# Viscosity

Topics: Viscosity, Fluid dynamics, Liquid Pages: 12 (2279 words) Published: March 23, 2013
Viscosity of Liquids
Part I: Low Viscosities
Mona Kanj Harakeh

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Objectives
• To measure and analyze the viscosities of ideal (Toluene/p-Xylene) and nonideal (Methanol/Water) binary solutions and their components. • To determine the Activation Energy to viscous flow. • The effect of temperature change on the viscosity will be studied. Method: The viscosities of liquids are determined by measuring the flow time for various liquids in an Ostwald viscometer. 2

Ostwald viscometer

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Viscosity
• The resistance of

a liquid to flow is called its

viscosity
• Viscosity is a property of liquids that is important in applications ranging from oil flow in engines to blood flow through arteries and veins. Measuring viscosity • How long a liquid takes to flow out of a pipette under the force of gravity. • How fast an object (steel ball) sinks through the liquid under gravitational force.

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Molecular properties contributing to viscosity
Viscosity arises from the directed motion of molecules past each other, it is a measure of the ease with which molecules move past one another. It is affected by many factors such as: • Molecular size. • Molecular shape. • Intermolecular interactions (attractive force between the molecules). • Structure of the liquid itself. • Temperature(Viscosity decreases with increasing temperature the increasing kinetic energy overcomes the attractive forces and molecules can more easily move past each other). 5

Viscosity
 The IUPAC symbol of viscosity is the greek symbol eta “”.  Viscosity “η” of a fluid is its resistance to flow.  When a Liquid flows, whether through a tube or as the result of pouring from a container. Layers of liquid slide over each other.

The force (f) required is directly proportional to the Area (A) and velocity (v) of the layers and inversely proportional to the distance (d) between them. Av Equ. 1

f 

fd gcms cm   gcm 1 s 1  1 piose  1P Av cm 2 cms  2

2

d

unit of viscosity
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Viscosity Units
The unit of viscosity is the poise named after Poiseuille Jean Louis Marie. It is most commonly expressed in terms of centipoise “cP”. The centipoise is commonly used because water has a viscosity of 1.0020 cP at 20oC; the closeness to one is a convenient coincidence. • The SI unit of viscosity is Pascal-second (Pa·s) = N·s m–2 or Kg m-1 s-1.

• In cgs unit

1 Poise “P” = 1 g.cm-1.s-1 (dyne .s) 10-2 Poise “P”= 1 centipoise “cP” 1 Pa.s = 103 cP 10 P = 1 Kg·m−1·s−1 = 1 Pa.s 1 cP = 0.001 Pa.s = 1 mPa.s

• The conversion between the units: 1 P = 0.1 Pa.s

For many liquids at room temperature the viscosity is very small 7 (0.002-0.04) therefore (10-2 P), centiP is often used.

Ostwald Method
• Time for fixed volume V of liquid to fall through a capillary into a reservoir Upper Fiducial mark – Depends on density. – Depends on viscosity. • Reference liquid is used. • This type can be used for liquids of viscosity up to 100 poise. Lower Fiducial mark

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Ostwald Method
The rate of flow R (cm3/sec) of a liquid through a cylindrical tube of radius r and length l under a pressure head P is given by the Pousille equation. Equ. 2

Measurement of P, r, t, V, and l permits the calculation of the viscosity: Equ. 3

It is easier to measure the viscosity of a liquid by comparing it with another liquid of known viscosity. Since P = gh Equ. 4

The viscosity of a solution can be determined relative to a reference liquid (de-ionized H2O). 9

Oswald viscometer
The Oswald viscometer is a simple device for comparing the flow times of two liquids of known density. If the viscosity of one liquid is known, the other can be calculated. Ostwald viscometer is used to measure the low viscosities’ liquid. After the reservoir is filled with a liquid, it is pulled by suction above the upper mark. The time required for the liquid to fall from mark 1 to mark 2 is recorded. Then the time required for the same volume of a liquid of known...