CiteULike: dchen's weitz http://www.citeulike.org/user/dchen/tag/weitz CiteULike.org en-gb Copyright © 2004-2008 citeulike.org http://www.citeulike.org/user/dchen/article/2767406 <i>Physical Review Letters, Vol. 100, No. 11. (2008)</i><br /><br />Biological activity gives rise to nonequilibrium fluctuations in the cytoplasm of cells; however, there are few methods to directly measure these fluctuations. Using a reconstituted actin cytoskeleton, we show that the bending dynamics of embedded microtubules can be used to probe local stress fluctuations. We add myosin motors that drive the network out of equilibrium, resulting in an increased amplitude and modified time dependence of microtubule bending fluctuations. We show that this behavior results from steplike forces on the order of 10 pN driven by collective motor dynamics. Nonequilibrium Microtubule Fluctuations in a Model Cytoskeleton Clifford Brangwynne Gijsje Koenderink Frederick Mackintosh David Weitz doi:10.1103/PhysRevLett.100.118104 Physical Review Letters, Vol. 100, No. 11. (2008) 2008-05-07T21:19:39-00:00 Physical Review Letters 100 11 APS 2008 biology weitz http://www.citeulike.org/user/dchen/article/2746695 <i>(2006)</i> Dynamics of dislocations in thin colloidal crystals Cohen Schall Weitz (2006) 2008-05-02T17:54:23-00:00 cohen colloid defect schall weitz http://www.citeulike.org/user/dchen/article/2746680 <i>(2004)</i> New developments in colloid science DA Weitz (2004) 2008-05-02T17:46:28-00:00 colloids review weitz http://www.citeulike.org/user/dchen/article/2735614 <i>Journal of Physics: Condensed Matter, Vol. 14, No. 33. (2002), pp. 7581-7597.</i><br /><br />We present novel measurements of the structure of colloidal gels. Using confocal microscopy, we obtain the precise three-dimensional positions of a large number of particles. We develop quantitative descriptions of the topology of the gel, including the number of bonds per particle, the chemical or bond fractal dimension, the number of flexible pivot points and other topological parameters that describe the chainlike structure. We investigate the dependence of these parameters on the particle volume fraction and the strength of the attraction that holds the particles together. While all samples have approximately the same fractal and chemical dimensions, we find that gels formed with stronger attraction or larger volume fraction have fewer bonds per particle, more filamentous chains and a greater number of flexible pivot points. Finally, we discuss the topological results in the context of the gel's elasticity. Measurements of the elastic constants of individual chainlike segments are explained with a simple model. The distribution of elastic constants, however, has a general form that is not understood. Direct imaging of three-dimensional structure and topology of colloidal gels AD Dinsmore DA Weitz doi:10.1088/0953-8984/14/33/303 Journal of Physics: Condensed Matter, Vol. 14, No. 33. (2002), pp. 7581-7597. 2008-04-29T22:57:53-00:00 Journal of Physics: Condensed Matter 14 33 7581 7597 colloids dinsmore gel structure weitz http://www.citeulike.org/user/dchen/article/822252 <i>Physical Review E, Vol. 65, No. 5. (20 May 2002), 051505.</i><br /><br />Experiments investigating the local viscoelastic properties of a simple uncross-linked flexible polymer are performed on polyethylene oxide solutions in the semidilute regime using polystyrene beads of varying sizes and surface chemistry as probes. We measure the thermal motions of the beads to obtain the elastic and viscous moduli of our sample. Two different dynamic light scattering techniques; diffusing wave spectroscopy and quasielastic light scattering (QELS); are used to determine the dynamics of the probe particles. Diffusing wave spectroscopy probes the short time dynamics of the scatterers while QELS or single scattering measures the dynamics at larger times. This results in a larger frequency overlap of the data obtained from the microrheological techniques with the data obtained from the conventional bulk measurements. The moduli are estimated using a modified algebraic form of the generalized Stokes-Einstein equation. Comparison of microrheology with bulk measurements shows excellent similarity confirming the applicability of this method for simple; uncross-linked polymeric systems. Microrheology of polyethylene oxide using diffusing wave spectroscopy and single scattering Bivash Dasgupta Shang-You Tee John Crocker BJ Frisken DA Weitz doi:10.1103/PhysRevE.65.051505 Physical Review E, Vol. 65, No. 5. (20 May 2002), 051505. 