Probing complex fluids with polarization contrast-matched scattering Randy Cush

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Probing complex fluids with polarization
contrast-matched scatteringRandy Cush Paul
RussoLSU Baton Rouge
Chicago ACS Meeting August 26, 2001
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Study of Complex Fluidsby DLS Prospects
Problems

• Wide-ranging autocorrelators
• gt 10 decades of time in one measurement!
• Contrast stinks everything scatters,
  esp.
• in aqueous systems where refractive index
  matching cannot hide matrix.

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Solution Use Polarizers to Hide Matrix
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Dynamic Light Scattering Setup
Uv Geometry (Polarized)
?
V
?Uv q2Dtrans
Hv Geometry (Depolarized)
?
V
H
?Hv q2Dtrans 6Drot
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ZADS PTFE latex microrheology of polyacrylamide
gel
See also Piazza, Tong, Weitz
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Strategy

• Find polymer that should entangle

• Random coil
• Polysaccharide
• Invisible in DDLS

Dextran

• Find polymer that should not entangle

• Highly-branched
• Polysaccharide
• Invisible in DDLS

Ficoll

• Find a rodlike probe that is visible in DDLS

• Rigid rod
• Virus
• Visible in DDLS

TMV

• Measure its diffusion in solutions of each
  polymer separately

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Doing our Part to Keep the A in LSU AM
Seedlings? Sick Plants ? And close-up of mosaic
pattern.
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TMV Characterization

• Sedimentation, Electron Microscopy and DLS
• Most TMV is intact.
• Some TMV is fragmented
• (weaker, faster mode in CONTIN)
• Intact TMV is easy to identify
• (stronger, slower mode in CONTIN)

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Hv correlation functions for 14.5 dextran and
28 ficoll with and without added 0.5mg/mL TMV

• The dilute TMV easily outscatters both matrices.

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Stokes-Einstein Plots if SE works, these would
be flat. Instead, deviations in different
directions for Drot and Dtrans
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Too-Good-to-be-True Conclusion?

• Below 6.5 dextran the diffusion of the rodlike
  TMV probe is controlled mostly by viscosity.
• Above 6.5 dextran a sharp transition suggests
  topological constraint for TMV rotation while
  translation is not much affected.
• The transition is more gradual in ficoll.
• The TMV probe senses something different for
  linear vs. highly branched polymers in solution.
• Looks good for topological models!

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Alternate Conclusion?

• The systems studied so far place (impossibly?)
  strict demands on geometric polarization
  alignment.
• Revised polarization placement
• Difficult zero angle measurements requiring even
  more TMV
• New systems must be studied
• TMV is OK
• Dextran/Ficoll must go!
• Depolarized probe diffusion has the potential, as
  yet unrealized, to assess strength of
  hydrodynamic vs. topological effects.

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Thank you!
Randy Cush David Neau Ding Shih Holly
Ricks Jonathan Strange Amanda Brown Zimei
Bu Zuhal Savas Kucukyavuz--METU Seth
FradenBrandeis Nancy ThompsonChapel Hill
NSF
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The chiral dextran and ficoll alter polarization
slightly before and after the scattering
center. Sign magnitude of Stokes-Einstein
failures depend on how one handles this tiny
effect.
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Misalignment from thick polarizer in active
part of detector train, exacerbated by tiny
cellsused to squelch optical rotation conserve
TMV
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Conditions for use as a Probe

• Is the TMV Probe Dilute?
• A TMV concentration of 0.5 mg/mL, well below the
  theoretical overlap concentration, was chosen.
  See Figure 2.
• Does dilute TMV overwhelm the matrix scattering?
• At 0.5 mg/mL the TMV easily outscatters both
  matrices. See Figure 3.
• Is the probe compatible with the matrix?
• -Solutions stable months after preparation
• -Angle dependent Hv SLS
• -Dtrans goes up, not down (Figures 6 8)

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Effect of Dextran Concentration

• The dependence of Drot and Dtrans upon added
  dextran is shown in Figure 4.
• The quotient Drot/Dtrans is plotted against
  viscosity in Figure 5. By combining both
  transport coefficients, each inversely
  proportional to viscosity in dilute solution, we
  can remove the effect of solution viscosity.
• Figure 6 reveals like positive deviations from
  the Stokes-Einstein continuum expectation that
  diffusion be inversely proportional to viscosity
  (below 6.5).
• Above 6.5 the deviations become greater for both
  Drot and Dtrans but in opposite directions

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There once was a theorist from France who
wondered how molecules dance. Theyre like
snakes, he observed, As they follow a curve,
the large ones Can hardly advance. D M -2
de Gennes
P.G. de Gennes Scaling Concepts in Polymer
Physics Cornell University Press, 1979
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Doi-Edwards-Onsager Reference Volumes for Rods n
number density of rods per unit volume
LC formation n 4/A2 ? 5/dL2
Reduced Density n/n ? ndL2/5
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Outline

• Characterize the TMV
• Is it intact and behaving properly?
• Establish conditions for use of TMV as probe
• Can the probe be dilute and still overwhelm the
  matrix scattering?
• Will the probe stay mixed with the matrix
  solutions without aggregating?
• Show the effect of the dextran and ficoll
  matrices on TMV diffusion

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Effect of Ficoll Concentration

• The dependence of Drot and Dtrans upon added
  dextran is shown in Figure 4.
• The quotient Drot/Dtrans is plotted against
  viscosity in Figure 7.
• Figure 8 shows slight like positive deviations
  from the Stokes-Einstein continuum expectation
  (below 11).
• Above about 11 ficoll the deviation slowly
  becomes greater for Drot and slightly greater for
  Dtrans but in opposite directions
• Figure 9 compares TMV behavior in ficoll to that
  in dextran.