# Details of Experiments

This web page is intended for instructors. Some of the experiments listed below also have "student web pages" which allow students to see movies of the experiments, but which don't have the descriptive details.

### Introduction to Brownian motion and viscosity

Coin flip activity: Students flip a coin. For heads they turn right and take a step, for tails they turn left and they take a step. They repeat this several times.

Student web page: Students watch a video of Brownian motion of polystyrene particles, 1.9 micrometer in diameter, in water. They notice that the motion of the particles in video is very similar to their random walk motion. The student web page has this video clip.

Web applet: Here is a link showing Brownian motion schematically: Einstein's Explanation of Brownian Motion

Prepare for class:

### Brownian Motion & Viscosity Lab

Microscopy: Students look at microscopy slides with polystyrene particles with diameters of 0.5 mm, 1.1 mm and 1.9 mm diffusing in water and water-glycerol mixtures.

Student web page: Click here for web movies of Brownian motion, which may be useful if you don't have access to a microscope and/or particles with controlled sizes.

Falling ball experiments: Students perform falling ball experiments. They measure the fall time of a glass sphere in glycerol and in water. Note that while the liquid's density does play a role (in changing the buoyant force on the sphere), with glass spheres this secondary effect doesn't change the results by much, as the density of glass = 2.6 g/cm^3 is much larger than the density of all of the liquids. In a more mathematically rigorous class, the velocity of the sphere can be measured and compared to the drag force, buoyant force, and weight of the sphere, to determine a value for viscosity. The Stokes drag force for slowly moving spheres is F_drag = 6 pi eta R v, with eta the viscosity, R the radius of the sphere, and v the velocity. For fast-moving spheres, this equation is not correct, but nonetheless can distinguish between the viscosities of these different liquids.

Drainage experiments: Students time how fast it takes water, mineral oil, and glycerol to drain out of a plastic beaker through a 1/8" hole drilled at the bottom of the beaker. Viscosities of these liquids vary significantly whereas their densities are similar. Density plays a role in determining the hydrostatic pressure within the draining container, which causes the fluid to flow out, but the main influence on draining time is the viscosity.

Prepare for class:

1. Brownian motion experiments
• slides with particles of different sizes
• microscope
2. falling ball experiments experiments
• need two graduated cylinders: one filled with water, second one filled with glycerol
• Spheres of various sizes and densities -- can be purchased from Small Parts Inc for example, for ~\$25.
• place a small piece of sponge at the bottom of the cylinders if necessary to avoid the cylinder being broken by the falling spheres
• use watches or stop watches to measure the time of the fall of the sphere
• use a magnet to get the steel spheres out of the cylinder filled with glycerol
3. three plastic beakers (or regular coffee cups) with a 1/8" hole in the bottom of the each beaker
5. lab report: lab report 1 (MS Word) (HTML format)

### Squishy Materials Lab

Squishy Materials: Students perform a set of experiments in four substances (we don't tell students what they are, we will tell them later). Click here to see a picture of the four substances:
• bentonite powder in water
• xanthane gum in water
• light mineral oil

Tools: We provided students with beakers, spatulas, spheres, microscopic slides - we do not tell them what experiments they should do. Instead we suggest some experiments that they can perform. Click here to see a picture of the experimental tools we provide for the students.

Student web page: If you wish, the students can view movies of the various substances being stirred. As we suggest the students shouldn't know the names of the materials, on the student web page we list them as squishy materials 1 - 4. The materials are listed below, and all movies linked to below can be found also on the student web page.

Details of squishy substances:

• acrylic beads in water -- squishy material #1
The substance is white with grainy feeling. When one tries to mix the substance it's easier to mix it if the mixing rate is slow. If one tries to mix it faster it's harder to mix the substance - this is a shear-thickening substance. Substance's viscosity increases with mixing rate. Corn starch is another example of a shear-thickening liquid. Click here for a movie of an acrylic beads - water mixture being mixed at different rates.
• bentonite in water -- squishy material #2
Bentonite powder (click here to see the picture of the powder) mixed with water is a Bingham liquid - it has a yield stress. This means that below a given value of a yield stress the liquid doesn't flow. If one exceeds this value of the yield stress the liquid will flow. Just like ketchup. If you take a glass bottle and shake it you will overcome the yield stress of ketchup and the ketchup will flow out of the bottle. It's not so easy to do that though. Remember the commercials of guys shaking ketchup bottles really hard? Maybe that's the reason why ketchup companies switched from glass to plastic squeezable bottles. It was just to hard to overcome the yield stress of ketchup. Click here for a movie where we place a metal spatula at an angle of less than 90 degrees with respect to the surface of the bentonite mixture - notice that the metal spatula doesn't tip over. The gravitational force acting on the spatula is either balanced by the yield stress or the yield stress is bigger than this force.
• xanthan gum in water -- squishy material #3
Xanthan gum is a food thickener. It's used in salad dressings. When you mix it with water, you get a thick, elastic and foggy substance. If you mix the substance it is easier to mix it when you mix it faster. It's a shear-thinning liquid. Substance's viscosity decreases with mixing rate. Moreover, the substance is elastic. If you poke the substance, it returns back to its original position -- click here to see a movie and a second movie. In these two movies notice what happens to the air bubbles in when we poke on them with spatula or when we gently shake the bottle. They return to their original position. This is elasticity. If we don't deform the substance too hard it will respond elastically.
• light mineral oil -- squishy material #4
This is a regular liquid. Just like water, except its viscosity is higher than that of water. If you mix it slow or fast the liquid's viscosity stays the same. Such liquids are called Newtonian liquids. Click here for a movie where we mix the mineral oil; of course, nothing funny happens here.
The first three substances have behavior that is much different from that of water. These substances are called non-Newtonian.

Prepare for class:

1. Each pair of students received the following:
• two beakers
• one metal spatula
• one acrylic sphere (inch in diameter) (we purchased from Small Parts Inc)
• two microscopy glass slides
• one eye dropper
2. four substances:
• bentonite powder in water
• xanthan gum in water
• light mineral oil
(HTML format)
4. lab report: lab report 2 (MS Word) (HTML format)

### Sand Lab

Sand: During this lab students experiment with sand. Students receive a Lab Sheet with experiments that we would like them to do. These include studying the Brazil nut effect, and learning about the concepts of force chains.

Prepare for class:

### Jamming

Prepare for class: