Fluid Dynamics

INTRODUCTION

In the fascinating field of fluid dynamics, three fundamental phenomena shape our understanding of liquids and gases: surface tension, capillarity, and viscosity. These forces govern how liquids behave, from the way water droplets dance on a leaf to the resistance you encounter when stirring honey.

In this article, we explore the principles and everyday applications of these forces.

SURFACE TENSION

The surface of a liquid at rest behaves as of it were a stretched, thin piece of membrane under tension. For example, a drop of water formed at the tip of a leaf in the early morning or at the end of a dripping tap looks like a spherical water balloon. The surface appears to be under tension, stretched like an elastic skin without breaking. We call this tension, "Surface Tension"

Surface tension arises due to the strong intermolecular forces that holds the liquid molecules at the surface together. It is the tendency of the liquid to reduced their exposed surface to the smallest possible area. If a line is drawn on the surface of a liquid, the force due to the surface tension acts perpendicular to the line and at a tangent to the surface of the liquid. When mercury spills on a clean glass plate, the smallest droplet are seen to be spherical and are also seen to roll over the glass surface as seen below 👇

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The larger drops due to their weight are flattened with curved sides.

Surface tension therefore is a property of liquids that arises from the cohesive forces between the molecules at the liquid's surface.

Application of the Effects of Surface Tension

1. Waterproof Materials: Such materials used for umbrella, Raincoat or tents are usually treated with oil-based substances which prevents water from wetting the materials. A thin water film is usually formed across the spaces between thee threads of the fabric due to the surface tensioon "skin" on the raindrops . This skin prevents water froom sipping through.

2. Cleaning action of soap and detergents: soaps and detergents lower the surface tension of water. This desirable for cleaning purposes because high surface tension of pure water can indeed impede its ability to easily move between the fibers of certain materials. with a lower surface tension, the water can wash away dirt particle. In addition, high temperature reduces the surface tension of water. Thus, it is more effective to use hot soapy water for cleaning purposes.

Also, detergents contains some chemicals with can combine with dirt and oil and make them soluble in water. With dirt or oil removed, the water can then wet the material and wash it clean.

Therefore, we reduce the surface tension of water by adding detergents(soap),alcohol, camphor or increasing water temperature by heating.

CAPILLARITY

Capillarity or capillary action is the tendency of a liquid to rise or fall in a narrow tube. When tubes with fine bores of different diameters are immersed in in a container of clean water, we observe that the water rises differently in the tube. The narrower the tube, the greater the height which the water rises.

When clean water is replaced with soapy water, it is observed that the rise in the liquid level is not as high as when clean water was used.

Whether a liquid rises or is depressed in a narrow tube or at the edge of a glass container depends on the relative strength of the cohesive or adhesive forces between the molecules of the liquid and those of the container.

• Cohesion is the force of attraction between molecules of the same kind e.g. the molecules of water

• Adhesion is the force of attraction between molecules of different kinds e.g. the molecules of glass and water.

While cohesion or adhesion determines the rise or fall of liquid when it is in contact with a surface, the degree of fall or rise depends on the surface tension of the liquid. When adhesive forces causes a liquid e.g. water to rise rise in a tube, surface tension acting on the circumference of the meniscus holds the water up as it creeps into the tube. Thus, cohesion, adhesion and surface tension are the forces responsible for the capillarity of liquid.

Applications of Capillarity

1. The ability of lamp wick to absorb the liquid paraffin from the base of the lamp is due to the capillary action of the liquid paraffin within the fibers of the wick.

2. The movement of water from the root of a plant to the upper part of its stem is due to the capillarity of water in the tiny tubes (xylem) in the plant.

3. The ability of a kitchen towel to absorb spilled fluid is also due to the capillary action of the fluid within the fabric of the towel.

VISCOSITY

Real fluids have internal frictions called "Viscosity".

Viscosity is the frictional force between the adjacent layers of fluids as they move past one another.

In liquids, viscosity is due to the cohesive force between the liquid molecules. In gases, it is due to the collision between the molecules. As the fluid layers begin to move at different speeds, they create a form of internal friction that prevents the fluid from flowing continuously without the aid of an external force. It is obvious that some liquids flow slower than others. These liquids have less internal friction, for example honey is more viscous than water and is harder to pour.

The viscosity of fluid is affected by temperature. the viscosity of a liquid tends to decrease with increasing temperature. As temperature increases, liquid molecules moves faster. They amount of time they spend in contact with each other decreases, thus the average intermolecular forces decreases, thereby also decreasing viscosity.

However, the viscosity of gas tends to increase with increasing temperature. This is because the frequency of intermolecular collision increases which also increases viscosity. The S.I unit of viscosity is pascal second (Pa S).

Viscosity (viscous force) gives rise to an opposing force that impedes the motion of objects through fluids. This opposing force is known ass drag. As an object move through a fluid, it drags the layers of fluid molecules adjacent to its surface by adhesion. This causes the immediate adjacent fluid layers to move at move at different speeds from the rest of the layers resulting in a viscous drag force on the object due to the liquids. Thus, a fluid with high viscosity produces a large drag force on an object travelling through it. Similarly, a fast moving object experiences a larger drag force.

Conclusion

In summary, surface tension, capillarity, and viscosity are essential in understanding how liquids and gases behave. These forces affect how liquids move and how molecules interact to create fascinating fluid behaviors.