Liquids: Viscosity

Introduction

A fluid’s viscosity is a measurement of its resistance to deformation at a specific rate. It corresponds to the informal sense of “thickness” in liquids: syrup, for example, has a higher viscosity than water. The internal frictional force between adjacent layers of fluid in relative motion is measured by viscosity. If a viscous fluid is driven through a tube, for example, it flows faster towards the tube’s axis than near the tube’s walls. Experiments have shown that a source of stress (such as a pressure difference between the tube’s two ends) is required to keep the flow going. It is due to the reason that a force is necessary to overcome the friction between the fluid layers that are in relative motion. In a tube with a constant rate of flow, the strength of the compensating force is proportional to the fluid’s viscosity.

Unit of Viscosity: 

The newton-second per square meter unit of viscosity is commonly expressed in SI units as pascal-second.

Formula of viscosity:

The most common formula and equation for calculating viscosity is Viscosity = (2 x (ball density – liquid density) x g x a2) (9 x v), where g = gravity acceleration = 9.8 m/s2, a = ball bearing radius, and v = ball bearing velocity through liquid.

Types of Viscosity:

Dynamic and kinematic viscosities are the two types of viscosity measurements used to describe fluids. These terms represent fluid flow in various ways depending on how they are measured, but they are interchangeable if the fluid density is known.

1. Dynamic Viscosity: 

Dynamic viscosity is measured by the ratio of the shear stress to the shear rate for a fluid. Newton’s equation indicates that the resulting shear of a fluid is directly proportional to the applied force and inversely proportional to its viscosity.

2. Kinematic Viscosity: 

 The ratio of the viscous force to the inertial force on a fluid is measured by kinematic viscosity. This is illustrated in the equation below, which may be used to convert between dynamic and kinematic viscosity if the fluid density is known. Kinematic viscosity is the diffusivity of momentum, similar to mass and heat diffusivity.

Significance of Viscosity:  

The viscosity of a fluid is the polar opposite of its fluidity, which indicates how easily it can flow. Internal friction between the molecules that make up the fluid is what it is all about. Viscosity is important in fluid transportation and lubrication engineering, injection moulding, spraying, and surface coating applications because it regulates the flow of the liquid. It is critical to understand the viscosity data in order to forecast fluid behaviour. 

Measuring viscosity: 

It is common knowledge that measuring the viscosity of fluids is critical for understanding their flow characteristics. The viscosity of a substance can be measured using a variety of equipment. Viscometers and rheometers are two types of viscosity measuring instruments. Generally,  widely used instruments for measuring viscosity are:

• Falling Sphere Viscometer
• Rotational Viscometer
• Zahn Cup
• Acoustic rheometer
• Capillary Viscometer
• Vibrating Viscometer
• Microfluidic Rheometers
• Fluorescence correlation spectroscopy

Factor affecting viscosity:

The viscosity of a fluid is affected by a number of things. These are the ones:

• Fluid Temperature: The viscosity of liquids usually reduces as the temperature rises.
• The viscosity of gases, on the other hand, increases as the temperature rises
• Flow Conditions: The viscosity of the liquid remains constant in laminar flow but changes in turbulent flow
• Pressure: The viscosity of gases often increases as pressure rises. The fact that liquids are incompressible has little effect
• Multiphase flow: The volume of each phase influences the viscosity of a multiphase flow
• Suspended Particles: Suspended materials increase the viscosity

Applications of viscosity:

 The term “viscosity” is commonly used in science and technology. The applications of viscosity are simply demonstrated by the following examples:

1. The molecular weight of organic liquids is calculated using viscosity information.
2. In lubrication engineering, viscosity data and its variation with temperature are critical in determining the best lubrication for a given piece of equipment. Light machinery, for example, employs low viscous liquids, whereas heavy machines require extremely viscous oils.
3. Viscosity data is essential for the preparation of certain medicines, such as syrups.
4. Cooking oils, fats, and butter, for example, are made to have a specified viscosity.
5. Gums, coolants, gasoline as a cleanser, braking fluid, cosmetics, food goods, and other products all require viscosity data to function properly during production.
6. The viscosity of blood determines how well it circulates within our bodies.

Conclusion 

The internal friction of a flowing fluid was defined as viscosity. Because its molecular constitution causes a lot of internal friction, a fluid with a high viscosity resists motion. A fluid with a low viscosity, on the other hand, flows readily due to its low friction molecular makeup. Fluid resists the relative motion of immersed objects as well as the motion of layers within the fluid with different velocities. The results of the original 5-trials – one of which was eliminated because the experiment only requires 4-trials per group – show that the time interval of the fall is indirectly related to the ball’s velocity, and the diameter of the steel ball is directly proportional to the velocity. It’s also worth noting that the viscosity of the solution varies, despite the fact that they should all be the same.