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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.

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Gel time of different volume fractions of polystyrene particles

Gel time of different volume fractions of polystyrene particles

Scattering of light in colloid

Rakshya Khatiwada

08/03/06


Outline

Outline

  • Introduction

  • Apparatus

  • Calibration

  • Sample Preparation

  • Measurements and Data Analysis

  • Results

  • Conclusion


Introduction

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


Gel time of different volume fractions of polystyrene particles

  • 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

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

CalibrationApparatus

  • Used single slit (D=10µm)

488nm laser beam


Single slit diffraction

Single slit diffraction:

Scattering wave vector

Unit: 1/m gives the size of the aggregate


Sample preparation

Sample preparation

  • Different volume fractions of polystyrene:

  • 9.14E-4

  • 7.03E-4

  • 5.41E-4

  • 4.16E-4

  • 3.20E-4

  • 2.46E-4

  • 35mM of MgCl2


50 l of polystyrene 50 l of mgcl 2

50μl of Polystyrene+50μl of MgCl2

MgCl2

Polystyrene

Sample Holder


Intensity vs wave vector q

Intensity vs. wave vector q

Measurements and Data Analysis


Gel time vs volume fraction

Gel time vs. volume fraction

Theoretical gel time:


Results gel time vs volume fraction

ResultsGel time vs. volume fraction


Conclusion

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

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

Acknowledgement

Supervisor: Tahereh Mokhtari & Dr. Christopher Sorensen

Thanks to

Rajan Dhoubhadel

& Hao Yan


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