Angle of Contact

When you fill the water in the glass vessels, the point where the water and glass touch will be concave or convex due to the angle of contact. This is formed between the liquid surface ( at the point of contact of liquid and solid) and the solid surface of the glass vessel.

Definition:

• Formation of the angle between liquid(water) and the solid surface(glass). Angle made when two mediums (glass and water) touch each other is called the angle of contact.
• It can also be defined by the tangent angle(at the point of contact) between the liquid surface and the solid surface.
• The angle of contact is determined by the nature of the liquid, the medium present between the liquid and solid and the nature of the solid material.
• If the temperatures increase, the angle of contact is also increasing.
• There might be some alteration in the contact angle due to the presence of impurities in the liquid.

Characteristics of the Angle of Contact:

• For a particular liquid-solid pair, the angle of contact remains constant.
• The angle of contact is determined by the composition of the liquid, the solid in contact and the medium above the liquid’s free surface.
• The liquid is said to wet the surface when the angle of contact between the liquid and the solid surface is small (acute). As a result, water wets the glass.
• The surface is not wetted if the angle of contact is too large.
• Mercury does not cause glass to become wet.
• If there are contaminants in the liquid, the angle of contact values will change.
• As the temperature rises, the angle of contact decreases.
• The angle of contact is 0 for a liquid that thoroughly wets the solid.
• It is constant for a particular liquid-solid pair and depends on the type of the solid in contact with the liquid. Other parameters stay unchanged.
• It is unaffected by the angle at which the solid meets the liquid surface.
• Surface tension has an effect on it. The temperature and purity of the liquid fluctuates as the surface tension changes. It is determined by the medium above the liquid’s free surface.

Points to be noted:

• When you smear glycerine on a glass, the angle of contact between the water and the glass shifts from acute to obtuse, resulting in a smeared window.
• The non-stick pan’s surface is coated with Teflon to shift the angle of contact from acute to obtuse.
• When the angle of contact is smaller, the detergent is more effective.

IMPORTANCE OF ANGLE OF CONTACT

Here are some important points about the angle of contact listed below

• When the angle of contact is obtuse, water will turn out into droplets.
• Water will spread over the solid surface – if the angle of contact is acute.

WATER CONTACT ANGLE

• When water is filled inside the glass vessels, the point where the glass and the water touch is concave upwards. In the case of mercury, it is convex upward.
• If the angle of contact is acute, the liquid is curved concave upward.
• The liquid surface turns out into a curved convex upwards when the contact angle is obtuse.

Dynamic Contact Angle

Contact angles that result from melting or de-melting of a solid are dynamic contact angles.

The advancing angle is utilized to investigate solid surfaces since the contact angle is measured at a surface that is re-wetted repeatedly as well as at various points that are quite close to one another. This reduces the impacts of time, like evaporation and averages out inhomogeneities in the local area.

When de-wetting, the receding angle is usually smaller than the approaching angle. This discrepancy, known as hysteresis, is utilized to determine the solid’s roughness.

The technical wetting process observed is one factor for distinguishing between dynamic and static contact angles. Dynamic measurements are superior for modeling dynamic processes like coating. When evaluating quasi-static processes, such as bonding in semiconductors, static contact angles are more useful.

In order to avoid misleading the measurement by introducing mechanical energy, the speed chosen for dynamic measurements must not be too high. On the other hand, it cannot be too tiny in order to rule out the above-mentioned time effects.

Methods to measure dynamic contact angle

During drop form analysis, increasing and decreasing the volume of a sessile drop.

• The Wilhelmy contact angle is calculated by measuring the force on a vertically immersed plate.

• Shape analysis of a drop moving across a solid’s sloped surface.

Conclusion:

According to the article, when a static quantity of liquid comes into contact with an impermeable solid surface, the liquid’s surface and the solid’s surface will normally rest at a specific angle (measured in the liquid). The contact angle is determined by the composition of both surfaces and is particularly susceptible to contamination. To wet the surface with water, a little amount of wetting agent or detergent may be necessary; only a small amount is required because only the surface of the liquid is involved, not the mass.