Gel time of different volume fractions of polystyrene particles

Scattering of light in colloid

08/03/06

Rakshya Khatiwada

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?

1. Network of smaller particles/monomers2. 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• Used single slit (D=10µm)

Sample

CCD camera

Filter

Small mirror

488nm laser beam

Single slit diffraction: 2))(0()( Sin

II 2

Dq

2)

4(

Sinq

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

Sample preparation

• Different volume fractions of polystyrene:

1. 9.14E-42. 7.03E-43. 5.41E-44. 4.16E-45. 3.20E-46. 2.46E-4

• 35mM of MgCl2

50μl of Polystyrene+50μl of MgCl2

Sample Holder

MgCl2 Polystyrene

Measurements and Data Analysis

Intensity vs. wave vector q

Gel time vs. volume fractionVolume 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

326 333 539 554

566 1078

Theoretical gel time:

)3/(3

DDvg f

K

at

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