1. Gel time of different volume fractions of polystyrene particles
Scattering of light in colloid
Rakshya Khatiwada
08/03/06

2. Outline Introduction Apparatus Calibration Sample Preparation
Measurements and Data Analysis
Results
Conclusion

3. Introduction 3D solid objects don’t coalesce like liquids
They form fractals with some different dimension
In dilute systems, including aerosols and colloids the dimension is 1.8

4. What is gelation of colloids?
Network of smaller particles/monomers
The particles exhibiting Brownian motion stop/slow down
How do we know when the gel point is reached?
The intensity of light doesn’t change much because the network stops growing

5. What is polystyrene? Micro spheres of 24nm diameter About same charge
MgCl2 forms ions which screen the coulomb charge repulsion allowing them to stick together with van der Waal’s force

6. Calibration Apparatus
Sample
CCD camera
Filter
Small mirror
Used single slit (D=10µm)
488nm laser beam

7. Single slit diffraction:
Scattering wave vector
Unit: 1/m gives the size of the aggregate

8. Sample preparation Different volume fractions of polystyrene: 9.14E-4
35mM of MgCl2

9. 50μl of Polystyrene+50μl of MgCl2
Sample Holder

10. Measurements and Data Analysis
Intensity vs. wave vector q

11. Gel time vs. volume fraction
Theoretical gel time (minutes)
Experimental gel time (minutes)
9.14E-4
0.300
7.03E-4
0.629
5.41E-4
1.21
4.16E-4
2.33
3.20E-4
4.50
2.46E-4
8.69
Theoretical gel time:

12. Results Gel time vs. volume fraction

13. Conclusion Reproducible data Lower volume fractions, longer gel time
Experimental gel times longer than theoretical
Literature values compared to our theory are consistent with our conclusions here.

14. Possibilities
Maybe theory has simplified so many details (example: same cluster size)
Maybe stoppage of system to evolve is not good indication of gel time.
Need a better way of finding gel point
Like Dynamic light scattering (can see particles moving)

15. Acknowledgement
Supervisor: Tahereh Mokhtari & Dr. Christopher Sorensen
Thanks to
Rajan Dhoubhadel
& Hao Yan