@article {1588575, title = {Noninvasive Probing of the Spatial Organization of Polymer Chains in Hydrogels Using Fluorescence Resonance Energy Transfer (FRET)}, journal = {Journal of the American Chemical Society}, volume = {129}, number = {15}, year = {2007}, pages = {4518-4519}, abstract = { Spatial organization of polymer chains upon ionic or covalent cross-linking to form a hydrogel is examined on the molecular scale using a fluorescent resonance energy transfer (FRET) technique. Minimal changes in the conformation in the polymer chains but significant changes of the intermolecular association of polymer chains are found when polymer chains are cross-linked with a different number of cross-links. This study provides a previously undescribed technique to analyze hydrogel structure in a noninvasive manner. }, url = {https://pubs.acs.org/doi/10.1021/ja0690058}, author = {Kong, Hyun J. and Kim, Chan J. and Huebsch, Nathaniel and Weitz, DavidA. and Mooney, David J.} } @article {156376, title = {Strain-rate frequency superposition: A rheological probe of structural relaxation in soft materials}, journal = {Physical Review Letters}, volume = {98}, number = {23}, year = {2007}, pages = {238303}, abstract = {The rheological properties of soft materials often exhibit surprisingly universal linear and nonlinear features. Here we show that these properties can be unified by considering the effect of the strain-rate amplitude on the structural relaxation of the material. We present a new form of oscillatory rheology, strain-rate frequency superposition (SRFS), where the strain-rate amplitude is fixed as the frequency is varied. We show that SRFS can isolate the response due to structural relaxation, even when it occurs at frequencies too low to be accessible with standard techniques.}, url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.98.238303}, author = {Wyss, Hans M. and Miyazaki, Kunimasa and Mattsson, Johan and Hu, Zhibing and Reichman, David R. and Weitz, David A} } @article {156121, title = {Short- and long-range correlated motion observed in colloidal glasses and liquids}, journal = {Journal of Physics-Condensed Matter}, volume = {19}, number = {20}, year = {2007}, pages = {205131}, abstract = {We use a confocal microscope to examine the motion of individual particles in a dense colloidal suspension. Close to the glass transition, particle motion is strongly spatially correlated. The correlations decay exponentially with particle separation, yielding a dynamic length scale of O(2-3 sigma) (in terms of particle diameter sigma). This length scale grows modestly as the glass transition is approached. Further, the correlated motion exhibits a strong spatial dependence on the pair correlation function g(r). Motion within glassy samples is weakly correlated, but with a larger spatial scale for this correlation.}, url = {https://iopscience.iop.org/article/10.1088/0953-8984/19/20/205131}, author = {Weeks, Eric R. and Crocker, John C. and Weitz, D. A.} } @article {156001, title = {Dripping, jetting, drops, and wetting: The magic of microfluidics}, journal = {Mrs Bulletin}, volume = {32}, year = {2007}, pages = {702-708}, abstract = {The following article is based on the Symposium X presentation given by David A. Weitz (Harvard University) on April 11, 2007, at the Materials Research Society Spring Meeting in San Francisco. The article describes how simple microfluidic devices can be used to control fluid flow and produce a variety of new materials. Based on the concepts of coaxial flow and hydrodynamically focused flow, used alone or in various combinations, the devices can produce precisely controlled double emulsions (droplets within droplets) and even triple emulsions (double emulsions suspended in a third droplet). These structures, which can be created in a single microfluidic device, have various applications such as encapsulants for drugs, cosmetics, or food additives.}, url = {https://link.springer.com/article/10.1557\%2Fmrs2007.145}, author = {Utada, A.S. and Chu, L. Y. and Fernandez-Nieves, A. and Link, D.R. and C. Holtze and Weitz, D. A.} } @article {156011, title = {Dripping to jetting transitions in coflowing liquid streams}, journal = {Physical Review Letters}, volume = {99}, number = {9}, year = {2007}, pages = {094502}, abstract = {A liquid forced through an orifice into an immiscible fluid ultimately breaks into drops due to surface tension. Drop formation can occur right at the orifice in a dripping process. Alternatively, the inner fluid can form a jet, which breaks into drops further downstream. The transition from dripping to jetting is not understood for coflowing fluid streams, unlike the case of drop formation in air. We show that in a coflowing stream this transition can be characterized by a state diagram that depends on the capillary number of the outer fluid and the Weber number of the inner fluid.