In wet cleaning, the workpieces are soaked in liquid cleaning medium, which can transfer the cleaning force and disperse and stabilize the dirt peeled off the surface of the workpieces. All this can be done only on the premise that the surface of the workpiece can be wetted by the cleaning medium. If the surface cannot be wetted by the cleaning medium, the workpiece cannot be wet cleaned.
1. Wetting effect
When the solid workpiece in the air is put into the liquid and the solid surface contacts with the liquid, the original solid-gas interface disappears and a new solid-liquid interface is formed. This phenomenon is called wetting. Wetting can also be said to be a process in which one fluid is replaced by another on a solid surface.
Wettability is the ability of a liquid to spread on a solid surface. That is to say, if the liquid spreads on the solid surface, the contact surface tends to expand, which is called wetting. If the liquid cannot spread on the solid surface, the contact surface tends to shrink into a spherical shape, which is called non-wetting. Wettability means that the adhesion of liquid to solid surface is greater than its cohesion, while non-wetting means that the adhesion of liquid to solid surface is less than its cohesion.
The essence of wetting is the adhesion of cleaning medium to workpiece surface, that is, the adhesion of pollutants is replaced by the wetting of cleaning medium to workpiece surface. Wetting is a prerequisite for cleaning. If the cleaning medium cannot wet the surface of the workpiece, it will be difficult to exert the decontamination effect of the cleaning medium. The wetting of the cleaning medium on the workpiece surface weakens the adhesion of pollutants to the workpiece surface and facilitates the stripping of pollutants.
When the liquid completely wets the solid, the liquid will cover the whole solid surface or form a monomolecular film on the solid surface. This phenomenon may also occur when two kinds of liquids are in contact with each other. For example, some oils can spread on the water surface as a monomolecular film of oil.
The wetting ability of cleaning medium also shows the ability of cleaning medium to penetrate and penetrate gaps and slits. The wetting ability of cleaning medium is not only related to the properties and morphology of workpiece surface, but also related to the density, viscosity and surface tension of cleaning medium. If the surface of the workpiece is unchanged, the wettability is directly proportional to the density of the cleaning medium and inversely proportional to the viscosity and surface tension of the cleaning medium.
2. Wetting angle
Contact angle is usually used to reflect the degree of wetting. At the junction of liquid, solid and gas, the tangent of liquid surface and the tangent of solid surface are made, and the included angle θ formed by the two tangents passing through the liquid interior is called contact angle, as shown in Figure 2-10. When θ is an acute angle, the liquid spreads on the solid surface, that is, the liquid wets the solid. When θ=0, it is called complete wetting. When θ is an obtuse angle, the liquid surface shrinks without expanding, and the liquid does not wet the solid. When θ = π, it is called completely non-wetting.
figure 2-10
The contact angle is also called wetting angle. When the wetting angle is less than 90, it means that the cleaning medium can wet the surface of the workpiece. When the wetting angle is 90, it is said that the cleaning medium cannot wet the surface of the workpiece. The smaller the wetting angle, the better the wetting.
The wetting angle θ1 of the cleaning medium in fig. 2-10(a) is smaller than the wetting angle θ2 shown in fig. 2-10(b) and the wetting angle θ3 shown in fig. 2-10(c), so the wettability of the cleaning medium in fig. 2-10(a) is better than that in figs. 2-10(b) and 2-10(c).
The wetting angle determines the degree of wetting. It can be seen from Figure 2-11(a) and Figure 2-11 (b) that two different shapes are formed at the contact between the liquid and the solid wall: concave and convex.
For example, water drops fall on the glass plate and slowly spread along the glass plate, and the contact surface tends to expand, that is to say, the wetting angle θ between water and the glass plate is acute, as shown in Figure 2-11(c), this phenomenon is wetting. However, when mercury falls on the glass plate, it appears spherical, that is to say, the wetting angle θ between mercury and glass plate is obtuse, as shown in Figure 2-11(d), this phenomenon is non-wetting. Although mercury can't infiltrate the glass plate, after wiping the zinc plate with dilute sulfuric acid and dropping mercury on the zinc plate, we will see that mercury slowly disperses along the zinc plate instead of being spherical. Therefore, the same liquid can soak some solids, but not others; Mercury can infiltrate zinc plate, but not glass plate. Water can wet glass plates, but not paraffin.
figure 2-11
In fig. 2-11, the wetting angle θ1 shown in fig. (a) is smaller than the wetting angle θ2 shown in fig. (b), so the wettability of the cleaning medium in fig. (a) is better than that in fig. (b). The wetting angle θ3 shown in figure (c) is smaller than the wetting angle θ4 shown in figure (d), so the wettability of the cleaning medium in figure (c) is better than that in figure (d). Therefore, the degree of wetting can be measured by measuring the wetting angle.
