Janus+Particles

Author: Daniel Sinkovits, 2011
 * Janus Particles Exploration **

Purpose
This lesson is an exercise in making and evaluating predictions based on the outcome of a simulation. The context for these predictions is the cutting-edge soft matter research topic of patchy particles, which are colloidal particles that have patches of different interactions on them. Janus particles, having only two patches, are the simplest patchy particles. The relative simplicity allows students to observe the systematic change of structure as the particles are changed and also allows the students to make predictions that are both reasonable and not obvious.

Overview
The lesson begins with colloidal particles and purely attractive interactions, demonstrating the structures that form based on changing the temperature. Then, the students explore how a small attractive sector leads to dimers, and they explore how external properties affect the formation of dimers. Then, the students observe trimers and are challenged to make //specific// predictions of what they will see as the width of the attractive sector (Janus balance) is increased based on their understanding of the attractive interactions. Finally, their predictions are tested as they observe the structures that form at different Janus balances.

Student Outcomes
Students will be able to identify that temperature, concentration, and Janus balance affect the structures formed, and explain the general affect of each. Students will be able to make a prediction of the structure that will form based on the attractive sectors on a particle. Students will be able to identify discrepancies between their predictions and the actual outcomes. Students will be able critically evaluate their reasoning behind the predictions based on the successes and failures of their predictions.

Illinois State Science Standards identified
 * 11.A.4a Formulate hypotheses referencing prior research and knowledge.
 * 11.A.3b Conduct scientific experiments that control all but one variable.
 * 11.A.4e Formulate alternative hypotheses to explain unexpected results.

Time
Two 40-minute blocks of time.

Level
Junior High Science or High School Chemistry

Materials and Tools
Computers, one per group of 2–4 students Internet access Java installed (worksheet links to the Janus Particle simulation, located here) Computer linked to a projector for demonstrations



Preparation
It is sufficient preparation to work through the worksheet that the students are provided with.

Prerequisites
The students should have done activities before involving simulations or applets on the computer, so they are familiar with using a computer in class. They should also have learned what crystallization looks like on the molecular level. A simulation activity would be perfect for this. Finally, one of the questions mentions polymers, so it would be helpful if the students had been exposed to that concept recently.

Background
To succeed in this lesson, you need to follow directions carefully and make careful observations. You will be asked to make predictions, and it is important that they are as specific as possible.

Teaching Notes
First, introduce the concept of colloidal particles and Brownian motion as a guided discussion with the Java applet as a demonstration. The students can use the worksheet as a note-taking assistant. Then continue the demonstration and discussion through showing how attractive interactions cause the system to reach gas, liquid, and solid phases at different temperatures.

Then, group the students into pairs or triplets to work on the worksheet up to the point marked STOP. They will explore how a particles with a small attractive sector lead to dimers, and how temperature, concentration, and size of the attractive sector control this. You can call attention back to the front to ensure everyone is on the same page regarding how and why dimers form. Before the end of the first day, make sure that students have sufficiently specific predictions with drawings of what structures they expect to appear when the attractive sector is wider.

The next day, recap what was learned the day before: that attractive interactions lead to structure when the temperature is low, and that a small attractive sector leads to dimers, whose formation is affected by temperature, concentration, and the size of the attractive sector. Remind them that they saw trimers appear at higher Janus balances (wider attractive sectors) and that they made predictions of what other structures they will see if the attractive sectors are made even wider.

Then, let the students return to their groups to continue the worksheet. It directs them to observe the structures that form and has them compare to their predictions, looking for discrepant events—both structures that they did not expect and structures that they predicted that didn’t form. They will also explore the large structures that form for Janus balance greater than 90 degrees, all the way up to full coverage of attractive interactions. Recap the lesson by emphasizing the value of making and testing predictions and soliciting what they learned from predictions that were not quite correct.

The last page of reflection questions can be done in class or as a homework assignment.

Additional Information
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