare our surface tension isotherms to others. Further, we will compare a wellknown wetting behavior, autophobing, to the pseudopartial wetting observed in these metallic systems. II. Experimental Section In this series of experiments,7-12 substrates in an ultrahigh vacuum (UHV) system were cleaned with an ion gun and a continuous film hundreds of nanometers thick of Pb was deposited by evaporation. We briefly review the procedure used in these experiments. The samples were transferred to another UHV system equipped with a scanning Auger microprobe (SAM). After cleaning, the sample was heated just above the melting temperature of lead for several minutes, during which time the lead dewetted into drops. The substrate was then cooled just below
10.1021/la035759d CCC: $27.50 © 2004 American Chemical Society Published on Web 03/03/2004
Partial Wetting of Substrates by Lead
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Table 1. Equilibrium Contact Angles and Film Thicknesses of Solid Lead Droplets on Various Substrates substrate
contact angle (deg)
film thickness (ML)a
graphite(0001) (ref 11) 6H-SiC(0001) (ref 12) Ni(111) (ref 7) Cu(111) (ref 8) Cu(100) (ref 8) Al(111) (ref 9)
119 103 54 48 34 27