}, url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.99.094502}, author = {Utada, Andrew S. and Fernandez-Nieves, Alberto and Stone, Howard A. and Weitz, David A} } @article {155946, title = {Velocity fluctuations of initially stratified sedimenting spheres}, journal = {Physics of Fluids}, volume = {19}, number = {11}, year = {2007}, pages = {113304}, abstract = {The study of velocity fluctuations in the sedimentation of spheres is complicated by the time evolution of the underlying particle distribution, both at the microscale and in the bulk. We perform a series of experiments and simulations to isolate the effect of an initial, stable stratification in the particle concentration. The directly observed dependence of velocity fluctuations on stratification agrees with a previously obtained scaling theory. (c) 2007 American Institute of Physics.}, url = {https://aip.scitation.org/doi/10.1063/1.2806597}, author = {Tee, Shang-You and Mucha, P. J. and Brenner, M.P. and Weitz, D. A.} } @inbook {155891, title = {A Stochastic Model of Glioblastoma Invasion}, booktitle = {Mathematical Modeling of Biological Systems, Vol I: Cellular Biophysics, Regulatory Networks, Development, Biomedicine, and Data Analysis}, volume = {31}, year = {2007}, pages = {217-224}, publisher = {Birkhauser Boston}, organization = {Birkhauser Boston}, abstract = {Glioblastoma is the most malignant form of brain cancer. It is extremely invasive; the mechanisms that govern invasion are not well understood. To better understand the process of invasion, we conducted an in vitro experiment in which a 3D tumor spheroid is implanted into a collagen gel. The paths of individual invasive cells were tracked. These cells were modeled as radially biased, persistent random walkers. The radial velocity bias was found to be 19.6 mu m/hr.}, url = {https://link.springer.com/chapter/10.1007/978-0-8176-4558-8_19}, author = {Stein, Andrew M. and Vader, David A. and Sander, Leonard M. and Weitz, David A} } @article {155681, title = {Mixing characterization inside microdroplets engineered on a microcoalescer}, journal = {Chemical Engineering Science}, volume = {62}, number = {4}, year = {2007}, pages = {1042-1048}, abstract = {We use a microdevice where microdroplets of reagents are generated and coalesce in a carrier continuous phase. The work focuses on the characterization of the mixing step inside the droplets, in the perspective to use them for chemical kinetic data acquisition. A dye and water are used, and an acid-base instantaneous chemical reaction is monitored thanks to a colored indicator. Acquisitions are done with a high-speed camera coupled to a microscope and a mixing parameter is calculated by image analysis. Different angles of bended channels and different ways of coalescence are compared. It is shown that the homogenization of the droplets can be reached in less than lOms after coalescence. This is achieved by forcing the droplets to coalesce in a "shifted" way, and later by adding 45 degrees angle bends along the channel. (c) 2006 Elsevier Ltd. All rights reserved.}, url = {https://www.sciencedirect.com/science/article/pii/S0009250906006634?via\%3Dihub}, author = {Sarrazin, F. and Prat, L. and Di Miceli, N. and Cristobal, G. and Link, D.R. and Weitz, D. A.} } @article {155701, title = {Structural rearrangements that govern flow in colloidal glasses}, journal = {Science}, volume = {318}, number = {5858}, year = {2007}, pages = {1895-1899}, abstract = {Structural rearrangements are an essential property of atomic and molecular glasses; they are critical in controlling resistance to flow and are central to the evolution of many properties of glasses, such as their heat capacity and dielectric constant. Despite their importance, these rearrangements cannot directly be visualized in atomic glasses. We used a colloidal glass to obtain direct three- dimensional images of thermally induced structural rearrangements in the presence of an applied shear. We identified localized irreversible shear transformation zones and determined their formation energy and topology. A transformation favored successive ones in its vicinity. Using continuum models, we elucidated the interplay between applied strain and thermal fluctuations that governs the formation of these zones in both colloidal and molecular glasses.