GEM4 Summer School OpenCourseWare

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1 GEM4 Summer School OpenCourseWare Lecture: Microrheology of a Complex Fluid by Dr. Peter So. Given August 10, 2006 during the GEM4 session at MIT in Cambridge, MA. Please use the following citation format: So, Peter. Microrheology of a Complex Fluid. Lecture, GEM4 session at MIT, Cambridge, MA, August 10, (accessed MM DD, YYYY). License: Creative Commons Attribution-Noncommercial-Share Alike. Note: Please use the actual date you accessed this material in your citation.

2 Microrheology of Complex Fluid Rheology: Science of the deformation & flow of matter Microrheology - Microscopic scale samples - Micrometer lengths Complex shear modulus G*(ω) σ = G* ε Image removed due to copyright restrictions. Storage modulus G Energy storage Elasticity ~ Solid - G* (ω) = G (ω) + j G (ω) Loss modulus G Image removed due to - Solid vs. fluid copyright restrictions. Energy dissipation - Resistance to deformation Viscosity ~ Fluid

3 High Frequency Microrheology Measurement Active Method: Magnetic microrheometer Baush, BJ 1998 Huang, BJ 2002 Passive Method: Single particle tracking Mason, PRL 1995 Yamada, BJ 2000 Multiple particle tracking Crocker, PRL 2000

4 Magnetic Microrheology Magnetic trap Computer Control signal Sample holder Current amplifier Distance (µm) data fit CCD Time (s) Figures by MIT OCW. Magnetic Microrheology 5 sec Step Response

5 Basic Physics of Magnetic Microrheometer Ferromagnetic particle 1 F = µ 0 ( m H) 2 Particles cluster together! Doesn t work! Paramagnetic particle no permanent magnetic moment F = µ χv ( H H ) 0 χ is suceptibility V is volume Note: (1) force depends on volume of particle (5 micron bead provide 125x more force) (2) force depends on magnetic field GRADIENT

6 Magnetic manipulation in 3D *Lower Force nn level *3D *Uniform gradien Amblad, RSI 1996 Huang, BJ 2002 Courtesy of Dr. Peter So. Used with permission.

7 Magnetic manipulation in 1D Sample chamber Magnet r Cover glass *High force >10 nn *Field non-uniform Needs careful alignment of tip to within microns *1D Figure by MIT OCW. After Bausch et al., The bandwidth of ALL magnetic microrheometer is limited by the inductance of the eletromagnet to about kilohertz

8 Magnetic Rheometer Requires Calibration Distance to pole piece (µm) Force on magnetic bead (pn) l = 2500 ma l = 250 ma Force on magnetic bead (pn) (a) Distance to pole piece (µm) Coil current (ma) (b) 10 µm 25 µm 50 µm 75 µm 100 µm linear fits Figure by MIT OCW. After Bausch, 1998.

9 Mag Rheometer Experimental Results 1.5 Displacement X(t)µm I II III Time (s) Force (pn) Figure by MIT OCW. After Bausch, Transient responses allow fitting to micro-mechanical model Image removed due to copyright restrictions. See Fig. 5 in Bausch et al. "Local Measurements of Viscoelastic Parameters of Adherent Cell Surfaces by Magnetic Bead Microrheometry."metry." Problem Magnetic bead rolling Biophys J 75 (1998): Solution Injection, Endocytosis Modeling (Karcher BJ 2003)

10 Model Strain Field Distribution New Text Image removed due to copyright reasons. See Fig. 9 in Bausch et al. "Local Measurements of Viscoelastic Parameters of Adherent Cell Surfaces by Magnetic Bead Microrheometry." Biophys J 75 (1998):

11 Single Particle Tracking Consider the thermal driven motion of a sphere in a complex fluid Langevin Equation t mv& ( t) = f ( t) + ξ ( t t') v( t') dt' 0 Inertial Random Memory function force thermal Material viscosity force Particle shape

