Publications by Year: 2004

2004
Wong, I. Y. ; Gardel, M. L. ; Reichman, D. R. ; Weeks, E. R. ; Valentine, M. T. ; Bausch, A. R. ; Weitz, D. A. Anomalous diffusion probes microstructure dynamics of entangled F-actin networks. Physical Review Letters 2004, 92, 178101. Publisher's VersionAbstract
We study the thermal motion of colloidal tracer particles in entangled actin filament (F-actin) networks, where the particle radius is comparable to the mesh size of the F-actin network. In this regime, the ensemble-averaged mean-squared displacement of the particles is proportional to tau(gamma), where 0
wong2004.pdf
Weitz, D. A. ; Russel, W. B. New developments in colloid science. Mrs Bulletin 2004, 29, 82-84. Publisher's VersionAbstract
This introductory article reviews the topics covered in this issue of MRS Bulletin on New Developments in Colloid Science. Colloidal particles have a long history of importance in a broad range of applications in technology and materials processing. They can be made from many different materials and suspended in a wide variety of solvents. The rheological properties of colloidal suspensions have traditionally been of primary concern in their technological applications, and our understanding of these properties continues to evolve. However, new uses of colloidal particles are also emerging. Because they can be produced to a precise size, colloidal particles are now also being used in novel ways as building blocks for engineering completely new materials, including high-precision filters, controlled-porosity substrates, and photonic devices. In addition, new methods are evolving to alter the shape of the particles and create controlled structures with nonspherical particles. New experimental techniques are allowing improved measurement and increased understanding of the structure, properties, and behavior of colloidal suspensions, Significant progress continues to be made, and the potential uses of colloidal particles continue to grow. This issue presents a snapshot summary of recent developments in this field.
weitz2004.pdf
Weitz, D. A. Packing in the spheres. Science 2004, 303, 968-969. Publisher's VersionAbstract

To study how objects pack together, physicists have long assumed that the objects are spherical. But this simplifying approximation may be leading researchers astray. As Weitz discusses in his Perspective, the work reported by Doney et al. in the same issue shows that how spheres pack may be the exception rather than the rule. Through a combination of experiments with a large collection of ellipsoidal candies and computer simulations, the researchers discover that a much higher volume can be filled with the ellipsoids than with spheres. Not just a theoretical puzzle, the ways in which objects pack are of vital concern to the shipping and manufacturing industries.

weitz2004.pdf
Valentine, M. T. ; Perlman, Z. E. ; Gardel, M. L. ; Shin, J. H. ; Matsudaira, P. ; Mitchison, T. J. ; Weitz, D. A. Colloid surface chemistry critically affects multiple particle tracking measurements of biomaterials. Biophysical Journal 2004, 86, 4004-4014. Publisher's VersionAbstract
Characterization of the properties of complex biomaterials using microrheological techniques has the promise of providing fundamental insights into their biomechanical functions; however, precise interpretations of such measurements are hindered by inadequate characterization of the interactions between tracers and the networks they probe. We here show that colloid surface chemistry can profoundly affect multiple particle tracking measurements of networks of fibrin, entangled F-actin solutions, and networks of cross-linked F-actin. We present a simple protocol to render the surface of colloidal probe particles protein-resistant by grafting short amine-terminated methoxy-poly(ethylene glycol) to the surface of carboxylated microspheres. We demonstrate that these poly(ethylene glycol)-coated tracers adsorb significantly less protein than particles coated with bovine serum albumin or unmodified probe particles. We establish that varying particle surface chemistry selectively tunes the sensitivity of the particles to different physical properties of their microenvironments. Specifically, particles that are weakly bound to a heterogeneous network are sensitive to changes in network stiffness, whereas protein-resistant tracers measure changes in the viscosity of the fluid and in the network microstructure. We demonstrate experimentally that two-particle microrheology analysis significantly reduces differences arising from tracer surface chemistry, indicating that modifications of network properties near the particle do not introduce large-scale heterogeneities. Our results establish that controlling colloid-protein interactions is crucial to the successful application of multiple particle tracking techniques to reconstituted protein networks, cytoplasm, and cells.
valentine2004.pdf
Shin, J. H. ; Gardel, M. L. ; Mahadevan, L. ; Matsudaira, P. ; Weitz, D. A. Relating microstructure to rheology of a bundled and cross-linked F-actin network in vitro. PNAS 2004, 101, 9636-9641. Publisher's VersionAbstract
The organization of individual actin filaments into higher-order structures is controlled by actin-binding proteins (ABPs). Although the biological significance of the ABPs is well documented, little is known about how bundling and cross-linking quantitatively affect the microstructure and mechanical properties of actin networks. Here we quantify the effect of the ABP scruin on actin networks by using imaging techniques, cosedimentation assays, multiparticle tracking, and bulk rheology. We show how the structure of the actin network is modified as the scruin concentration is varied, and we correlate these structural changes to variations in the resultant network elasticity.
