Gel time of different volume fractions of polystyrene particles

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Gel time of different volume fractions of
polystyrene particles

• Scattering of light in colloid

Rakshya Khatiwada
08/03/06
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Outline

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

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Introduction

• 3D solid objects dont coalesce like liquids
• They form fractals with some different dimension
• In dilute systems, including aerosols and
  colloids the dimension is 1.8

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• 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 doesnt change much
  because the network stops growing

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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 Waals force

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Calibration Apparatus

• Used single slit (D10µm)

488nm laser beam
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Single slit diffraction
Scattering wave vector Unit 1/m gives the
size of the aggregate
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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

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50µl of Polystyrene50µl of MgCl2
MgCl2
Polystyrene
Sample Holder
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Measurements and Data Analysis

• Intensity vs. wave vector q

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Gel time vs. volume fraction
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
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Results Gel time vs.
volume fraction
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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.

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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)

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Acknowledgement

• Supervisor Tahereh Mokhtari Dr. Christopher
  Sorensen
• Thanks to
• Rajan Dhoubhadel
• Hao Yan