2006-08-30T16:29:17-00:00 Physical Review E 65 5 051505 American Physical Society dws microrheology weitz http://www.citeulike.org/user/dchen/article/2730434 <i>Physical Review Letters, Vol. 69, No. 16. (19 October 1992), 2439.</i> Stability criteria for emulsions J Bibette DC Morse TA Witten DA Weitz doi:10.1103/PhysRevLett.69.2439 Physical Review Letters, Vol. 69, No. 16. (19 October 1992), 2439. 2008-04-28T16:23:12-00:00 Physical Review Letters 69 16 2439 American Physical Society emulsion mason weitz http://www.citeulike.org/user/dchen/article/2730409 <i>Journal of Colloid and Interface Science, Vol. 179, No. 2. (10 May 1996), pp. 439-448.</i><br /><br />We have measured the yield transition of monodisperse emulsions as the volume fraction, [phi], and droplet radius,a, are varied. We study the crossover from the perturbative shear regime, which reflects the linear viscoelastic properties, to the steady shear regime, which reflects nonlinear, plastic flow. For small oscillatory strains of peak amplitude [gamma], the peak stress, [tau], is linearly proportional to [gamma]. As the strain is increased, the stress becomes nonlinear in [gamma] at the yield strain, [gamma]y. The [phi] dependence of [gamma]yis independent ofaand exhibits a minimum near the critical volume fraction, [phi]c[approximate] 0.635, associated with the random close packing of monodisperse spheres. We show that the yield stress, [tau]y, increases dramatically as the volume fraction increases above [phi]c; [tau]yalso scales with the Laplace pressure, [sigma]/a, where [sigma] is the interfacial tension. For comparison, we also determine the steady shear stress over a wide range of strain rates, [gamma]. Below [phi] [approximate] 0.70, the flow is homogeneous throughout the sample, while for higher [phi], the emulsion fractures resulting in highly inhomogeneous flow along the fracture plane. Above [phi] [approximate] 0.58, the steady shear stress exhibits a low strain rate plateau which corresponds with the yield stress measured with the oscillatory technique. Moreover, [tau]yexhibits a robust power law dependence on [gamma] with exponents decreasing with [phi], varying from to . Below [phi] [approximate] 0.58, associated with the colloidal glass transition, the plateau stress disappears entirely, suggesting that the equilibrium glassy dynamics are important in identifying the onset of the yield behavior. Yielding and Flow of Monodisperse Emulsions TG Mason J Bibette DA Weitz doi:10.1006/jcis.1996.0235 Journal of Colloid and Interface Science, Vol. 179, No. 2. (10 May 1996), pp. 439-448. 2008-04-28T16:14:56-00:00 Journal of Colloid and Interface Science 179 2 439 448 emulsion mason weitz http://www.citeulike.org/user/dchen/article/2722885 <i>Physical Review E, Vol. 52, No. 6. (1 December 1995), 6289.</i> Thermal fluctuations of the shapes of droplets in dense and compressed emulsions Hu Gang AH Krall DA Weitz doi:10.1103/PhysRevE.52.6289 Physical Review E, Vol. 52, No. 6. (1 December 1995), 6289. 2008-04-27T00:26:02-00:00 Physical Review E 52 6 6289 American Physical Society emulsion fluctuation temperature weitz http://www.citeulike.org/user/dchen/article/2722867 <i>Physical Review Letters, Vol. 76, No. 18. (29 April 1996), 3448.</i> Model for the Elasticity of Compressed Emulsions Martin-D Lacasse Gary Grest Dov Levine TG Mason DA Weitz doi:10.1103/PhysRevLett.76.3448 Physical Review Letters, Vol. 76, No. 18. (29 April 1996), 3448. 2008-04-27T00:13:18-00:00 Physical Review Letters 76 18 3448 American Physical Society elasticity emulsion weitz http://www.citeulike.org/user/dchen/article/1706975 <i>Physical Review E, Vol. 59, No. 1. (January 1999), 715.