The surface tension of metals

Hogness,1 Burdon,2 Bircumshaw, and Sauerwald have done a great deal to render accurate measurements possible the best method is probably the maximum bubble pressure method, but the measurement of sessile drops (see Chap. IX), and of drop volumes, are also useful. Metals always have a very high surface tension. Table X gives typical results. 

The most remarkable features are the high surface tensions, greater, often many times greater, than those of any other substances and the occasional positive temperature coefficients of tension. There is no obvious correlation between the tension and other physical properties though (except for the last four metals in the table) the surface tension is higher for the metals with the higher melting-points, a fact which indicates that the cohesional forces in the liquid and in the solid state are similar in kind. 

For the remarkable rise in tension sometimes found with increasing temperature there is no satisfactory explanation as yet there is no reasonable doubt of the fact, although quantitatively the measurement of temperature coefficients is so difficult that there is often considerable 1 J.A.CJS. (1921), 1621. See Chap. Ill, 9. 

Metal y dynes per cm. Tem- perature dy/dT Temperature range for dy/dT Reference 

The author knows of no other cases of a positive temperature coefficient of tension, except some rather discordant results of Zickendraht,1 with sulphur, and the observations of Jaeger2 on liquid crystals. In every one of five cases the tension increases abruptly by a small amount, up to 

---

Kozakivitch, P. Measurement of the surface tension of metals. Seite IE in Nat. Phys. Lab. Symp. No. 9. London Her Majesty s Stationery Office 1959. 

Secondary effects of the presence of hydrogen have also been suggested. Thus, e.g., hydrogen lowers the surface tension of metals, and hence, the flow of dislocations in the region of grain boundaries could be made easier. 

It is immediately evident that in the case of metals there are problems connected with the free energy AG of condensation nuclei in Eq. (6.12). Note that the magnitude of the nucleation rate is very sensitive to the surface tension (cubic in the exponential expression). Not only is this quantity often poorly known for bulk liquids, but the surface tensions of small droplets almost certainly differ substantially from those of the bulk. Furthermore, the surface tensions of metals are typically much... 

Those components with small sublimation heats can segregate on surfaces because the surface tensions of metals are connected with its sublimation heats. There are no such connections between the surface tension and the sublimation heats on the surface of oxides. 

A liquid of density 2.0 g/cm forms a meniscus of shape corresponding to /3 = 80 in a metal capillary tube with which the contact angle is 30°. The capillary rise is 0.063 cm. Calculate the surface tension of the liquid and the radius of the capillary, using Table II-l. 

Because the surface tension of oxides such as Fe O, AI2O2, etc, is much lower than that of the majority of metals and alloys, the presence of... 

Adhesion to Metals. For interaction between coating and substrate to occur, it is necessary for the coating to wet the substrate (107). Somewhat oversimplified, the surface tension of the coating must be lower than the surface tension of the substrate. In the case of metal substrates, clean metal surfaces have very high surface tensions and any coating wets a clean metal substrate. 

This potential depends on the interfacial tension am of a passivated metal/electrolyte interface shifting to the lower potential side with decreasing am. The lowest film breakdown potential AEj depends on the surface tension of the breakdown site at which the film-free metal surface comes into contact with the electrolyte. A decrease in the surface tension from am = 0.41 J m"2 to nonmetallic inclusions on the metal surface, will cause a shift of the lowest breakdown potential by about 0.3 V in the less noble direction. 

Figure 2.1 (a) A schematic representation of the apparatus employed in an electrocapillarity experiment, (b) A schematic representation of the mercury /electrolyte interface in an electro-capillarity experiment. The height of the mercury column, of mass m and density p. is h, the radius of the capillary is r, and the contact angle between the mercury and the capillary wall is 0. (c) A simplified schematic representation of the potential distribution across the metal/ electrolyte interface and across the platinum/electrolyte interface of an NHE reference electrode, (d) A plot of the surface tension of a mercury drop electrode in contact with I M HCI as a function of potential. The surface charge density, pM, on the mercury at any potential can be obtained as the slope of the curve at that potential. After Modern Electrochemistry, J O M. 

In terms of understanding the mercury/electrolyte interface, it is clear from the above discussion that the measurement of the surface free energy (in terms of the surface tension), is central. If the clectrocapillarity technique could be applied to solid electrodes, then it is capable of supplying information extremely difficult to obtain by any other technique. Sato has indeed developed a technique to measure the surface tension of a metal electrode which he terms piezoelectric surface stress measurement and is based upon the previous work of Gokhshtein (1970). 

The stable form of arsenic is the gray or metallic form, although other forms are known. Cooling the vapor rapidly produces yellow arsenic, and an orthorhombic form is obtained if the vapor is condensed in the presence of mercury. Arsenic compounds are used in insecticides, herbicides, medicines, and pigments, and arsenic is used in alloys with copper and lead. A small amount of arsenic increases the surface tension of lead, which allows droplets of molten lead to assume a spherical shape, and this fact is utilized in the production of lead shot. 

Periodic variations in the surface tension of liquid metals, c1 , are shown in Figure 6.5. The much higher surface tension of rf-block metals compared to the s- and p-block metals suggests that the surface tension relates to the strength of interatomic bonding. Similar periodic trends can be found also for the melting temperature and the enthalpy of vaporization, and the surface tension of liquid metals is strongly... 

In Figure 6.7, different interfacial tensions or energies of metals are correlated with the fusion temperature in the form 7 fus-Vrm. In general the ratio of the average surface energy of the solid to the surface tension of the liquid is around 1.2... 

---