}, url = {https://science.sciencemag.org/content/318/5858/1895.full}, author = {Schall, Peter and Weitz, David A and Spaepen, Frans} } @article {155186, title = {Target-locking acquisition with real-time confocal (TARC) microscopy}, journal = {Optics Express}, volume = {15}, number = {14}, year = {2007}, pages = {8702-8712}, abstract = {We present a real-time target-locking confocal microscope that follows an object moving along an arbitrary path, even as it simultaneously changes its shape, size and orientation. This Target-locking Acquisition with Realtime Confocal (TARC) microscopy system integrates fast image processing and rapid image acquisition using a Nipkow spinning-disk confocal microscope. The system acquires a 3D stack of images, performs a full structural analysis to locate a feature of interest, moves the sample in response, and then collects the next 3D image stack. In this way, data collection is dynamically adjusted to keep a moving object centered in the field of view. We demonstrate the system{\textquoteright}s capabilities by target-locking freely-diffusing clusters of attractive colloidal particles, and actively-transported quantum dots (QDs) endocytosed into live cells free to move in three dimensions, for several hours. During this time, both the colloidal clusters and live cells move distances several times the length of the imaging volume. (c) 2007 Optical Society of America}, url = {https://www.osapublishing.org/oe/fulltext.cfm?uri=oe-15-14-8702\&id=138985}, author = {Lu, Peter J. and Sims, Peter A. and Oki, Hidekazu and Macarthur, James B. and Weitz, David A} } @article {155146, title = {Visualizing the strain field in semiflexible polymer networks: Strain fluctuations and nonlinear rheology of F-actin gels}, journal = {Physical Review Letters}, volume = {98}, number = {19}, year = {2007}, pages = {198304}, abstract = {We image semiflexible polymer networks under shear at the micrometer scale. By tracking embedded probe particles, we determine the local strain field, and directly measure its uniformity, or degree of affineness, on scales of 2-100 mu m. The degree of nonaffine strain depends upon the polymer length and cross-link density, consistent with theoretical predictions. We also find a direct correspondence between the uniformity of the microscale strain and the nonlinear elasticity of the networks in the bulk.}, url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.98.198304}, author = {Liu, J. and Koenderink, G. H. and Kasza, K. E. and MacKintosh, F. C. and Weitz, D. A.} } @article {155081, title = {Viscoelastic properties of microtubule networks}, journal = {Macromolecules}, volume = {40}, number = {21}, year = {2007}, pages = {7714-7720}, abstract = {Micrombules are filamentous protein biopolymers found in eukaryotic cells. They form networks that guide active intracellular transport and support the overall cell structure. Microtubules are very rigid polymers, with persistence lengths as large as a millimeter. As such, they constitute an example of rodlike polymers, whose mechanical and theological properties are as yet poorly understood. We measure the linear and nonlinear viscoelastic properties of isotropic solutions of purified microtubules, as well as networks permanently cross-linked with biotin-NeutrAvidin. In the linear regime both solutions and networks are soft elastic materials with elastic moduli on the order of a few pascals. The elastic moduli show a power-law dependence on tubulin concentration, c(T), with G{\textquoteright} similar to c(T)(nu), where v approximate to 1.4 for solutions and increases slightly to nu approximate to 1.6-1.8 for networks. At large deformations, we observe a concentration-dependent yield stress. The rheology of microtubule solutions cannot be explained by the Doi-Edwards model, which treats noninteracting rigid rods. Instead, they show behavior very similar to the permanently cross-linked networks, suggesting the presence of effective cross-linking even in pure microtubule solutions. We develop a simple model based on transient cross-linking interactions between microtubules to interpret the rheological response. We also calculate a lower bound estimate of the strength of this interaction. Our data provide a framework with which to understand the dynamics and mechanics of more physiological networks of microtubules with microtubule-associated cross-linking and motor proteins, and ultimately to understand the role of microtubules in cell mechanics.