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Patrick Spicer
Procter and Gamble Company
Microstructures Group
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Title:
Structured Fluid Consumer Products and Microstructural Heterogeneity
Abstract:
The criterion used to decide a fluid consumer product is sufficiently mixed varies significantly depending
on the fluid and its application. Previous work has focused on quantifiable measures like conductivity,
composition, and color. In the case of structured fluids, however, the situation is complicated by the fact
that molecular-scale homogeneity may never be achieved and might not even be desirable. Given that
most practical structured fluids are always heterogeneous at certain length scales we are motivated to
determine the heterogeneity of the fluid, how processing affects heterogeneity, and the effects of
heterogeneity on product quality and performance. I'll discuss how we are using microrheological techniques to try and
answer these questions, show some examples, and suggest some paths forward for increasing our understanding of this
open research area.
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Tom Lubensky
University of Pennsylvania
Department of Physics & Astronomy
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Title:
Foundations and Applications of One- and Two-Point Microrheology
Abstract:
This talk will review the theoretical foundations of one- and two-particle microrheology and discuss its
applications to equilibrium and non-equilibrium systems including, semidilute polymer solutions, living
cells, and bacterial baths.
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Daniel Ou-Yang
Lehigh University
Department of Physics
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Title:
A Comparative Study of Living Cell Micromechanical Properties Using Oscillatory Optical Tweezers
Abstract:
Micromechanical properties of biological cells are crucial for cells functions. Despite
extensive study by a variety of approaches, a full understanding of the subject remains illusive. We
conducted a comparative study of the micromechanical properties of cultured alveolar epithelial cells
with an oscillatory optical tweezer-based cytoreheometer. In this study, the frequency-dependent
viscoelasticity of these cells was measured by optical trapping and forced oscillation of either a
submicron endogenous intracellular organelle (probes in an intracellular configuration), or a 1.5μm
silica bead located atop of plasma membrane but attached to the cytoskeleton through
transmembrane integrin receptors (probes in an extracellular configuration). From both the intracellular and extracellular
probe configurations, the storage modulus and the magnitude of the complex shear modulus followed weak power-law
dependence with frequency. These data are comparable to data obtained by other measurement techniques. The
exponents of power-law dependence of the data from the intra- and extra- cellular measurements are similar; however,
the differences in the magnitudes of the moduli from the two measurements are statistically significant.
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Penger Tong
Hong Kong University of Science & Technology
Department of Physics
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Title:
Diffusion of Colloidal Particles at Soft Interfaces
Abstract:
Colloidal particles have been used as tracer particles to study the rheological properties of soft
interfaces, but interpretation of the experimental results is not straightforward. In this talk, I will review
the recent development in this area and report our experimental study of Brownian dynamics of hard
spheres at an oil-water interface. Optical microscopy and multi-particle tracking are used to measure
the mean square displacement of the interfacial particles. The measured short-time self-diffusion
coefficient DS
S has the form, DS
S = αD0(1 - β n), where n is the area fraction occupied by the particles
and D0 is the Stokes-Einstein diffusion coefficient. The fitted values of α and β] are found to be different from those for bulk
colloidal suspensions. Implications of the results to microrheology of soft interfaces will be discussed.
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Frank Scheffold
University of Fribourg
Physics Department
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Title:
Glassy Dynamics and Network Elasticity in Complex Fluids Probed by DWS
Abstract:
We characterize the linear viscoelastic shear properties of complex fluids using diffusing wave
spectroscopy (DWS) based microrheology covering the frequency range from 0.1 to 1 Mrad/s. We
present results on two different network forming systems. Shear moduli of an aqueous wormlike
micellar solution are studied by a combination of DWS as well as various mechanical techniques such
as rotational rheometry, oscillatory squeeze flow, and torsional resonance. Since DWS as well as
mechanical oscillatory squeeze flow and torsional resonance oscillation cover a sufficiently high frequency range, the
persistence length of wormlike micelles could be determined directly from rheological measurements for the first time [1].
In the second part we study an charge stabilized aqueous suspensions of PNIPAM microgel particles consisting of
polyelectrolyte cross-linked gels. The solvent quality for polymers depends on temperature and thus allows tuning of the
particle size. The rather high charge of the particles is used to prevent aggregation in the collapsed state. We study these
systems with a combination of Static and Dynamic Light Scattering (SLS/DLS) and Diffusing Wave Spectroscopy (DWS).
The tunable interaction potential provides external control of the micro-structural properties. Slow relaxation properties of
dense suspensions around the liquid-solid transition are studied by a Two Cell DWS. From optical microrheology we
obtain information about the viscoelastic flow properties in the arrested state [2]. Our results show that the build up of
elasticity in this regime can be directly linked to the increase in rigidity of the underlying polymer network.
