Sam Houston State University

Undergraduate Research
Summer 2011

Sphere Packing
Antonio Carrillo
Huntsville High School
Sam Houston State University
Antonio's Summer Research is supported by the ACS project SEED program
and the generosity of Project SEED supporters.

This summer my main goal was to make well packed hexagonal arrays of polystyrene nanospheres. The spheres would later on be coated with gold or silver and be used for Raman Spectroscopy to detect cyanide. My first experiments involved mainly adding different amounts of water to sphere solution to see if volume had an effect on the sphere spreading. I did this in a controlled humid environment by using the Head Space Partial Pressure Control (HPC) apparatus. This apparatus worked great but it only allowed us to deposit 1 sphere solution at a time and it took 30-60 minutes to get deposited spheres to dry. So to get more sphere packing done we tried using Petri Dishes as individual chambers for the spheres to dry on. Other significant changes that were done to the original sphere packing progress were swirling the spheres to get more slide coverage as well as larger sphere domains, adding sodium deodecylsulfate to our sphere solutions, and using ethanol as well. Now the solution volume did have an effect on the spheres spreading, but swirling the sphere solution allowed us to spread the sphere solution even more and had a big impact on the sphere packing results. And using Petri dishes allowed us to pack a week’s amount of spheres in a day. Since we need a way to predict how the spheres will pack before they’re laid down we started looking into surface tension measurements using the DuNoüy ring method. However this method would require us to use way more sphere solution than we can afford. So Megan Konarik and I collaborated on finding a correlation between surface tension and the contact angle of a drop. That way we could measure the surface tension using microns rather than milliliters. We had problems getting precise contact angle measurements however. Eventually we found out that if we let the spheres spread for 10 seconds we would get more accurate results. With the contact angle measurements and the surface tension measurements we were able to start a Zisman plot. However we still need to improve the contact angle measurement’s precision and accuracy. In the end I was able to help get better and faster sphere packing as well as start laying the foundation for a method that will allow us to predict how our sphere solutions will pack.

Antonio designing a microscope schematic for surface tension experiments