}, url = {https://pubs.acs.org/doi/10.1021/ma070862l}, author = {Lin, Yi-Chia and Koenderink, Gijsje H and MacKintosh, Frederick C and Weitz, David A} } @article {155016, title = {A new microrheometric approach reveals individual and cooperative roles for TGF-beta 1 and IL-1 beta in fibroblast-mediated stiffening of collagen gels}, journal = {Faseb Journal}, volume = {21}, number = {9}, year = {2007}, pages = {2064-2073}, abstract = {The stiffness of the extracellular matrix can profoundly influence cell and tissue behaviors. Thus there is an emerging emphasis on understanding how matrix mechanical environments are established, regulated, and modified. Here we develop a microrheometric assay to measure the mechanical properties of a model extracellular matrix (type I collagen gel) and use it to explore cytokine-induced, cell-mediated changes in matrix mechanical properties. The microrheometric assay uses micron-scale ferrimagnetic beads embedded within collagen gels during fibrillogenesis. The beads are magnetized, then subjected to a twisting field, with the aggregate rotation of the beads measured by a magnetometer. The degree of bead rotation reflects the stiffness of the surrounding matrix. We show that the microscale assay provides stiffness measures for collagen gels comparable to those obtained with standard macroscale rheometry. To demonstrate the utility of the assay for biological discovery, we measure stiffness changes in fibroblast-populated collagen gels exposed to three concentrations of six cytokines over 2 to 14 days. Among the cytokines tested, transforming growth factor-beta 1 and interleukin-1 beta enhanced matrix stiffness, and together exerted cooperative effects on cellular modulation of matrix mechanics. The microrheometry approach developed here should accelerate the discovery of biological pathways orchestrating cellular modulation of matrix mechanics.}, url = {https://faseb.onlinelibrary.wiley.com/doi/abs/10.1096/fj.06-7510com}, author = {Leung, Lester Y. and Tian, David and Brangwynne, Clifford P and Weitz, David A and Tschumperlin, Daniel J.} } @article {154836, title = {Colloidal assembly route for responsive colloidosomes with tunable permeability}, journal = {Nano Letters}, volume = {7}, number = {9}, year = {2007}, pages = {2876-2880}, abstract = {We present a robust and straightforward approach for fabricating a novel colloiclosome system where colloidal particles are assembled to form colloidal shells on the surface of stimuli-responsive microgel scaffolds. We demonstrate that the structural properties of the colloidal shells can be controlled through the colloidal particle size and modulus, and the state of supporting microgel particles. This technique offers a new way to engineer colloidosomes, enabling fine control over their permeability over a wide range of length scales.}, url = {https://pubs.acs.org/doi/10.1021/nl0715948}, author = {Kim, Jin-Woong and Fernandez-Nieves, Alberto and Dan, Nily and Utada, Andrew S. and Marquez, Manuel and Weitz, David A} } @article {154831, title = {Controlled production of emulsion drops using an electric field in a flow-focusing microfluidic device}, journal = {Applied Physics Letters}, volume = {91}, number = {13}, year = {2007}, pages = {133106}, abstract = {We describe a flexible emulsification method using an electric field to generate droplets in a hydrodynamic-flow-focusing geometry in microchannels. The droplet size is controlled by the ratio of inner and outer flow rates as well as by the electric field. As the voltage increases, the droplet size decreases. A Taylor cone is formed and generates very fine droplets, less than 1 mu m in diameter. Small inner flow rates and high electric fields are required to form a stable Taylor cone in a dc electric field. An ac electric field produces tiny droplets periodically. (C) 2007 American Institute of Physics.}, url = {https://aip.scitation.org/doi/10.1063/1.2790785}, author = {Kim, Haejune and Luo, Dawei and Link, Darren and Weitz, David A and Marquez, Manuel and Cheng, Zhengdong} } @article {154856, title = {Fabrication of monodisperse gel shells and functional microgels in microfluidic devices}, journal = {Angewandte Chemie-International Edition}, volume = {46}, number = {11}, year = {2007}, pages = {1819-1822}, abstract = { Microgel structures\ such as spherical microgel shells (picture on the upper left) and spherical microgel particles that incorporate quantum dots, magnetic nanoparticles, and polymer microparticles (other images) have been prepared by a capillary microfluidic technique. Because these particles change their volume with changes in temperature, they may find application in, for example, drug delivery. }, url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.200604206}, author = {Kim, Jin-Woong and Utada, Andrew S. and Fernandez-Nieves, Alberto and Hu, Zhibing and Weitz, David A} } @article {154826, title = {Gravitational stability of suspensions of attractive colloidal particles}, journal = {Physical Review Letters}, volume = {99}, number = {2}, year = {2007}, pages = {028303}, abstract = {Colloidal suspensions are susceptible to gravitationally induced phase separation. This can be mitigated by the formation of a particle network caused by depletion attraction. The effectiveness of this network in supporting the buoyant weight of the suspension can be characterized by its compressional modulus. We measure the compressional modulus for emulsion networks induced by depletion attraction and present a model that quantitatively predicts their gravitational stability. We also determine the relationship between the strength of the depletion attraction and the magnitude of the compressional modulus.