Teaching viscosity can be sweetened by using chocolate.
Chocolate is one of the only foodstuffs that stays solid at room temperature but easily melts at body temperature. This peculiar behaviour is due to cocoa butter, a fatty substance obtained from the seeds of cacao, which is solid under 25 °C but liquid at 37 °C.
As thousands of children around the world could assure you, chocolate quality is an important issue. When chocolate is liquid, its quality is determined mainly by its viscosity. In this article we present a method, designed by our students, of measuring chocolate viscosity using a viscometer constructed from simple and easily obtainable materials.
After building the apparatus, which should take 2–3 hours, you can use it to measure the viscosity of water, syrup, honey and chocolate, and compare the values with published data.
The viscosity of liquids and gases is defined as the material’s resistance to deformation under stress, which is determined by the friction between particles in the material. The thicker a material is, the greater its viscosity. According to Poiseuille’s Law, the viscosity factor of a fluid that flows out of a syringe nozzle (meaning that the flow of liquid is laminar instead of turbulent, figure 1) is calculated as:
n = πr4ρt / 8V
where:
n is the viscosity factor
r is the radius of the syringe’s nozzle
ρ is the special weight of the fluid, where ρ = dg, d is the density of the fluid (d = m / V) and g is the acceleration due to gravity (9.8 ms-2)
t is the time required for the fluid to be emptied from the syringe
V is the volume of the fluid (in our experiments, we used 60 ml for all fluids).
The viscosity is measured with a special instrument called a viscometer. The unit of measurement of viscosity is the Poiseuille (Pl), equivalent to a Pascal second, 1 Pa s = 1 N m–2 s = 1 kg m−1 s−1. The unit poise (P) = 0.1 Pa s is also used.
The following experiment outlines how to build a viscometer (figure 3) using a method that we developed with our students. The central idea was proposed by the students, who asked, given Poiseuille’s law, how can we build a viscometer using everyday materials? The device in question had to be able to measure viscosity at different temperatures. We chose to develop a flow-cup type of viscometer, because the dark colour of chocolate makes a ball-falling type of viscometer – which measures the time taken for a ball of known volume to fall through a liquid – completely unsuitable.
experimental design showing
the thermometer in the water
bath (A), the thermometer in
the syringe (B), and the
styrofoam insulation (C).
The actual construction of the viscometer by the students is already a valuable piece of experiential learning. In the next step, the students encounter important research questions concerning the role of water in the shampoo bottle as a water bath and the necessity of using a pair of thermometers to make sure that thermal equilibrium between the measured liquid and the water bath is reached.
Students can measure the viscosity values for different materials at two different temperatures (20 °C and 80 °C) and rank the liquids in order of increasing viscosity. You can discuss what might cause differences in viscosity between the liquids.
Our students’ results are shown in figures 4 and 5, and tables 1 and 2.
Substance | Flow time t (s) | Viscosity n (Pa s) |
---|---|---|
Water | 4.2 | 0.07 |
Plain syrup | 6.3 | 0.5 |
Caramel syrup | 23.5 | 1.9 |
Honey | 32.5 | 2.5 |
Substance | Flow time t (s) | Viscosity n (Pa s) |
---|---|---|
Water | 2.3 | 0.05 |
Honey | 4.5 | 0.3 |
Milk chocolate | 38.0 | 2.3 |
Dark chocolate | 43.0 | 2.8 |
Ask your students to measure the viscosity values of chocolate, honey and water at five or more temperatures to study the change in viscosity in relation to temperature.
The time required for water, honey and chocolate to pass through the viscometer at different temperatures, along with their calculated viscosity values, are shown in tables 3, 4 and 5, respectively. In figure 6, changes in viscosity as a function of temperature can be seen for these fluids.
Temperature Θ (°C) | Flow time t (s) | Viscosity n (Pa s) |
---|---|---|
30 | 4.207 | 0.07 |
40 | 3.92 | 0.06 |
50 | 3.5 | 0.06 |
60 | 3.05 | 0.05 |
70 | 2.73 | 0.04 |
80 | 2.3 | 0.04 |
Temperature Θ (°C) | Flow time t (s) | Viscosity n (Pa s) |
---|---|---|
35 | 30 | 2.3 |
45 | 24.38 | 1.9 |
50 | 17.5 | 1.3 |
55 | 15.5 | 1.2 |
60 | 7.1 | 0.5 |
Temperature Θ (°C) | Flow time t (s) | Viscosity n (Pa s) |
---|---|---|
40 | 800 | 52.6 |
50 | 660 | 43.4 |
60 | 590 | 38.8 |
70 | 480 | 31.6 |
80 | 43 | 2.8 |
To extend the activity, you can ask your students additional questions such as:
Molten chocolate represents a dense blend of phospholipid-coated sucrose and cocoa particles in liquid fat. Because of this, the viscosity of chocolate has a complex pattern that is described as non-Newtonian. A particular force is required for chocolate to begin flowing; once the fluid begins flowing, as this force increases, the fluid thins.
Essentially there are two parameters that describe how chocolate flows. The first one is the limit in elasticity, the force that chocolate needs to begin flowing. The second parameter is the plastic viscosity, which is related to the energy that the chocolate requires in order to remain in motion at a constant rate (Beckett, 2000).
Understanding how chocolate flows is not just an interesting lesson for students but also of course very important for chocolate manufacturers.
We express our deepest gratitude to our students, Zoe Efthimiadou, Viktoria Kelanastasi and Aggeliki Kosma, for their conscientiousness, bright ideas and hard work.
We also express our thanks to Professors KG Efthimiadis, H Polatoglou and K Melidis from the Physics Department at Aristotle University, Thessaloniki, Greece, for their useful suggestions.
Finally we express our deepest appreciation to Mr N Kyriakides, a school parent who undertook the task of constructing the metal base upon which the whole apparatus was built.