Suspensions: Brownian Motion


Audience: Chemistry Course

Running time: 60 min

Content Standards: The structure and levels of organization of matter

Expectations: matter and their interactions Q.CF1: structure and properties

National Standards for science content:

    UCP.1 System, order and organization

   UCP.2 Evidence, models and explanation

   B.2 Structure and properties of matter

   B.4 Motions and forces


Objectives:

  1. Described the structure of matter based on its properties and characteristics.

  1. Compare and contrast the properties of solutions, suspensions and colloids, and provides examples of these types of mixtures.

  2. They will explain the concept of temperature in terms of the content of the average kinetic energy of the particles.

  3. They connect the contents of thermal energy of a material with the movement of the particles that constitute it according to kinetic-molecular theory.


I. Brownian Motion

Materials:

  • 1 Wooden box - dimensions (1.5ft x 1.5ft x 0.4 ft)

  • 1 Bungee Safety

  • 1 Bag of marbles

  • 1 Disc (this depends on the roughness of the surface)

  • 4 Round Headed Bolt (5/16-6”)

  • 1 Washer and Nut Kit (5/16)

  • 4 Slotted Angle Zinc (1.5ft)

  • 4 Clamps


Activity Instructions:

To make the concept clearer, it is recommended, to have explained before hand the concept of colloidal suspension and given an example of it (eg. milk). If it were to make groups to interact with the Brownian motion simulator, it is advisable for them to be of 4 people, to evenly distribute the tasks well.

Start:

After the explanation, we proceed to the model of Brownian motion, which refers to the example previously explained. This is done in order to give the audience the understanding of the relationship of the marbles and the disk in the previous example (colloidal suspension).

Development:

After the comparison of the two systems, we proceed to explain the type of random motion (Brownian motion) which involves a colloidal suspension. Finally, Brownian movement begins in the model so that the audience perceives the phenomenon at a macro scale. To start such movement we must pull the elastic strips and simultaneously view the disk displacement or development. It is advisable to do the simulation with four people, to distribute the work and have a more uniform distribution.

Conclusion:

Students must answer the following questions on the student sheet:

  1. Does the displacement occurred in one direction? Why?


Background information:

In 1827, Robert Brown, a British botanist, was observing a few grains of pollen that were blooming. While observing the pollen under a microscope, to his surprise, he saw small particles suspended by the pollen that had a continuous and random motion. This random motion is was its known today as Brownian motion. Brownian motion is caused by the thermal fluctuation of the molecules surrounding the bigger particle (colloidal). In the case of Brown’s observation, the bigger particles and molecules were the pollen, and air (oxygen, nitrogen, and hydrogen molecules) respectively. Since Brownian motion is affected by the thermal fluctuation, is already implicit that temperature has a direct impact to this motion. At higher temperature, higher the thermal fluctuation and therefore, greater is the diffusivity of the colloids.

Diffusivity is way to describe how easily a particle suspended in a fluids (gas or liquid) can move through the molecules of the system. Einstein describe (1906) the diffusivity of a particle in a system by were D is the diffusivity of the particle taken into account, k is the Boltzmann constant, T is the fluid temperature, μ is the fluid viscosity, and a is the particle radius. In conclusion, what is shown here is that for any particle not big enough, that gravity can have a effect on it, the displacement of the particle will decrease as the temperature decrease (for the thermal fluctuation), as viscosity increase (more resistance to the colloid), and an increase of particle radius (bigger particle has difficulty to pass through molecules).


Suggested Video:

http://www.youtube.com/watch?v=cDcprgWiQEY&feature=related

http://www.youtube.com/watch?v=anlVe_7W3T0




II. Phoretic Movement (Optional Activity)

Activity Instructions:

This activity requires that the people who took the workshop, the concept of Brownian motion should had been explained to them. If it were to divide them in groups in order to interact with the phoretic motion simulator, you should do them in groups of 4, to evenly distribute the tasks well.

Development Activity:

Unlike Brownian motion module, in this case, of the four people chosen, three of them must exert a force quite similar when they pulled the spring. These three springs should be chosen so that they are next to each other. The fourth person will be in charge of pulling the fourth spring, which is in front of the other 3 springs, with a much lower intensity than that of the other three. The purpose of this is to create an imbalance of forces through the interactions that witness the object used as the colloid. Thus, it can create a movement in one direction (phoretic motion) in the system.

Background Information:

Brownian motion as mention before, is the random movement of colloidal particles in a fluid. However, the necessity to induce a directional motion, has open a new area of research. This directional motion is also known as phoretic motion.

Until now, there have been several viable mechanism to cause directional movement. Among them are diffusiophoresis (concentration gradient), and thermophoresis (temperature gradient). As seen, these phoretic motion are induce by a gradient. As example, diffusiophoresis provides a directional motion by a high concentration of a species in one side of the system which want to diffuse to the other side to establish a equilibrium. The diffusivity of the molecules, carry the colloidal particle, inducing to the colloid a net movement to one direction.  

References:

● Bhalerao, Rajeev S. Brownian Motion for the School-Going Child. Rep. no. TIFR/TH/04-31. Print.

● Fowler, Michael. "Brownian Motion." Brownian Motion. Virginia University, 1 Aug. 2008. Web. 21 May 2012. <http://galileo.phys.virginia.edu/classes/152.mf1i.spring02/BrownianMotion.htm>.