UNIT 2/EAT 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!)

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

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: F drag = 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): M sphere g = 6  r  + Mf luid g (in practice, we would say Mass = Density x Volume)