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

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

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

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

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

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

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

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

50μl of Polystyrene+50μl of MgCl2 Sample Holder

Measurements and Data Analysis Intensity vs. wave vector q

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:

Results Gel time vs. volume fraction

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.

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)

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