shin2004.pdf
Schall, P. ; Cohen, I. ; Weitz, D. A. ; Spaepen, F. Visualization of dislocation dynamics in colloidal crystals. Science 2004, 305, 1944-1948. Publisher's VersionAbstract
The dominant mechanism for creating large irreversible strain in atomic crystals is the motion of dislocations, a class of line defects in the crystalline lattice. Here we show that the motion of dislocations can also be observed in strained colloidal crystals, allowing detailed investigation of their topology and propagation. We describe a laser diffraction microscopy setup used to study the growth and structure of misfit dislocations in colloidal crystalline films. Complementary microscopic information at the single-particle level is obtained with a laser scanning confocal microscope. The combination of these two techniques enables us to study dislocations over a range of length scales, allowing us to determine important parameters of misfit dislocations such as critical film thickness, dislocation density, Burgers vector, and lattice resistance to dislocation motion. We identify the observed dislocations as Shockley partials that bound stacking faults of vanishing energy. Remarkably, we find that even on the scale of a few lattice vectors, the dislocation behavior is well described by the continuum approach commonly used to describe dislocations in atomic crystals.
schall2004.pdf
Nikova, A. T. ; Gordon, V. D. ; Cristobal, G. ; Talingting, M. R. ; Bell, D. C. ; Evans, C. ; Joanicot, M. ; Zasadzinski, J. A. ; Weitz, D. A. Swollen vesicles and multiple emulsions from block copolymers. Macromolecules 2004, 37, 2215-2218. Publisher's VersionAbstract
We engineer novel structures by "stuffing" the aliphatic regions of self-assembled aggregates with hydrophobic homopolymer. These "stuffed" vesicles and multiple emulsions are formed in a one-step process when we rehydrate stuffed films made of amphiphilic block copolymer and hydrophobic homopolymer. Without such homopolymer, this system forms micelles. With homopolymer, vesicles form; varying vesicle membrane thicknesses show that these structures incorporate different amounts of homopolymer. Multiple emulsions, containing more homopolymer than stuffed vesicles, are also fabricated using this single-amphiphile system. The system's incorporation of homopolymer to modify the properties and morphology of the resultant structures is a convenient strategy for preparing self-assembled macromolecular structures with controllable properties.
nikova2004.pdf
Mucha, P. J. ; Tee, S. Y. ; Weitz, D. A. ; Shraiman, B. I. ; Brenner, M. P. A model for velocity fluctuations in sedimentation. Journal of Fluid Mechanics 2004, 501, 71-104. Publisher's VersionAbstract
We present a model for velocity fluctuations of dilute sedimenting spheres at low Reynolds number. The central idea is that a vertical stratification causes the fluctuations to decrease below those of an independent uniform distribution of particles, such a stratification naturally occurring from the broadening of the sedimentation front. We use numerical simulations, scaling arguments, structure factor calculations, and experiments to show that there is a critical stratification above which the characteristics of the density and velocity fluctuations change significantly. For thin cells, the broadening of the sediment front (and the resulting stratification) is small, so the velocity fluctuations are predicted by independent-Poisson-distribution estimates. In very thick cells, the stratification is significant, leading to persistent decay of the velocity fluctuations for the duration of the experiment. Estimated stratifications quantitatively agree with the simulations, and indicate the likelihood that previous experimental measurements were also affected by stratification. The Velocity fluctuations in sedimentation are therefore not universal but instead depend on both the cell shape and developing stratification.
mucha2004.pdf
Manley, S. ; Cipelletti, L. ; Trappe, V. ; Bailey, A. E. ; Christianson, R. J. ; Gasser, U. ; Prasad, V. ; Segre, P. N. ; Doherty, M. P. ; Sankaran, S. ; et al. Limits to gelation in colloidal aggregation. Physical Review Letters 2004, 93, 108302. Publisher's VersionAbstract
We show that the dynamics of large fractal colloid aggregates are well described by a combination of translational and rotational diffusion and internal elastic fluctuations, allowing both the aggregate size and internal elasticity to be determined by dynamic light scattering. The comparison of results obtained in microgravity and on Earth demonstrates that cluster growth is limited by gravity-induced restructuring. In the absence of gravity, thermal fluctuations ultimately inhibit fractal growth and set the fundamental limitation to the lowest volume fraction which will gel.
manley2004.pdf
Link, D. R. ; Anna, S. L. ; Weitz, D. A. ; Stone, H. A. Geometrically mediated breakup of drops in microfluidic devices. Physical Review Letters 2004, 92, 054503. Publisher's VersionAbstract
Microfluidic technology offers capabilities for the precise handling of small fluid volumes dispersed as droplets. To fully exploit this potential requires simultaneous generation of multiple size droplets. We demonstrate two methods for passively breaking larger drops into precisely controlled daughter drops using pressure-driven flow in simple microfluidic configurations: (i) a T junction and (ii) flow past isolated obstacles. We quantify conditions for breakup at a T junction and illustrate sequential breakup at T junctions for making small drops at high dispersed phase volume fractions.
