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Viscometer

Three physical states of matter have different intermolecular distance and intermolecular forces between constituent particles. It affects the physical properties of these states and makes them different from each other. 

In solid state, the constituent particles are very close to each other that provide a close packed structure to this state of matter. Unlike solid state, the gaseous particles are far away from each other due to their kinetic energy. They have large intermolecular space between them. Liquid state is considered as the intermediate state of solid and liquid state. Liquids have definite volume due to moderate intermolecular force of attraction between particles that confine them in a definite space. Liquids acquire the shape of the container and can flow from a higher to lower level. The flow of liquids is due to weaker intermolecular force in a liquid than solid. 

Liquids are compressible due to intermolecular distance between particles which is larger than in a solid. They can also diffuse into another liquid. No doubt the rate of diffusion is much slower than gases. This is because the liquid particles move faster than solid but slower than gases. Heating of liquids converts them to gaseous state that is called ass vaporization. At high temperature, the intermolecular distance between liquid molecules increases that reduces the intermolecular attraction between particles whereas at low temperature the vapor converted to liquid and further cooling can convert it to solid state.

So we can say that liquids exhibit many unique physical properties which make them different from solid and gaseous state of matter. One of the properties of liquid is viscosity. It can be defined as the measurement of resistance of flow in fluids. It is basically internal friction of a moving fluid so fluid with large viscosity resists motion of fluid layers due to internal friction. Low viscous fluid can flow easily due to low internal friction between fluid layers. There are various methods to measure the viscosity of fluids.  DIY Method can be used to measure the viscosity. Viscometers are also used to determine the viscosity of different liquids.

 

Viscometer Principle

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The term 'viscosity' is related to fluid's thickness. Two-plate model is used to define this term mathematically. It can be determined by viscometry which is part of rheometry.  

Rheometry can be defined as the science of materials' flow behavior and deformation. We know that solid materials are elastic whereas liquids are viscous. On the basis of viscosity, matters can be classified as visco-elastic solids such as gels and visco-elastic liquids like shampoo or honey. Mathematically viscosity is inversely proportional to force and displacement. 

$\tau$ = $\frac{F}{A}$ $\frac{N}{m^{2}}$

High force with a low displacement results is measurement of high viscosity. Unit of viscosity is $\frac{N}{m^{2}}$ or pascal [Pa]. Viscometers are designed to measure the viscosity of fluids, semi-solids and gases. We know that viscosity is the measurement of the internal friction so viscometers function as a measurement tool within rheology.

Viscometer Diagram

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The diagram of Ostwald viscometer which is also known as -tube viscometer or capillary viscometer is given below. 

Ostwald Viscometer

How Does a Viscometer Work?

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It is a device which is used to measure the viscosity of the liquid. We must know the density of that liquid. With the help of viscometer, we measure the time for a known volume of the liquid to flow through the capillary between the given marks of viscometer. The liquid flows under the influence of gravity. The Ostwald viscometers named after the German chemist Wilhelm Ostwald. Usually pure water is used to calibrate the instrument as we know the viscosity of pure water. The time must be calculated for both known and unknown liquids that can be used to determine the viscosity of unknown liquid with the help of given formula.

$\eta 1$ = $\eta 2 * \frac{\rho1t1}{\rho2t2}$

Here:

•    $\eta 2$ = Viscosity coefficient for water or known liquid
•    $\eta 2$ = Viscosity coefficient for unknown liquid
•    $\rho 2$ = Density of water or known liquid
•    $\rho 2$ = Density of unknown liquid 

Viscometer Calibration

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In Ostwald viscometer the viscosity is calculated by the comparison of time to require for given volume to move across the given capillary tube at constant temperature. For calibration the instrument must be immersed in temperature controlled water bath.  The Poiseuille’s law for capillary tube is used to determine the viscosity of liquid. 

Q = V / t =  ∆Pπ r4 / 8 η l

Here:
•    t= time required for given liquid
•    V= Volume of liquid
•    ρ = density of liquid
•    η = Viscosity of liquid 
•    ∆P  = Pressure gradient
•    l = length of tube

V / t =  π r4ρgl / 8 ηl
So η = π r4ρgt / 8 V

Types of Viscometer

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Different types of viscometers are listed below. Out of them, Ostwald viscometer is most common viscometer. 
  • Ostwald viscometer
  • Vibrational viscometers
  • Falling sphere viscometer
  • Bubble viscometer
  • Oscillating piston viscometer
  • Rotational viscometers
  • Falling piston viscometer

Viscometer Experiment

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For the determination of viscosity of given unknown liquid, we use water as reference liquid and measure the viscosity with respect to water at room temperature with the help of viscometer. During viscometer experiment, first record the laboratory temperature. Take washed R.D.bottle and dry to measure of empty and filled bottles with distilled water. Always record the weight with stopper. Repeat same with filled the R.D. bottle with unknown given liquid. Now use these data to calculate the densities of given liquid as we know the mass and volume of both liquids.

In the next step, clean and rinse the viscometer with distilled water and fix it vertically on the stand. Filled this viscometer with specific amount of mixture (water and given liquid) and record the time of flows for each solutions. Repeat same experiment for 4-5 readings. Determine of the density of the liquid with give formula.

$\frac{Density of liquid}{density of water}$   = $\frac{Weight of liquid}{Weight of water}$

Now use the density of liquid for the determination of viscosity of unknown liquid.

Viscosity of the liquid = $\frac{tl * dl}{tw *dw}$ * viscosity of water 

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