[1] N. Willenbacher, C. Oelschlaeger, M. Schopferer, P. Fischer, F. Cardinaux and F. Scheffold, Broad Bandwidth Optical
and Mechanical Rheometry of Wormlike Micelle Solutions, Physical Review Letters 99, 068302 (2007)
[2] Frank Scheffold, Pedro Diaz-Leyva, Mathias Reufer, and Iseult Lynch, in preparation
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John Crocker
University of Pennsylvania
Chemical and Biomolecular Engineering, Institute for Medicine and Engineering, Pennsylvania Muscle Institute
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Title:
What Do Cell Rheology Experiments Really Measure?
Abstract:
While cells' responses to mechanical stimuli are seen as increasingly important for understanding cell biology, how to best measure, interpret and model cells' mechanical properties remains unclear. We
determine the frequency-dependent shear modulus of cultured mammalian cells using four different methods, both novel and well established. This approach clarifies the effects of ATP-dependent processes, cell regional variations and cytoskeletal heterogeneity on the interpretation of such measurements.
We apply a passive technique two-point microrheology (TPM), to measure cells' dynamic shear modulus for the first time. TPM has the advantage that it does not depend on details of the tracer coupling or assumed deformation geometry, providing a uniquely interpretable and quantitative result. We also apply an active method using externally-attached magnetic tracers, magnetic twisting cytometry (MTC), and passive methods using the same tracers (either internalized or externally attached), laser tracking microrheology (LTM). Our results clearly indicate two qualitatively similar but distinct
mechanical responses, corresponding to the cortical and intracellular networks, each having an unusual, weak power-law form at low frequency. Comparison with recent reconstituted biopolymer models suggests such a response is determined by the dynamics of network cross-linking proteins which we further hypothesize are directly related to mechano-sensing by the cell.
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David Weitz
Harvard University
Department of Physics
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Title:
Microrheology in Bionetworks and Cells
Abstract:
This talk will describe the applicability of different methods of microrheology to probe the mechanical properties of the cell, both using cells, and using reconstituted networks of biomolecular proteins.
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Todd Squires
University of California, Santa Barbara
Chemical Engineering
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Title:
Active and Nonlinear Microrheology -- Issues, Challenges, and Ideas
Abstract:
In passive microrheology, the linear viscoelastic properties of complex fluids are inferred from the Brownian motion of colloidal tracer particles. Active (but gentle) forcing may also be used to obtain such linear-response information. More significant forcing may drive the material significantly out of equilibrium, thus potentially providing a window into the nonlinear response properties of the material. In leaving the linear-response regime, however, the theoretical underpinning for passive microrheology is lost, and a variety of issues arise. Most generally, what exactly can be measured, and how can such measurements be interpreted? Here we motivate and discuss a variety of theoretical issues facing the interpretation of active microrheology. Using a colloidal suspension as a model material, we examine the different sources of stress upon the probe particle (e.g. direct collisions between the probe and bath colloids, as well as microstructural deformations within the bulk suspension) and discuss their analog (or lack thereof) in the corresponding macro-rheological system. In particular, we analyze experiments in active (but linear) oscillatory microrheology (I. Gopal and E. Furst), explicitly computing the microstructural response and the resulting force on the probe. This allows us to elucidate those microstructural processes that give the 'true' rheology in linear systems, and those which appear as artifacts. We explicitly derive the one- and two-point microrheological response for this system, by analogy with Levine and Lubensky (2000). Having elucidated these 'artifacts,' we track those processes as the probe is driven into steady motion in the hopes of probing nonlinear microrheology. We discuss several crucial issues for the interpretation of nonlinear microrheology: 1) how to interpret the inhomogeneous and non-viscometric nature of the deformation field around the probe, 2) the distinction between direct and bulk stresses and their deconvolution, and 3) the (Lagrangian) time-dependent nature of the stress histories experienced by material elements as they advect past the probe. Having identified these issues, we briefly discuss adaptations of the basic technique to more faithfully recover bulk rheology. While we specifically discuss a model colloidal suspension, we ultimately envision a technique capable of measuring the nonlinear rheology of general materials.
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Christoph Schmidt
Georg-August-Universität
Biophysics
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Title:
Active and Passive Microrheology: Non-Linear and Non-Equilibrium Systems
Abstract:
Microrheology can be used to probe the viscoelastic properties of materials on micrometer scales and is thus, for example, applicable in cells. Living cells are out-of-equilibrium systems and cytoskeletal networks also typically show strongly non-linear responses. Non-linear response can not be probed by "passive" microrheology which relies on thermal fluctuations. To separate the non-thermal from the thermal fluctuations in non-equilibrium systems, it is necessary to measure fluctuations and mechanical properties separately. I will report on a high-bandwidth technique for active 1- and 2-particle microrheology with which we can probe linear and nonlinear responses of soft materials. With the same method in conjunction with passive microrheology, we can characterize motor-driven active systems that mimic aspects of the cell cytoskeleton.
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