link2004.pdf
Janmey, P. A. ; Weitz, D. A. Dealing with mechanics: Mechanisms of force transduction in cells. Trends in Biochemical Sciences 2004, 29, 364-370. Publisher's VersionAbstract
Physical cues, such as forces applied to a cell membrane or the stiffness of materials to which cells adhere, are increasingly recognized as essential determinants of biological function, and mechanical stimuli can be as important as chemical stimuli in determining tissue fate or contributing to pathological states. The physical environment of the cell can act in concert with, or sometimes override, the signals given by proteins and other cellular ligands to change cell morphology, growth rates and transcriptional programs. Recent developments in technology and techniques have facilitated studies of how forces trigger cellular events on the molecular level. As the mechanisms of force transduction are identified, methods and concepts from the physical sciences might become as important as those of biochemistry in elucidating how cells function and how these functions might be altered or corrected in therapeutic and biotechnological contexts.
janmey2004.pdf
Hiddessen, A. L. ; Weitz, D. A. ; Hammer, D. A. Rheology of binary colloidal structures assembled via specific biological cross-linking. Langmuir 2004, 20, 6788-6795. Publisher's VersionAbstract
The selectivity and range of energies offered by specific biological interactions serve as valuable tools for engineering the assembly of colloidal particles into novel materials. In this investigation, high affinity biological interactions between biotin-coated "A" particles (R-A = 0.475 mum) and streptavidin-coated "B" particles (R-B = 2.75 mum) drive the self-assembly of a series of binary colloidal structures, from colloidal micelles (a large B particle coated by smaller A particles) to elongated chain microstructures (alternating A and B particles), as the relative number of small (A) to large (B) particles (2 less than or equal to N-A/N-B less than or equal to 200) is decreased at a low total volume fraction (10(-4) less than or equal to phi(T) less than or equal to 10(-3)). At a significantly higher total volume fraction (phi(T) greater than or equal to 10(-1)) and a low number ratio (N-A/N-B = 2), the rheological behavior of volume-filling particle networks connected by streptavidin-biotin bonds is characterized. The apparent viscosity (eta) as a function of the shear rate (gamma), measured for networks at phi(T) = 0.1 and 0.2, exhibits shear-rate-dependent flow behavior, and both the apparent viscosity and the extent of shear thinning increase upon an increase of a factor of 2 in the total volume fraction. Micrographs taken before and after shearing show a structural breakdown of the flocculated binary particle network into smaller flocs, and ultimately a fluidlike suspension, with increasing shear rate. Rheological measurements provide further proof that suspension microstructure is governed by specific biomolecular interactions, as control experiments in which the streptavidin molecules on particles were blocked displayed Newtonian flow behavior. This investigation represents the first attempt at measuring the rheology of colloidal suspensions where assembly is driven by biomolecular cross-linking.
hiddessen2004.pdf
Gordillo, J. M. ; Cheng, Z. D. ; Ganan-Calvo, A. M. ; Marquez, M. ; Weitz, D. A. A new device for the generation of microbubbles. Physics of Fluids 2004, 16, 2828-2834. Publisher's VersionAbstract
In this paper we present a new method for the production of bubble-liquid suspensions (from now on BLS) composed of micron-sized bubbles and with gas to liquid volume ratios larger than unity. We show that the BLS gas fraction lambda=Q(g)/Q(l), being Q(g) and Q(l) the flow rates of gas and liquid, respectively, is controlled by a dimensionless parameter which accounts for the ratio of the gas pressure inside the device to the liquid viscous pressure drop from the orifices where the liquid is injected to the exit, where the BLS is obtained. This parameter permits the correct scaling of the BLS gas volume fraction of all the experiments presented. (C) 2004 American Institute of Physics.
gordillo2004.pdf
Gordon, V. D. ; Xi, C. ; Hutchinson, J. W. ; Bausch, A. R. ; Marquez, M. ; Weitz, D. A. Self-assembled polymer membrane capsules inflated by osmotic pressure. Journal of the American Chemical Society 2004, 126, 14117-14122. Publisher's VersionAbstract
We fabricate and characterize capsules that are composite membranes, made of a polymer network stabilized by adsorption to colloids and inflated by osmotic pressure from internal free polyelectrolyte; here, poly-L-lysine forms the network and inflates the capsules. To assess these capsules' properties and structure, we deform capsules using microcantilevers and use finite element modeling to describe these deformations. Additional experimental tests confirm the model's validity. These capsules' resilient response to mechanical forces indicates that loading and shear should be good triggers for the release of contents via deformation. The osmotic pressure inflating these capsules has the potential to trigger release of contents via deflation in response to changes in the capsules' environment; we demonstrate addition of salt as a trigger for deflating capsules. Because these capsules have a variety of release triggers available and the technique used to fabricate them is very flexible and allows high encapsulation efficiency, these capsules have very high potential for application in many areas.