</i><br /><br />A liquid-glass transition was observed experimentally in a new system; an oil-in-water emulsion. Dynamic light scattering was employed to obtain the intermediate scattering function f(q;t) for a range of volume fractions φ and scattering vectors q . The results are compared with predictions of the mode coupling theory. While the usual idealized version of the theory provides accurate fits to the data on the liquid side of the transition; fits for volume fractions near the transition and in the glass phase were found to require the extended version; presumably due to an additional decay mechanism related to the deformability of the oil droplets. Emulsion glasses: A dynamic light-scattering study Hu Gang AH Krall HZ Cummins DA Weitz doi:10.1103/PhysRevE.59.715 Physical Review E, Vol. 59, No. 1. (January 1999), 715. 2007-09-29T02:17:32-00:00 Physical Review E 59 1 715 American Physical Society emulsion weitz http://www.citeulike.org/user/dchen/article/2722836 <i>Physical Review Letters, Vol. 75, No. 10. (1995), 2051.</i> Elasticity of Compressed Emulsions TG Mason J Bibette DA Weitz doi:10.1103/PhysRevLett.75.2051 Physical Review Letters, Vol. 75, No. 10. (1995), 2051. 2008-04-26T23:57:13-00:00 Physical Review Letters 75 10 2051 American Physical Society elasticity emulsion mason weitz http://www.citeulike.org/user/dchen/article/2714504 <i>Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 213, No. 2-3. (12 February 2003), pp. 189-197.</i><br /><br />Suspensions of clay particles (laponite), mixed with poly(ethylene oxide) (PEO) undergo a dramatic shear thickening when subjected to vigorous shaking, which transforms them from a low viscosity fluid into a [`]shake-gel', a solid with elasticity sufficient enough to support its own weight. The shake-gel is reversible, relaxing back to a fluid with a relaxation time that is strongly dependent on PEO concentration. Shake-gels are observed for PEO concentrations slightly below the threshold for complete saturation of the laponite particles by the polymer. Light scattering measurements confirm that the PEO is adsorbed on the surface of the laponite particles, and suggests that shear induces a bridging between the colloidal particles, resulting in a gel network which spans the system. Desorption of the polymer reduces the bridging and thus relaxes the network. Shake-gels: shear-induced gelation of laponite-PEO mixtures J Zebrowski V Prasad W Zhang LM Walker DA Weitz doi:10.1016/S0927-7757(02)00512-5 Colloids and Surfaces A: Physicochemical and Engineering Aspects, Vol. 213, No. 2-3. (12 February 2003), pp. 189-197. 2008-04-24T22:24:38-00:00 Colloids and Surfaces A: Physicochemical and Engineering Aspects 213 2-3 189 197 gel shear weitz http://www.citeulike.org/user/dchen/article/2714497 <i>Science, Vol. 299, No. 5613. (14 March 2003), pp. 1716-1718.</i><br /><br />10.1126/science.1081160 Grain Boundary Scars and Spherical Crystallography AR Bausch MJ Bowick A Cacciuto AD Dinsmore MF Hsu DR Nelson MG Nikolaides A Travesset DA Weitz doi:10.1126/science.1081160 Science, Vol. 299, No. 5613. (14 March 2003), pp. 1716-1718. 2008-04-24T22:18:23-00:00 Science 299 5613 1716 1718 2d crystal surface weitz http://www.citeulike.org/user/dchen/article/2714466 <i>Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol. 361, No. 1805. (15 April 2003), pp. 753-766.</i><br /><br />We discuss the behaviour of the dynamics of colloidal particles with a weak attractive interparticle interaction that is induced through the addition of polymer to the solvent. We briefly review the description of their behaviour in terms of the jamming phase diagram, which parametrized the fluid-to-solid transition due to changes in volume fraction, attractive energy or applied stress. We focus on a discussion of ageing of the solid gels formed by these colloid-polymer mixtures. They exhibit a delayed collapse induced by gravity. The time evolution of the height of the sediment exhibits an unexpected scaling behaviour, suggesting a universal nature to this delayed collapse. We complement these measurements of the scaling of the collapse with microscopic investigations of the evolution of the structure of the network using confocal microscopy. These results provide new insight into the origin of this ageing behaviour. Dynamics of weakly aggregated colloidal particles Maria Kilfoil Eugene Pashkovski James Masters DA Weitz doi:10.1098/rsta.2002.1163 Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences, Vol. 361, No. 1805. (15 April 2003), pp. 753-766. 