Solid surface tension, contact angle

Since the surface tension of water is the same in the two systems, the difference in contact angle can only arise due to the surface tension of solids being different. The surface tension of liquids can be measured directly (as described in Chapter 2). However, this is not possible in the case of solid surfaces. Experiments show that, when a liquid drop is placed on a solid surface, the contact angle, 0, indicates that the molecules interact across the interface. This shows that these data can be used to estimate the surface tension of solids. 

If a liquid is placed on a solid surface then, depending on the nature of surface, the liquid will spontaneously form a droplet or spread out into a film. If a droplet is formed then there will be a value of contact angle, which is the angle which the tangent to the surface makes with the solid surface. The contact angle can be correlated with the surface tension values of the corresponding solid and liquid in the equilibrium condition ... 

The solid-liquid interfacial tensions of solids have been studied using contact angle measurements. It is one of the oldest techniques devised by Young in 1805 for studying the nature of solid surfaces. A contact angle 6 is the angle between a solid surface and a liquid meniscus, as shown in the figure beiow. 

Fig. 1 Lefi Sessile drop in equilibrium on a solid surface, with contact angle 0 and surface tensions >t, ys. and ysL- Right Action of liquid surface tension and Laplace pressure A P...

Table 1 provides an overview of a small sampling of liquids of the relevant properties for a few common optofluidic liquids. Note that this represents a non-comprehensive list, other parameters of particular interest include fluid-fluid surface tension, fluid-solid surface energy, contact angle, fluid-solid electroosmotic mobility, compatibility with soft elastomers (or other materials of interest), and numerous others. 

For example, although many reports revealed that the rare earth additions change the shape, size and composition of non-metallic inclusions formed in the steels and that they play a very important role in improving the various properties of steels, research is still not adequate. Thermodynamic data are still too scattered to accurately predict these characteristics of non-metallic inclusions and the conditions under which the shape, size and composition of inclusions are controlled. Data for the effects of rare earths on the surface and interfacial properties surface tension, contact angle, etc., of steels, are also unsatisfactory. Precise solubilities of rare earths in molten and solid iron and steels are still not available. Studies are also required on the nucleation process of the non-metallic inclusions themselves in the steels. Knowledge of all of these is essential to understand the metallurgical process and the various properties of steels. 

Contact Angle and Surface Tension Contact angle is a critical parameter in two-phase flow defined as the angle between the gas-liquid interface and the soUd surface. It is measured at the triple point where all three points intersect This can be interpreted as a measure of the wetting of the solid surface by a liquid on different surfaces. Contact angles depend on the base material of surface temperature, surface impurities, and surface morphology (roughness). 

Keywords Solid surface tension Solid surface energy Contact angle Work of adhesion Zisman method Surface tension component mefliod Fowkes method Owais-Wendt-Rabel-Kaelble mefliod Extended Fowkes mefliod Equation of state... 

Valency of an ion effective number of electrons per ion Angle on a crystal surface numerical constant an angle Contact angle at the edge of a solid Surface tension of a liquid Critical surface tension"... 

Young s equation is the basis for a quantitative description of wetting phenomena. If a drop of a liquid is placed on a solid surface there are two possibilities the liquid spreads on the surface completely (contact angle 0 = 0°) or a finite contact angle is established.1 In the second case a three-phase contact line — also called wetting line — is formed. At this line three phases are in contact the solid, the liquid, and the vapor (Fig. 7.1). Young s equation relates the contact angle to the interfacial tensions 75, 7l, and 7sl [222,223] ... 

At the three-phase contact line the surface tension exerts strong forces on the surface. For instance, if we consider a water drop on a polymer surface, typical contact angles are 90°. The surface tension pulls upwards on the solid surface. If we estimate the wetting line to have a width of 6 = 10 nm, the force F per unit length l can be related to the effective pressure exerted on the solid surface ... 

The pressure required to prevent liquid from entering a plug of a finely divided solid is twice as great for a liquid of surface tension 50 mN m-1, which completely wets the solid, as it is for a liquid of surface tension 70 mN m-1, which has a finite contact angle with the solid. Calculate this contact angle. 

The advancing contact angles, measured on smooth silicon surfaces coated with the polymer brushes or the silane, were used to calculate the solid surface tension ysv according to the equation of state approach (EQS) [39] ... 

Contact angle measurement is probably the most common method for solid surface tension measurement in condensed state. Young [71] described the equilibrium at three-phase boundary in terms of the vectorial sum, as shown in Fig. 3, resulting in the following equation of equilibrium forces balance... 

In many areas of applied surface thermodynamics, measurement of contact angles plays an important role. The range of applications of contact angle measurement is remarkable. It can be used as a simple tool to assess, for example, the cleanliness of the surfaces, or it can be a highly sensitive scientific measurement aimed at obtaining information on the solid surface tension and the... 

The measurement of contact angles on solids in powder or particle form is a challenging task, and numerous methods have been proposed 7. In this section, three successful indirect approaches are discussed capillary penetration into columns of powders, sedimentation volume of particles, and solidification fronts of particles. These methods are indirect because they provide the solid surface tension rather than the contact angle. For the sake of completeness, various direct approaches are also summarized briefly. 

In a praetical sense, it is not necessary to eliminate completely all evidence of roughness and heterogeneity. If the average contact angle and liquid surface tension measurements for a variety of liquids all give the same solid surface tension via the equation of state (11. then effectively the surface is sufficiently smooth and homogeneous. The required level of accuracy will determine the acceptable solid surface quality. 

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