Effect of Surface Conditions on Material Properties

24.12.2018

Surface is a term most people use or hear in different occasions almost every day. If one asks about the meaning of this term, they usually face trouble in finding eloquent definitions, and the most of those inadequate answers can be entertaining: “The outside of an entity.” “The part of an object we can see.” “The side of any material we can touch.” “The upper side of an object, like top of a table.” “Anything in contact with air.” It is obvious that these are not satisfactory answers to define the term “surface.” An acceptable description shouldn’t be restricted by any of the above-mentioned conditions. A surface can be defined as a transition layer between two immiscible or partially miscible phases. These phases could be solids, liquids, gases or vacuum. It is apparent that we cannot speak about a gas-gas or gas-vacuum surface for those phases are completely miscible. The remaining possibilities are liquid-gas, solid-gas, solid-solid, solid-liquid, solid-vacuum and immiscible or partially miscible liquids (e.g., water-oil or water-butyl alcohol).

Liquids with very high boiling points (Say 350oC or above) and vacuum can also form surfaces for the miscibility (namely, the evaporation) of those liquids in vacuum would be very restricted.

The term interface has the same meaning with surface, although it is usually used in the circumstances if none of the phases is a gas. Investigating the surface properties of the materials is an ever increasing field of study and has the potential to offer beneficial information on involved material properties [1-11]. Short-range forces of attraction exist among the molecules and they are responsible for the existence of liquids and solids. The molecules located within the bulk of each phase are subjected to equal forces of attractions in all directions. However, those touching the molecules of the other phase experiences unbalanced attractive forces (Fig. 1a). This unbalance condition is reduced toward each phase and after a certain distance it becomes ignorable, ending the border layer. In many industrial, even daily life processes, the properties of this unbalanced surface condition can result in a number of benefits or create numerous problems especially when at least one of the involved phases is a fluid. For example, if more stable phase (usually solid) holds atoms, ions or molecules from the other phase to balance its molecular attraction forces, this process of holding constituent parts of the other phase on its surface is called adsorption. Adsorption is a valuable process commonly used in many applications such as in air or water purification and recovering dissolved valuables (Fig.1b).

a b

Figure 1. The magnitude of unbalanced attraction forces on the first layer molecules is steadily reduced downward, and then disappears (a). If the first phase is a fluid (i.e., liquid or gas), top layer molecules holds atom, molecules or ions from it to reduce the inward pulling force on them, resulting in an process called adsorption (b).

Surface tension is defined as the force acting at right angle to any line of unit length on a surface. It is an important feature especially for liquid-liquid and liquid-gas systems as in the cases of cleaning dirt with soaps and detergents and, by adjusting the surface tension between two liquids, the formation or prevention of emulsions. Soaps and detergents are members of a group of chemicals called surfactants, and are used to make water-hating (hydrophobic) dirt water-loving (hydrophilic). Lubricants are used to alter the surface properties of solids, such as to reduce friction forces or lessen abrasion (Fig 2).

Figure 2. With lubrication of moving metal parts, the friction forces and wear will be less than those of metal-metal frictions.

The surfaces of most materials gain an electrical charge in polar liquids (e.g., in water) and its sign and magnitude is a major parameter in governing numerous industrial processes; adsorption, surface cleaning, flocculation, flotation, adjusting suspension stability and reducing particle sedimentation time in waste water treatment to name a few (Fig. 3).

a b c

Figure 3. Most solids gain an electrical charge in polar liquids and the sign and the magnitude of the charge is imposed by the liquid and solid properties. When solids particles are added into water (a), if all the particles gain similarly signed charge, they repel each other to form a stable suspension (b). If the particles are differently charged, or altered to be so, they attract each other and increase particle sizes result in their sedimentation. It can also be utilized in the formulation of paints with improved stability ad shelf lives. Surface preparation before protective coverage is another area of surface treatment. Even the reactions by unprotected surfaces toward threats can be different based on their conditions (Fig. 4).

Figure 4. The speed of metal corrosion is higher on rough surfaces in comparison to that of smooth ones due to the increased surface area, especially at early stages of corrosion [5].

Before the twentieth century, surfaces were regarded as the world of unknown dimension since the technical means were not advanced enough to study them. Now days, there are numerous technical means to understand surfaces down to the nanometric scales. The names of these new techniques and relevant instruments can form extensive lists consisting of usually shortened forms of long, multiple-word names like SEM (scanning electron microscopy), TEM (transmission electron microscopy), STEM (scanning transmission electron microscopy), LEED (low energy electron diffraction), HEED, AES, ESCA, STM, TPD, AFM, FEM and FIM. A better understanding of surface physical and chemical properties should be a basic requirement in many areas like the improvement of efficiency in surface alterations to painting, polishing, attaching materials and protection against corrosion or wear.