Showing papers on "Sessile drop technique published in 1987"

Wettability of SiC by aluminium and Al-Si alloys

Abstract: The variations with time of the contact angle formed by molten pure aluminium or Al-Si alloys with single crystalline SiC were measured by the sessile drop method in a vacuum of 10−4 to 10−5 Pa at temperatures ranging from 933 to 1200 K. In the Al/SiC system, a “non-wetting-wetting” transition was observed at a temperature that decreases as time increases. After holding times of about 2 h, contact angles were stabilized to acute angles even at the aluminium melting point. Although additions of silicon to aluminium were in such amounts as to prevent Al4C3 formation at the interface, wettability in both pure Al/SiC and Al-Si alloys/SiC systems was not observed to differ appreciably.

The dependence of contact angles on drop size and line tension

Abstract: We report contact angle measurements of five n-alkanes, dodecane through hexadecane, on Teflon (FEP) as a function of drop size. In all cases the contact angles decreased by approximately 5° when the drop size was increased from approximately 1 to 4 mm contact radius. A complete solution to the problem of mechanical equilibrium of a sessile drop on a solid surface indicates that the dependence of the contact angle on drop size may be explained by including the effect of line tension in the Young equation. The observed drop size dependence of the contact angle yields a line tension of (2.5 ± 0.5) × 10−6 J/m. Over the range of n-alkanes studied it was not possible to discern any dependence of the line tension on liquid surface tension.

Surface properties of liquid 3 He and 4 He: A density-functional approach

Abstract: We present a comparative study of the free surface of $^{3}\mathrm{He}$ and $^{4}\mathrm{He}$ at zero temperature using phenomenological density functionals. The functional for each fluid is adjusted on experimentally known properties; the surface profile is then obtained variationally. Simple analytical formulas are derived for the relevant quantities characterizing the surface. Our results are compared with other available theoretical calculations.

Estimation of the Surface Energy of Polymer Solids

Abstract: The methods to estimate the surface tension of polymer solids using contact angles have been reviewed in the first part. They are classified into the following three groups depending on the theories or the equations applied: (1) the methods using the Young's equation alone, (2) the methods using the combined equation of Young and Good-Girifalco, and (3) the methods using the equations of work of adhesion. Some notes and comments are given for each method and results are compared with each other. The two-liquids method for rather high energy surface is also introduced. Next, some new possibilities to evaluate the surface tension of polymer solids are presented by our new contact angle theory in consideration of the friction between a liquid drop and a solid surface. The advancing and receding angles of contact (θ a and θ r ) are explained by the frictional tension γF and accordingly two kinds of the critical surface tension γC (γCa and γCr ) are given. This work has shown that one of the recommend...

Equilibrium of the Triple Line Solid/Liquid/Fluid of a Sessile Drop

Abstract: The equilibrium at the triple line where a liquid and a fluid (either vapour or a second liquid immiscible with the first) meet on a solid surface was originally described nearly two centuries ago1. By using a simple vectorial argument, the well-known Young equation may be obtained by resolution of the three interfacial tensions, γ, parallel to the solid surface: $${\gamma _{s2}} = {\gamma _{s1}} + {\gamma _{12}}\cos \theta$$ (1) where 1, 2 and S represent respectively the liquid, the fluid and the solid and θ is the contact angle measured in phase 1. Nevertheless, an objection has on occasion been presented. Although everything is balanced parallel to the solid surface, nothing would seem to counteract the vertical component γ12 sin θ.2,3 When the solid is treated as perfectly rigid, it is possible to apply variational calculus and the criterion of minimum free energy at equilibrium. The result is that equation 1 is perfectly correct.4–9 When the solid is considered to be elastic, but very thin, a variational treatment leads us to take into account, in addition to interfacial effects, those due to elastic strain energy and (implicitly) gravity10. The approach invokes the modelling of the solid either by thin plate or membrane theory. This treatment leads to modified equilibrium conditions although in practice the effect will be very small except for very thin solids (cell walls?).

Equilibrium of liquid drops on thin plates; plate rigidity and stability considerations

Abstract: Following an earlier paper, the equilibrium of an axisymmetric sessile drop resting on a thin, elastic plate has been considered using the minimum free energy hypothesis. A consequence is that although Young's equation is still obeyed at the triple line when considering the true contact angle, the apparent contact angle (i.e. with respect to the horizontal) invokes parameters other than simply the interfacial free energies, such as the drop volume and elastic properties of the solid. A quantitative estimate of apparent contact angle dependence on plate thickness has been made in the case of small drops. Finally, consideration of stability, again using the minimum free energy hypothesis, suggests that the axisymmetric configuration may not always be the most likely. This, it is conjectured, may have consequences in cell biology.