3. Surface tension
In a multiphase system, there are interfaces between phases. Traditionally, people only call gas-liquid and gas-solid interfaces surfaces. Because of the different environments, the forces on the molecules at the interface are different from those in the phase body. For example, the resultant force of a water molecule inside water subjected to the action of surrounding water molecules is 0, but that of a water molecule on the surface of water is not the case. Because the attraction of the upper space gas phase molecule to it is less than that of the internal liquid phase molecule, the resultant force of the molecule is not equal to 0, and its resultant force direction points vertically to the inside of the liquid, resulting in the liquid surface having a tendency of automatic shrinkage, which is called surface tension.
As we know, a sphere is a geometric body with the smallest surface area under a certain volume. Therefore, under the action of surface tension, liquid droplets always try to keep spherical shape. The following drops are spherical, and the drops on our common leaves are also approximately spherical. Surface tension can also be regarded as a manifestation of cohesion between liquid molecules. This is because the molecular forces on the surface of liquid are different from those on the inside of liquid, and the intermolecular forces on the inside molecules cancel each other out and are in equilibrium. However, the intermolecular forces on the molecules on the surface of the liquid are unbalanced, and they are attracted by a kind of attraction vertically pointing to the inside of the liquid. Therefore, in order to extend the liquid surface, it is necessary to resist the shrinking force of the surface.
When water is dispersed into mist droplets, that is, its surface is enlarged, and many internal water molecules move to the surface, it is necessary to overcome the surface work of this force on the system. Obviously, such a dispersion system stores more surface energy.
Under the condition of constant temperature and pressure, the work consumed to increase the unit surface area of liquid by 1cm2 is called surface free energy. Surface tension can be defined as the force applied along the unit length of the surface. The unit of surface tension is NewtoN/meter (n/m) in SI system, but it is still commonly used as dyne/cm (1 dyn/cm = 1 Mn/m).
Because of the surface tension, the liquid surface tends to shrink automatically, just like an elastic film, which can keep the minimum surface area of the liquid. Therefore, if the surface tension is low, the liquid can expand and do less work, that is, it consumes less energy and is easy to wet. The surface tension of cleaning medium plays an important role in cleaning. If the surface tension is low, it will be easy to wet and penetrate, and weaken and cut off the adhesion of pollutants to the workpiece. The surface tension of some cleaning media is shown in Table 2-2.
table 2-2
The surface tension of liquid changes with the change of temperature, and the surface tension of liquid decreases with the increase of temperature. Heating the cleaning medium in wet cleaning is to reduce the surface tension of the liquid, so that the cleaning liquid can better wet the surface of the workpiece, which is beneficial to the cleaning of the workpiece.
4. Interfacial tension
Interfacial tension refers to the tension between two phases which are immiscible. Surface tension is only a special form of interfacial tension, which refers to the tension between gas-liquid or gas-solid interface, and sometimes interfacial tension refers to the tension between liquid-liquid interface.
The surface tension of liquid is the interfacial tension between liquid and air. When a liquid comes into contact with another immiscible liquid, the force generated at the interface is called the interfacial tension between liquid phases. When a liquid comes into contact with a solid surface, the force generated at its interface is called the interfacial tension between the liquid phase and the solid phase. The surface tension of liquid, that is, the free energy of liquid surface. The interfacial tension between the surface and air is the free energy of the solid surface. Different solid surface materials have different surface free energy. And the energy on the surface of metal and general inorganic matters is over 100mN/m, which is called high-energy surface. The energy on the surface of organic matter such as plastic is low, which is called low energy surface.