Publications by Year: 2010

2010
Broedersz, C. P. ; Depken, M. ; Yao, N. Y. ; Pollak, M. R. ; Weitz, D. A. ; MacKintosh, F. C. Cross-link-governed dynamics of biopolymer networks. Physical Review Letters 2010, 105, 238101. Publisher's VersionAbstract

Recent experiments show that networks of stiff biopolymers cross-linked by transient linker proteins exhibit complex stress relaxation, enabling network flow at long times. We present a model for the dynamics controlled by cross-links in such networks. We show that a single microscopic time scale for cross-linker unbinding leads to a broad spectrum of macroscopic relaxation times and a shear modulus G similar to omega(1/2) for low frequencies omega. This model quantitatively describes the measured rheology of actin networks cross-linked with alpha-actinin-4 over more than four decades in frequency.

broedersz2010.pdf
Shum, H. C. ; Abate, A. R. ; Lee, D. ; Studart, A. R. ; Wang, B. ; Chen, C. - H. ; Thiele, J. ; Shah, R. K. ; Krummel, A. ; Weitz, D. A. Droplet microfluidics for fabrication of non-spherical particles. Macromolecular Rapid Communications 2010, 31, 108-118. Publisher's VersionAbstract

We describe new developments for controlled fabrication of monodisperse non-spherical particles using droplet microfluidics. The high degree of control afforded by microfluidic technologies enables generation of single and multiple emulsion droplets. We show that these droplets can be transformed to non-spherical particles through further simple, spontaneous processing steps, including arrested coalescence, asymmetric polymer solidification, polymerization in microfluidic flow, and evaporation-driven clustering. These versatile and scalable microfluidic approaches can be used for producing large quantities of non-spherical particles that are monodisperse in both size and shape; these have great potential for commercial applications.

shum2010.pdf
Yao, N. Y. ; Broedersz, C. P. ; Lin, Y. - C. ; Kasza, K. E. ; MacKintosh, F. C. ; Weitz, D. A. Elasticity in ionically cross-linked neurofilament networks. Biophysical Journal 2010, 98, 2147-2153. Publisher's VersionAbstract

Neurofilaments are found in abundance in the cytoskeleton of neurons, where they act as an intracellular framework protecting the neuron from external stresses. To elucidate the nature of the mechanical properties that provide this protection, we measure the linear and nonlinear viscoelastic properties of networks of neurofilaments. These networks are soft solids that exhibit dramatic strain stiffening above critical strains of 30-70%. Surprisingly, divalent ions such as Mg(2+), Ca(2+), and Zn(2+) act as effective cross-linkers for neurofilament networks, controlling their solidlike elastic response. This behavior is comparable to that of actin-binding proteins in reconstituted filamentous actin. We show that the elasticity of neurofilament networks is entropic in origin and is consistent with a model for cross-linked semiflexible networks, which we use to quantify the cross-linking by divalent ions.

yao2010.pdf
Broedersz, C. P. ; Kasza, K. E. ; Jawerth, L. M. ; Muenster, S. ; Weitz, D. A. ; MacKintosh, F. C. Measurement of nonlinear rheology of cross-linked biopolymer gels. Soft Matter 2010, 6 4120-4127. Publisher's VersionAbstract

One of the hallmarks of biopolymer gels is their nonlinear viscoelastic response to stress, making the measurement of the mechanics of these gels very challenging. Various rheological protocols have been proposed for this; however, a thorough understanding of the techniques and their range of applicability as well as a careful comparison between these methods are still lacking. Using both strain ramp and differential prestress protocols, we investigate the nonlinear response of a variety of systems ranging from extracellular fibrin gels to intracellular F-actin solutions and F-actin cross-linked with permanent and physiological transient linkers. We find that the prestress and strain ramp results agree well for permanently cross-linked networks over two decades of strain rates, while the protocols only agree at high strain rates for more transient networks. Surprisingly, the nonlinear response measured with the prestress protocol is insensitive to creep; although a large applied steady stress can lead to significant flow, this has no significant effect on either the linear or nonlinear response of the system. A simple model is presented to provide insight into these observations.

broedersz2010.pdf
Stein, A. M. ; Vader, D. A. ; Weitz, D. A. ; Sander, L. M. The micromechanics of three-dimensional collagen-I gels. Complexity 2010, 16, 22-28. Publisher's VersionAbstract

We study the micromechanics of collagen-I gel with the goal of bridging the gap between theory and experiment in the study of biopolymer networks. Three-dimensional images of fluorescently labeled collagen are obtained by confocal microscopy, and the network geometry is extracted using a 3D network skeletonization algorithm. Each fiber is modeled as an elastic beam that resists stretching and bending, and each crosslink is modeled as torsional spring. The stress-strain curves of networks at three different densities are compared with rheology measurements. The model shows good agreement with experiment, confirming that strain stiffening of collagen can be explained entirely by geometric realignment of the network, as opposed to entropic stiffening of individual fibers. The model also suggests that at small strains, crosslink deformation is the main contributer to network stiffness, whereas at large strains, fiber stretching dominates. As this modeling effort uses networks with realistic geometries, this analysis can ultimately serve as a tool for understanding how the mechanics of fibers and crosslinks at the microscopic level produce the macroscopic properties of the network. (C) 2010Wiley Periodicals, Inc. Complexity 16: 22-28, 2011

stein2010.pdf
Ramsteiner, I. B. ; Weitz, D. A. ; Spaepen, F. Stiffness of the crystal-liquid interface in a hard-sphere colloidal system measured from capillary fluctuations. Physical Review E 2010, 82, 041603. Publisher's VersionAbstract

Face-centered cubic single crystals of sigma = 1.55 mu m diameter hard-sphere silica colloidal particles were prepared by sedimentation onto (100) and (110) oriented templates. The crystals had a wide interface with the overlaying liquid that was parallel to the template. The location of the interface was determined by confocal microscopic location of the particles, followed by identification of the crystalline and liquid phases by a bond-orientation order parameter. Fluctuations in the height of the interface about its average position were recorded for several hundred configurations. The interfacial stiffness (gamma) over tilde was determined from the slope of the inverse squared Fourier components of the height profile vs the square of the wave number, according to the continuum capillary fluctuation method. The offset of the fit from the origin could quantitatively be accounted for by gravitational damping of the fluctuations. For the (100) interface, (gamma) over tilde = (1.3 +/- 0.3) k(B)T/sigma(2); for the (110) interface, (gamma) over tilde = (1.0 +/- 0.2)k(B)T/sigma(2). The interfacial stiffness of both interfaces was found to be isotropic in the plane. This is surprising for the (110), where crystallography predicts twofold symmetry. Sedimentation onto a (111) template yielded a randomly stacked hexagonal crystal with isotropic (gamma) over tilde = 0.66k(B)T/sigma(2). This value, however, is less reliable than the two others due to imperfections in the crystal.

ramsteiner2010.pdf

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