}, url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.99.028303}, author = {Kim, Chanjoong and Liu, Yaqian and Kuehnle, Angelika and Hess, Stephan and Viereck, Sonja and Danner, Thomas and Mahadevan, L and Weitz, David A} } @article {154846, title = {Uniform nonspherical colloidal particles with tunable shapes}, journal = {Advanced Materials}, volume = {19}, number = {15}, year = {2007}, pages = {2005-2009}, abstract = {A framework for large-scale synthesis of a variety of uniform nonspherical particle types (see figure) is introduced. The technique involves controlling the directionality of phase separations in the seeded-polymerization technique by manipulating the crosslinking density gradients of the dimer seed particles, thus allowing the obtainment of novel nonspherical particle shapes and the production of sufficient quantities to characterize their bulk properties.}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adma.200602345}, author = {Kim, Jin-Woong and Larsen, RyanJ. and Weitz, David A} } @article {154801, title = {The cell as a material}, journal = {Current Opinion in Cell Biology}, volume = {19}, number = {1}, year = {2007}, pages = {101-107}, abstract = {To elucidate the dynamic and functional role of a cell within the tissue it belongs to, it is essential to understand its material properties. The cell is a viscoelastic material with highly unusual properties. Measurements of the mechanical behavior of cells are beginning to probe the contribution of constituent components to cell mechanics. Reconstituted cytoskeletal protein networks have been shown to mimic many aspects of the mechanical properties of cells, providing new insight into the origin of cellular behavior. These networks are highly nonlinear, with an elastic modulus that depends sensitively on applied stress. Theories can account for some of the measured properties, but a complete model remains elusive.}, url = {https://www.sciencedirect.com/science/article/pii/S0955067406001839?via\%3Dihub}, author = {Kasza, Karen E. and Rowat, Amy C. and Liu, Jiayu and Angelini, Thomas E. and Brangwynne, Clifford P and Koenderink, Gijsje H and Weitz, David A} } @article {154431, title = {Novel defect structures in nematic liquid crystal shells}, journal = {Physical Review Letters}, volume = {99}, number = {15}, year = {2007}, pages = {157801}, abstract = {We use double-emulsion drops to experimentally investigate the defect structures of spherical shells of nematic liquid crystals. We uncover a rich scenario of coexisting defect structures dictated by the unavoidable finite thickness of even the thinnest shell and by the thickness variation around the sphere. These structures are characterized by a varying number of disclination lines and pairs of surface point defects on the inner and outer surfaces of the nematic shell. In the limit of very thick shells the defect structure ultimately merges with that of a bulk nematic liquid crystal drop.}, url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.99.157801}, author = {Fernandez-Nieves, A. and Vitelli, V. and Utada, A.S. and Link, D.R. and Marquez, M. and Nelson, D.R. and Weitz, D. A.} } @article {154416, title = {Topological changes in bipolar nematic droplets under flow}, journal = {Physical Review Letters}, volume = {98}, number = {8}, year = {2007}, pages = {087801}, abstract = {Bipolar liquid crystal drops moving inside microchannels exhibit periodic director field transformations due to induced circulating flows inside them. These modifications are characterized by changes in the type of point surface disclinations; they periodically change from splay to bend disclinations, implying the drop changes between bipolar and escaped concentric configurations. Upon stopping the flow, this structure does not relax to the lower energy bipolar configuration; we argue this is due to drop flattening inside the channels.}, url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.98.087801}, author = {Fernandez-Nieves, A. and Link, D.R. and Marquez, M. and Weitz, D. A.