12 Langevin Equation in Frequency Domain Laplace transform of Langevin Equation v ~ ( s) = ~ f ( s) + mv(0) ~ ξ ( s ) + ms Multiple by v(0), taking a time average, Ignoring inertial term Random force < ~ f ( s) v(0) >= ξ ( s ) 0 Equipartition of energy m < v(0) v(0) >= kt Generalized Stokes Einstein = 6πa ~ η ( s) ~ G( s) = s ~ η ( s) ~ G ( s) = π as kt < ~ r 2 ( s ) > Definition and Laplace transform of mean square displacement < v (0) v ~ ( s) >= s 2 < ~ r 2 ( s) > / 6

13 (2) Fluorescence Laser Tracking Microrheometer Approach: Monitoring the Brownian dynamics of particles embedded in a viscoelastic material to probe its frequencydependent rheology trajectory Image removed due to copyright restrictions. mean squared displacement shear modulus Yamada, Wirtz, Kuo, Biophys. J. 2000

14 (2) Nanometer Resolution for the Bead s Trajectory Collecting enough light from a fluorescent bead is critical σ = 0.5 µm A x c B x Photons detected per measurement Uncertainty on Uncertainty on x c (nm) Nanometer resolution 10 4 photons per measurement

15 (2) Calibrating the FLTM y Ch3 Ch2 Ch1 Ch0 x 5-nm stepping at 5 or 50 khz Curve fitting matches theory RatioX = (ChO + Ch 2 ) / (Ch 1 +Ch 3 ) nm steps at 50 khz 5-nm steps at 5 khz Ratio Y = (Ch2+Ch3) / (ChO + Ch1) y-position of the bead (nm) Fit y e-006 y 2 Figure by MIT OCW. Figure by MIT OCW.

16 Characterizing the FLTM Using polyacrylamide gels (w/v 2% to 5%) of known properties 9 Good agreement with previously published data Image removed due to copyright restrictions. See. Fig. 11(a) in Schnurr, B., F. Gittes, F. C. MacKintosh, and C. F. Schmidt. "Determining Microscopic Viscoelasticity in Flexible and Semiflexible Polymer Networks from Thermal Fluctuations." Macromolecules 30 (1997): Image removed due to copyright restrictions.

17 Single Particle Tracking Data Image removed due to copyright restrictions. See Fig. 4(a) and 7 in Yamada, Soichiro, Denis Wirtz, and Scot C. Kuo. "Mechanics of Living Cells Measured by Laser Tracking Microrheology." Biophys J 78 (2000): Image removed due to copyright restrictions. See Fig. 4(a) and 7 in Yamada, Soichiro, Denis Wirtz, and Scot C. Kuo. Image removed due to copyright restrictions. "Mechan." Biophys ics See of Living Fig. 4(a) Cells and Measured 7 in Yamada, by Laser Soichiro, Tracking Denis Microrheology Wirtz, and Scot C. Kuo. "Mechanics of Living Cells Measured by Laser Tracking Microrheology." Biophys J 78 (2000): J 78 (2000):

18 Two- and Multiple Particle Tracking SPT responses can be influence by local processes (adhesion, active, etc) and not represents global cytoskeleton behavior Solution: Look at the correlated motion of two particles under thermal force D D rr rr i r ( r, τ ) =< r ( t, τ ) r ( t, τ ) δ ( r R ( t)) >, kt ( r, s) = ~ 2π rsg( s) j r ij i j t The major difference is that the correlation signal is a function of r the separation of the particles but not their size Instead of using fast quadrant detectors, multiple particle tracking uses a wide field camera which is slower

19 SPT vs MPT Triangle: SPT Circle: MPT 0.01 G' G" Figure by MIT OCW. After Crocker, SPT and MPT results can be quite different specially in cells

20 A Comparison of Microrheometry Methods Magnetic SPT MPT Bandwidth khz MHz khz Signal Amplitude µm nm nm Local Effects Yes Yes No Nonlinear regime Yes No No Instrument Intermediate Intermediate Simple