 Graph 2 - the weight used had a weight of 5.0 newtons. |
Shunsuke Kariya: 2005
Introduction: background information
Some liquids, such as syrup, honey, and water based glue, have
high viscosity. These liquids create drag-forces on any object
moving through the liquid. The purpose of this experiment is to
examine how the change in concentration (water content) of a viscous
liquid affects the drug force on an object moving through the
liquid with a constant known velocity.
Research Question
What is the relationship between the water content of honey and
the drag force on a moving object?
Hypothesis
If the drag force is related to concentration of the solution,
then drag force must decrease as the percentage of water rises.
Explanation
When any high viscosity solution is diluted with water, the solution
is more easily poured. It is expected that the drag force will
be lowered accordingly. Without taking measurements it is not
possible to predict the relationship between drag and the percentage
of water.
Materials and Procedure
There are many water miscible liquids that have high viscosity,
but in this experiment honey was used because of its low price
and availability.
A 250 ml cylinder was filled with 200 ml of the original honey,
which is the 100% concentration. This was done to have enough
depth so that that the weight could be moved for the long time
when it is pulled upward.
A weight that fitted inside the cylinder was chosen. The relative
dimensions are shown on figure
1 at right. A string was tied to the weight and to the force
probe as shown. The weight was sunk to the bottom of cylinder.
The force probe reading was then recorded as the weight was pulled
upward slowly by hand at a constant velocity.
The weight was consistently pulled upwards 15 cm in two seconds, ensuring that the velocity was the same for each trial. A typical force-time graph is shown at right. The same process was repeated to obtain at least three similar graphs for each concentration. The solution was then diluted by 5%, keeping the same volume, and the process was repeated.
Data
Table 1 lists the force probe measurements for many trials at each concentration. Drag-force values obtained in this way from the force-time graphs had an uncertainty of ± 0.2 Newtons.
Graph 2 below is the force-time graph when the 0.5 N weight was hung on the force probe. This value was subtracted from the force probe measurements before the data was plotted in Graph 3.
|
Graph 2 - the weight used had a weight of 5.0 newtons. |
Graph 3 is a plot of all the force probe measurements minus the weight of the object. The drag forces for concentrations of less than 50% were very small and could not be detected with the force-probe.
Analysis
Graph 4 shows drag force plotted against concentration with error bars and a power law fit to the data points. The drag force versus percentage concentration of honey follows a ninth power law within errors.
Evaluation and suggestions for further work
From this experiment, it can be concluded that the viscosity of
honey solutions is decreased significantly as the solution is
diluted to lower percentages. Decrease in drag-force is most prominent
between 80% -100% when drag-force is decreased by almost 80%.
There are several factors that may have affected the uncertainty
in this lab.
The first factor is the acceleration due to pulling the weight by hand. It made slight ups and downs on the force while the weight was being pulled.
The second factor was temperature. Both the honey and the water used were at room temperature (25°C) but even small variations would have affected the data significantly. In future the temperature of the solution should be monitored with a thermometer with one tenth of a degree markings.
The third factor is buoyancy. Small changes in density due to dilution made the buoyancy change and may have affected the result. The volume of the mass is small and the effect would have been much less than 0.1 Newtons.
Fourthly - the cylinder was relatively narrow. Edge effects could have increased the drag somewhat above that in a more extended body of liquid.
Fifthly, of most importance, is the difficulty of pulling the weight with exactly the same velocity each time. This major difficulty, pulling the weight at the same constant velocity could be made much less significant by making a motor driven apparatus to pull the weight vertically at a constant known velocity. A more sensitive force-probe could then be used to detect the drag forces in less than 50% solution cases.
In spite of the uncertainties the shape of graph represents
the clear relationship.. The drag force is proportional to the
9th power of the concentration.
For the further suggestion, it may be good to try the same experiment
with different viscous solutions such syrup, ketchup, water based
glue, etc.