gordon2004.pdf
Gardel, M. L. ; Shin, J. H. ; MacKintosh, F. C. ; Mahadevan, L. ; Matsudaira, P. ; Weitz, D. A. Elastic behavior of cross-linked and bundled actin networks. Science 2004, 304, 1301-1305. Publisher's VersionAbstract
Networks of cross-linked and bundled actin. laments are ubiquitous in the cellular cytoskeleton, but their elasticity remains poorly understood. We show that these networks exhibit exceptional elastic behavior that reflects the mechanical properties of individual. laments. There are two distinct regimes of elasticity, one reflecting bending of single. laments and a second reflecting stretching of entropic fluctuations of filament length. The mechanical stiffness can vary by several decades with small changes in cross-link concentration, and can increase markedly upon application of external stress. We parameterize the full range of behavior in a state diagram and elucidate its origin with a robust model.
gardel2004.pdf
Gardel, M. L. ; Shin, J. H. ; MacKintosh, F. C. ; Mahadevan, L. ; Matsudaira, P. A. ; Weitz, D. A. Scaling of F-actin network rheology to probe single filament elasticity and dynamics. Physical Review Letters 2004, 93, 188102. Publisher's VersionAbstract
The linear and nonlinear viscoelastic response of networks of cross-linked and bundled cytoskeletal filaments demonstrates remarkable scaling with both frequency and applied prestress, which helps elucidate the origins of the viscoelasticity. The frequency dependence of the shear modulus reflects the underlying single-filament relaxation dynamics for 0.1-10 rad/sec. Moreover, the nonlinear strain stiffening of such networks exhibits a universal form as a function of prestress; this is quantitatively explained by the full force-extension relation of single semiflexible filaments.
gardel2004.pdf
Fernandez-Nieves, A. ; Link, D. R. ; Rudhardt, D. ; Weitz, D. A. Electro-optics of bipolar nematic liquid crystal droplets. Physical Review Letters 2004, 92, 105503. Publisher's VersionAbstract
We directly visualize the response and relaxation dynamics of bipolar nematic liquid crystal droplets to an applied electric field E. Despite strong planar anchoring, there is no critical field for switching. Instead, upon application of E, the surface region first reorients, followed by movement of the disclinations and the bipolar axis. After removing E, elastic forces restore the drop to its original state. The collective electro-optic properties of ordered hexagonal-close-packed monolayers of drops are probed by diffraction experiments confirming the proposed switching mechanism.
fernandez-nieves2004.pdf
Cohen, I. ; Mason, T. G. ; Weitz, D. A. Shear-induced configurations of confined colloidal suspensions. Physical Review Letters 2004, 93, 046001. Publisher's VersionAbstract
We show that geometric confinement dramatically affects the shear-induced configurations of dense monodisperse colloidal suspensions; a new structure emerges, where layers of particles buckle to stack in a more efficient packing. The volume fraction in the shear zone is controlled by a balance between the viscous stresses and the osmotic pressure of a contacting reservoir of unsheared particles. We present a model that accounts for our observations and helps elucidate the complex interplay between particle packing and shear stress for confined suspensions.
cohen2004.pdf
Aray, Y. ; Manuel, M. ; Rodriguez, J. ; Vega, D. ; Simon-Manso, Y. ; Coll, S. ; Gonzalez, C. ; Weitz, D. A. Electrostatics for exploring the nature of the hydrogen bonding in polyethylene oxide hydration. Journal of Physical Chemistry B 2004, 108, 2418-2424. Publisher's VersionAbstract

Binding between water and models of poly(ethylene oxide), (CH2−CH2−O)nn = 2−40, has been studied using the topographic features of the electrostatic potential, V(r), and standard density functional theory methods. It was found that, in general, the contour around the minima of the oxygen atoms overlap forming a negative-valued spiral coiled around a positive-valued helix. The positive zone defines a helical groove in the O−C−C−O units where minima lone pairs critical points are located. Topological analysis of the water molecule has also suggested that the attractive electrostatic effect between the positive water O−H zone and the negative PEO lone pairs plays an important role in the hydrogen bonding of the PEO−water system. Thus, the V(r) topology predicts a coil of water molecules around the PEO chain forming hydrogen bonding with two sites of ether oxygens. This coil is formed in such a way that more water molecules accumulate on the cavities surrounding the poly(ethylene oxide)'s oxygen atoms where the minima of the negative zone are located.

aray2004.pdf