Probing complex fluids with polarization contrast-matched scattering Randy Cush & Paul Russo LSU – Baton Rouge Chicago ACS Meeting August 26, 2001

Study of Complex Fluids by DLS: Prospects & Problems Wide-ranging autocorrelators > 10 decades of time in one measurement! – – – Contrast stinks: everything scatters, esp. in aqueous systems where refractive index matching cannot hide matrix.

Solution: Use Polarizers to Hide Matrix

Dynamic Light Scattering Setup  Hv = q 2 D trans + 6D rot LASERV H  PMT Hv Hv Geometry ( Depolarized ) Uv Geometry (Polarized) V   Uv = q 2 D trans PMT LASER

ZADS PTFE latex microrheology of polyacrylamide gel See also: Piazza, Tong, Weitz

Strategy Find polymer that should not “entangle” Find a rodlike probe that is visible in DDLS Measure its diffusion in solutions of each polymer separately Random coil Polysaccharide Invisible in DDLS Highly-branched Polysaccharide Invisible in DDLS Rigid rod Virus Visible in DDLS Dextran Ficoll TMV Find polymer that should “entangle”

Seedlings  Sick Plants  And close-up of mosaic pattern. Doing our Part to Keep the “A” in LSU A&M

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)

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.The dilute TMV easily “outscatters” both matrices.

Stokes-Einstein Plots: if SE works, these would be flat. Instead, deviations in different directions for D rot and D trans

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!

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.

Thank you! LSULSU Randy Cush David Neau Ding Shih Holly Ricks Jonathan Strange Amanda Brown Zimei Bu Zuhal & Savas Kucukyavuz--METU Seth Fraden—Brandeis Nancy Thompson—Chapel Hill NSF

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.

Misalignment from thick polarizer in “active” part of detector train, exacerbated by tiny cells used to squelch optical rotation & conserve TMV

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)

Effect of Dextran Concentration The dependence of D rot and D trans upon added dextran is shown in Figure 4. The quotient D rot /D trans 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 D rot and D trans but in opposite directions

There once was a theorist from France who wondered how molecules dance. “They’re like snakes,” he observed, “As they follow a curve, the large ones Can hardly advance.” D ~ M -2 P.G. de Gennes Scaling Concepts in Polymer Physics Cornell University Press, 1979 de Gennes

L d LC formation  = 4/A 2  5/dL 2 Reduced # Density   dL 2 /5 Doi-Edwards-Onsager Reference Volumes for Rods = number density = # of rods per unit volume

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

An ear of corn has about as many kernels as TMV has protein subunits (ca. 2130). The protein subunits enfold a spiral-wound strand of RNA which will encode the next generation. TMV is more extended than an ear of corn.

Effect of Ficoll Concentration The dependence of D rot and D trans upon added dextran is shown in Figure 4. The quotient D rot /D trans 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 D rot and slightly greater for D trans but in opposite directions Figure 9 compares TMV behavior in ficoll to that in dextran.