} } @article {154181, title = {Controllable monodisperse multiple emulsions}, journal = {Angewandte Chemie-International Edition}, volume = {46}, number = {47}, year = {2007}, pages = {8970-8974}, abstract = { A drop within a drop within a drop: A microfluidic technique is used to generate highly controlled multiple emulsions (see picture). The high degree of control and scalability afforded by this method makes it a flexible and promising route for engineering designer emulsions and microcapsules with multiphase structures. Moreover, its generality will enable fabrication of novel materials containing complex internal structures. }, url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/anie.200701358}, author = {Chu, Liang-Yin and Utada, Andrew S. and Shah, Rhutesh K. and Kim, Jin-Woong and Weitz, David A} } @article {154176, title = {Monodisperse thermoresponsive microgels with tunable volume-phase transition kinetics}, journal = {Advanced Functional Materials}, volume = {17}, number = {17}, year = {2007}, pages = {3499-3504}, abstract = {A facile method to control the volume-phase transition kinetics of thermo-sensitive poly(N-isopropylacrylamide) (PNIPAM) microgels is presented. Monodisperse PNIPAM microgels with spherical voids are prepared using a microfluidic device. The swelling and shrinking responses of these microgels with spherical voids to changes in temperature are compared with those of voidless microgels of the same size and chemical composition prepared using the same microfluidic device. It is shown that the PNIPAM microgels with voids respond faster to changes in temperature as compared with their voidless counterparts. Also, the induced void structure does not have a detrimental effect on the equilibrium volume change of the microgels. Thus, the volume phase transition kinetics of the microgels can be finely tuned by controlling the number and size of the voids. The flexibility, control, and simplicity in fabrication rendered by this approach make these microgels appealing for applications that range from drug delivery systems and chemical separations to chemical/biosensing and actuators.}, url = {https://onlinelibrary.wiley.com/doi/abs/10.1002/adfm.200700379}, author = {Chu, Liang-Yin and Kim, Jin-Woong and Shah, Rhutesh K. and Weitz, David A} } @article {154076, title = {Force fluctuations and polymerization dynamics of intracellular microtubules}, journal = {Proceedings of the National Academy of Sciences of the United States of America}, volume = {104}, number = {41}, year = {2007}, pages = {16128-16133}, abstract = {Microtubules are highly dynamic biopolymer filaments involved in a wide variety of biological processes including cell division, migration, and intracellular transport. Microtubules are very rigid and form a stiff structural scaffold that resists deformation. However, despite their rigidity, inside of cells they typically exhibit significant bends on all length scales. Here, we investigate the origin of these bends using a Fourier analysis approach to quantify their length and time dependence. We show that, in cultured animal cells, bending is suppressed by the surrounding elastic cytoskeleton, and even large intracellular forces only cause significant bending fluctuations on short length scales. However, these lateral bending fluctuations also naturally cause fluctuations in the orientation of the microtubule tip. During growth, these tip fluctuations lead to microtubule bends that are frozen-in by the surrounding elastic network. This results in a persistent random walk of the microtubule, with a small apparent persistence length of approximate to 30 mu m, approximate to 100 times smaller than that resulting from thermal fluctuations alone. Thus, large nonthermal forces govern the growth of microtubules and can explain the highly curved shapes observed in the microtubule cytoskeleton of living cells.}, url = {https://www.pnas.org/content/104/41/16128}, author = {Brangwynne, Clifford P and MacKintosh, F. C. and Weitz, David A} } @article {154101, title = {Rheology and microrheology of a microstructured fluid: The gellan gum case}, journal = {Journal of Rheology}, volume = {51}, number = {5}, year = {2007}, pages = {851-865}, abstract = {Particle tracking microrheology is used to study the effect of a constant applied shear during gelation of aqueous gellan gum with a monovalent salt. Shear modifies the gellan gum hydrogel microstructure and the bulk rheological properties of the system, depending on whether shear is applied during gelation or afterwards. The microstructure determines the linear elastic response of the gel, as well as the critical strain and stress above which the response becomes nonlinear. Bulk oscillatory rheology is used to study microstructured gellan gum hydrogels at different polymer and salt concentrations. The similarity between our system and concentrated microgel particle suspensions can be explained by considering the microstructured gellan system to be composed of microgel particles whose size is set by the applied shear stress magnitude during gelation. Polymer concentration and ionic strength control the individual microgel particles{\textquoteright} elastic properties. We also find the gellan system exhibits an isoenergetic transition from the jammed to un-jammed state when sheared, similar to jammed colloidal systems [C. G. Robertson and X. R. Wang], "Isoenergetic jamming transition in particle-filled systems{\textquoteright}{\textquoteright}.