On the use of water in the measurement of solid surface tension

Abstract: Characterization of surfaces which undergo biological fluid contact is often aided by contact angle analysis. A comparison of solid surface tension evaluation methods, particularly addressing a sessile drop technique including and excluding water as a diagnostic liquid, and one in which the surface itself is equilibrated with water is reported for four materials. A particular non-linear relationship between macromolecular deposition and solid surface tension has been observed in our laboratory and elsewhere. This relationship may not be predicted if surface analysis is performed outside an aqueous environment.

Wettability of transition metal boride eutectic alloys to graphite

Abstract: The wetting of nickel, platinum and palladium boride eutectics (Ni55B45, Ni62B38, Pd72B28, and Pt72B28) to polycrystalline graphite has been studied by sessile drop and surface analytical techniques. The wettability is critically influenced by surface segregation of low-level bulk impurities in the alloys. At the melting temperature, the impurities segregate to the surface of the liquid alloy and inhibit wetting. A poorly-wetted droplet with a large contact angle results. Good wetting results by introducing materials into the contact system which possess high chemical reactivity to the major segregant species, either as a surface coating or by incorporation into the alloy during manufacture. The materials act to suppress segregation by tying up the alloy impurities through compound formation. The results indicate that the use of surface sensitive techniques in the study of wetting is essential.

A Reappraisal of Wetting in the System A1-A12O3from 750–1000°C

Abstract: Wetting plays an important role in many technological aspects of the aluminum industry including: fluxless brazing,1 the selection of refractories for containment2 or filtration and the fabrication of metal-matrix composites.3 While the importance of the oxide film on aluminum in this context was recognized early,4,5 the extremely low equilibrium oxygen pressure of the Al-Al2O3 system (10−48 atm at 800°C) precluded the study of this system under the conditions of controlled oxygen concentration as has since been done with other metal-oxide systems.6,7Therefore, kinetic factors such as gas flow, metal impurities, heating rates and gas composition all contributed to the structure and thickness of the oxide film on the aluminum, and consequently to disparate results. Early studies of interfacial phenomena in the Al-Al2O3 system made use of high vacuum techniques to clean the metal surface of oxide films by the formation and removal of a volatile suboxide, Al2O5,8,9 Despite various experimental precautions, contact angles reported for this system at temperatures below approximately 1000°C exhibit wide scatter. Little attempt has been made to date to reconcile these cjgflicting data. Recent reevaluation of the surface tension of aluminum,10 and its contact angle with alumina substrates,11 suggests that most prior low-temperature measurements represented the interaction of aluminum with its substrate in the presence of an oxide film at least a monolayer thick and in other cases probably 50–100 Angstroms or more in thickness.

Stability of polymeric surfaces subjected to ultraviolet irradiation

Abstract: Polymers are generally low surface free energy materials. Their polar component of the surface free energy is rather low in comparison to that of water. As a result, such solids tend to maintain undesirably high values of the solid-water interfacial free energy when placed in contact with biological fluid environments. One of the surface modification techniques to increase the polar component of the surface free energy, and thereby to reduce the solid-water interfacial energy of a polymeric solid, is the ultraviolet irradiation of the solid in air. A model polymeric surface, smooth, chemically homogeneous, and tightly adherent to the substrate, was prepared in this study by radiofrequency sputter deposition of polymethylmethacrylate onto a polished, single-crystal, silicon substrate. The stability of this polymeric surface when subjected to ultraviolet irradiation for various times was monitored by measuring the time changes in the contact angle of a water drop on the solid which was equilibrated in a nonpolar environment such as octane. It is inferred that the UV irradiation can degrade the polymer matrix and generate photooxidation products which are slowly extracted by water. The incorporation of the extractables in the probe liquid drop (water) deformed the latter and after a certain time the top portion of the drop's interface became planar. Under such conditions, the drop appeared to be gel-like and had an elastic response to small external forces such as the touching of its surface with a needle. After a long period of time, the water drop finally disappeared by penetrating into the pores generated in the polymer as a result of its degradation and the extraction of the photooxidation products.

The wetting of an alumina substrate by liquid silver.

Abstract: The wetting of the Al2O3 substrate by liquid silver was investigated by means of the sessile-drop technique in an atmosphere of argon. An empirical relation was established between the contact angle and the temperature or surface tension of the liquid drop by the use of the diameter and the height of the sessile drop. The contact angle was constant as the time increased while it decreased linearly with the increase in the temperature. The variation in the contact angle with the temperature is correlated with the surface tension of the sessile drop. The critical surface tension (γc) of Al2O3 is 361 dyn cm−1, while the surface tension of liquid silver at 0 K is 790 dyn cm−1.