The effect of the temperature on the viscosity of the syrup.

The effect of the temperature on the viscosity of the syrup

Aim: In this investigation I will be investigating the effect of temperature on the viscosity of golden syrup by measuring the rate of descent of a sphere through the syrup at different temperatures.

Viscosity is an internal property of a fluid that offers it resistance to flow. The flow of fluid involves different components of the fluid sliding against each other, as if they were in layers at different velocities. The ease at which the fluid flows is dependant on how easily the layers slide against each other. The lower the viscosity is the less the occurrence of friction between the layers as they slide against each other. Hence the easier the fluid flow. In contrast when there is a greater occurrence of friction between the layers, the flow of the fluid becomes more difficult to maintain and as a result they slide each other with less velocity, which gives it a higher viscosity.

Flow is harder to control in syrup whereas in water the flow is much simpler. Hence the syrup is more viscous than water.

In fluid the molecules move randomly. This includes the molecules which are at rest as well as those which are moving with a velocity. In addition to the random motion of the molecules a drift motion also occurs and it carries the molecules along with the flow. During the drift motion, molecules tend to move from layer to layer due to their random movement. Consequently for each molecule that shifts into another layer in one direction, another molecule will shift layer in the opposite direction. For instance, a molecule which is moving at high velocity layer shifts to an adjacent layer which flows with a lower velocity. In turn, a molecule that moves in a low velocity layer will shift to the adjacent layer which is at high velocity.

The variation in temperature affects the viscosity of a fluid. Increasing the temperature lowers the viscosity of the fluid. The fluid becomes thinner, less viscous and flows more freely. This is because molecules gain energy when the temperature is increased, so that they are moving at a high velocity and collide more successfully. However, when the fluid becomes cooler the viscosity increases and as the fluid becomes thicker, the rate of velocity becomes lower and doesn’t flow as easily. This is due to the reduction in energy from the molecules, which in turn reduces the movement of molecules. Less collisions occur, so that the molecules are more closely packed together.

Stoke’s law can be used in situations of steady motion. Therefore it relates to the motion of sphere through viscous liquid. It can be used to find the viscosity of the same liquid at different temperature.

delta p = difference in density between the sphere and the liquid

g = acceleration of gravity

r2 = radius of sphere

v = velocity = (distance sphere falls) / (time taken for it to be fall)

When a sphere falling vertically under gravity in a viscous fluid, there are three forces act on it known as:

it’s weight W, acting downwards
the upthrust U due to the weight of fluid displaced, acting upwards
the viscous drag F, acting upwards

The resultant downward force is (W – U – F) and causes the sphere to accelerate until its velocity is zero, and so the viscous drag reaches a value of, W – U – F = 0. at this point the sphere reaches it’s terminal velocity and so it continues to move with a constant velocity.

Variables

Mass of the sphere – Changing the mass of the sphere will change the weight of the sphere. An inconstant weight will have an impact on the velocity at which the sphere travels since the sphere travel with a different force each time.

Size / surface area of object – Varying the surface area of the sphere will affect the velocity at which the sphere travels. This is because when the surface area is increased it means the friction force opposing the motion of the sphere will be increased too. In effect, the time it takes for the terminal velocity to be reached alters.

The volume of Syrup – Volume should be kept the same, because changing the volume will change the density of the syrup, which is involved in Stoke’s law equation.

Position of descend – According to the ‘laminar flow’ theory, which I have stated previously, the position of descent should also be the same in each case. Since the layers travel in different velocities, from zero at the wall to maximum at the centre, positioning the descent of the sphere in different places each time will affect the rate of increase in velocity due to the increase in temperature.

Initial speed at which the sphere descends – Releasing the sphere at same velocity every time is important. Otherwise the increase in rate of velocity will vary since the temperature will also vary. This is because the time taken for terminal velocity to reach varies. Hence it will also affect the viscosity of the syrup.

Temperature of the syrup – When the temperature is altered, the speed at which the molecule travels also changes. In effect the viscosity of the syrup also changes.

Preliminary work

To have an idea of the effect of different temperatures on syrup I will carry out preliminary work. This would enable me to determine appropriate ranges such as choosing suitable sized equipment and correct volume of syrup for the final experiment. It would also enable me to make my experiment as efficient as possible in order to obtain the best possible results.