}, url = {https://sor.scitation.org/doi/10.1122/1.2751385}, author = {Caggioni, M. and Spicer, P. T. and Blair, D. L. and Lindberg, S. E. and Weitz, D. A.} } @article {154051, title = {Bending dynamics of fluctuating biopolymers probed by automated high-resolution filament tracking}, journal = {Biophysical Journal}, volume = {93}, number = {1}, year = {2007}, pages = {346-359}, abstract = {Microscope images of. uctuating biopolymers contain a wealth of information about their underlying mechanics and dynamics. However, successful extraction of this information requires precise localization of. lament position and shape from thousands of noisy images. Here, we present careful measurements of the bending dynamics of filamentous ( F-) actin and microtubules at thermal equilibrium with high spatial and temporal resolution using a new, simple but robust, automated image analysis algorithm with subpixel accuracy. We. nd that slender actin. laments have a persistence length of similar to 17 mu m, and display a q(-4) -dependent relaxation spectrum, as expected from viscous drag. Microtubules have a persistence length of several millimeters; interestingly, there is a small correlation between total microtubule length and rigidity, with shorter filaments appearing softer. However, we show that this correlation can arise, in principle, from intrinsic measurement noise that must be carefully considered. The dynamic behavior of the bending of microtubules also appears more complex than that of F-actin, reflecting their higher-order structure. These results emphasize both the power and limitations of light microscopy techniques for studying the mechanics and dynamics of biopolymers.}, url = {https://www.sciencedirect.com/science/article/pii/S0006349507712870}, author = {Brangwynne, Clifford P and Koenderink, Gijsje H and Barry, Ed and Dogic, Zvonimir and MacKintosh, Frederick C and Weitz, David A} } @article {154001, title = {Spinodal decomposition in a model colloid-polymer mixture in microgravity}, journal = {Physical Review Letters}, volume = {99}, number = {20}, year = {2007}, pages = {205701}, abstract = {We study phase separation in a deeply quenched colloid-polymer mixture in microgravity on the International Space Station using small-angle light scattering and direct imaging. We observe a clear crossover from early-stage spinodal decomposition to late-stage, interfacial-tension-driven coarsening. Data acquired over 5 orders of magnitude in time show more than 3 orders of magnitude increase in domain size, following nearly the same evolution as that in binary liquid mixtures. The late-stage growth approaches the expected linear growth rate quite slowly.}, url = {https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.99.205701}, author = {Bailey, A. E. and W.C.K. Poon and Christianson, R. J. and Schofield, A. B. and Gasser, U. and Prasad, V. and S. Manley and Segre, P. N. and Cipelletti, L. and Meyer, W. V. and Doherty, M. P. and Sankaran, S. and Jankovsky, A. L. and Shiley, W. L. and Bowen, J. P. and Eggers, J. C. and Kurta, C. and Lorik, T., Jr. and Pusey, P. N. and Weitz, D. A.} } @article {153931, title = {Optical manipulation and rotation of liquid crystal drops using high-index fiber-optic tweezers}, journal = {Applied Physics Letters}, volume = {91}, number = {9}, year = {2007}, pages = {091119}, abstract = {We report an optical fiber tweezer based on high-index material for trapping and optical manipulation of microscale particles in water. The use of a high-index material increases the trapping force with respect to the more common silica, through tighter focusing of light. We demonstrate the potential of this simple and versatile device by trapping and rotating nematic liquid crystal drops. We monitor the rotation of the drop by detecting light modulation observed with the same fiber using backscattered light, which exhibits modulation in intensity due to the rotation of the drop; this further extends the capabilities of the fiber tweezers. (c) 2007 American Institute of Physics.}, url = {https://aip.scitation.org/doi/10.1063/1.2775321}, author = {Abedin, Kazi Sarwar and Kerbage, Charles and Fernandez-Nieves, Alberto and Weitz, David A} }