Transcript Viscosity

UNIT 2/EAT
Good Enough to Eat
In this topic we look at:
• Fluid flow (VISCOSITY)
• Material properties
• Refraction & sugar content of liquids
(REFRACTOMETRY)
• POLARISATION of light
…in the context of food (especially sweets!)
VISCOSITY
We can say that viscosity is the resistance a material
has to flowing (or changing form). It affects the speed
at which a fluid flows – a viscous liquid is more ‘runny’.
This property can be thought of as an internal friction.
We can compare viscosities with a viscometer, but to
measure it we use the falling ball test (see later).
When a fluids moves slowly, its flow is orderly and we call
it LAMINAR FLOW, represented by STREAMLINES:
Streamlines close to the sides of the edge of the flow will
indicate slower velocity, as there is more friction with the
sides. The fastest flow is in the centre.
Fast moving fluids do not flow orderly – the streamlines
become chaotic & unstable, producing TURBULENT FLOW.
Layers move past each other creating friction, and this
increases if a liquid is more viscous. The flow forms loops,
whirls and eddies, wasting energy, causing more ‘drag’ and
heating the fluid up:
• Designers of cars or submarines want air or water to
flow past them in laminar flow, to reduce drag and save
energy.
• Chocolate flow in factories needs to be laminar to
prevent uneven coating and formation of air bubbles.
• Oil flowing in pipes across hundreds of miles of Saudi
Arabia would heat up if it did not have laminar flow.
Viscosity is affected by temperature:
DATA FOR WATER
perature 0
10
20
30
40
C
cosity
1.81 1.31 1.00 0.80 0.64
3
2
Ns/m
50
60
70
80
90
100
0.54 0.47 0.40 0.37 0.32 0.29
Engine lubrication:
Car engines use oil to prevent friction, but as the car
engine heats up during use, the oil viscosity will
change. An ideal oil at cold temperatures would be too
runny when hot to lubricate properly. The solution is to
design a special lubricant with additives that prevent
the viscosity changing with temperature.
STOKE’S LAW:
When a spherical object, moves through a viscous liquid
there is a viscous drag force upon it:
Fdrag = 6r
where r = radius of sphere,  = viscosity and  = velocity
of sphere.
We can therefore find this viscosity by dropping a
sphere in a fluid, and measuring its terminal velocity. At
this point:
Weight (downwards) = Viscous Drag + Upthrust
(upwards)
Since the Upthrust on sphere = weight of fluid
displaced (Archimedes’ Principle):
Msphereg = 6r + Mfluidg
(in practice, we would say Mass = Density x Volume)