2008-04-24T22:01:56-00:00 Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences 361 1805 753 766 aging gel gravity weitz http://www.citeulike.org/user/dchen/article/2714211 <i>Applied Physics Letters, Vol. 82, No. 16. (2003), pp. 2610-2612.</i><br /><br />View This Record in Scopus Phase switching of ordered arrays of liquid crystal emulsions D Rudhardt Fernández Nieves DR Link DA Weitz Applied Physics Letters, Vol. 82, No. 16. (2003), pp. 2610-2612. 2008-04-24T19:51:28-00:00 Applied Physics Letters 82 16 2610 2612 AIP emulsion liquidcrystal phase weitz http://www.citeulike.org/user/dchen/article/2714010 <i>Nature, Vol. 420, No. 6913. (21 November 2002), pp. 299-301.</i> Electric-field-induced capillary attraction between like-charged particles at liquid interfaces MG Nikolaides AR Bausch MF Hsu AD Dinsmore MP Brenner C Gay DA Weitz doi:10.1038/nature01113 Nature, Vol. 420, No. 6913. (21 November 2002), pp. 299-301. 2008-04-24T19:25:39-00:00 Nature 420 6913 299 301 colloids interaction interface nature weitz http://www.citeulike.org/user/dchen/article/2713987 <i>Nature, Vol. 424, No. 6952. (2003), pp. 1014-1014.</i> Capillary attraction (communication arising): Like-charged particles at liquid interfaces MG Nikolaides AR Bausch MF Hsu AD Dinsmore MP Brenner C Gay DA Weitz doi:10.1038/4241014b Nature, Vol. 424, No. 6952. (2003), pp. 1014-1014. 2008-04-24T19:16:11-00:00 Nature 424 6952 1014 1014 interaction interface microrheology nature weitz http://www.citeulike.org/user/dchen/article/2713983 <i>Proceedings of the National Academy of Sciences, Vol. 100, No. 19. (16 September 2003), pp. 10718-10721.</i><br /><br />Vesicles are bilayers of lipid molecules enclosing a fixed volume of aqueous solution. Ubiquitous in cells, they can be produced in vitro to study the physical properties of biological membranes and for use in drug delivery and cosmetics. Biological membranes are, in fact, a fluid mosaic of lipids and other molecules; the richness of their chemical and mechanical properties in vivo is often dictated by an asymmetric distribution of these molecules. Techniques for vesicle preparation have been based on the spontaneous assembly of lipid bilayers, precluding the formation of such asymmetric structures. Partial asymmetry has been achieved only with chemical methods greatly restricting the study of the physical and chemical properties of asymmetric vesicles and their use in potential applications for drug delivery. Here we describe the systematic engineering of unilamellar vesicles assembled with two independently prepared monolayers; this process produces asymmetries as high as 95%. We demonstrate the versatility of our method by investigating the stability of the asymmetry. We also use it to engineer hybrid structures comprised of an inner leaflet of diblock copolymer and an independent lipid outer leaflet. 10.1073/pnas.1931005100 Engineering asymmetric vesicles Sophie Pautot Barbara Frisken DA Weitz doi:10.1073/pnas.1931005100 Proceedings of the National Academy of Sciences, Vol. 100, No. 19. (16 September 2003), pp. 10718-10721. 2008-04-24T19:14:13-00:00 Proceedings of the National Academy of Sciences 100 19 10718 10721 emulsion technique weitz http://www.citeulike.org/user/dchen/article/2713958 <i>Langmuir, Vol. 19, No. 24. (25 November 2003), pp. 10281-10287.</i><br /><br />Abstract: We report the spontaneous formation of emulsion droplets and multilamellar concentric onions when a water drop is immersed into dodecane containing phospholipids. We show that the origin of the spontaneous emulsification is the formation of a semicrystalline multilamellar film at the dodecane-water interface, which swells with water, shedding the emulsion and onion droplets. We use coherent anti-Stokes Raman scattering microscopy to determine that the shell of the onion structures is composed of partially hydrated concentric bilayers, and the core is composed of lipids, water, and dodecane. Spontaneous Formation of Lipid Structures at Oil/Water/Lipid Interfaces S Pautot BJ Frisken JX Cheng XS Xie DA Weitz doi:10.1021/la034532f Langmuir, Vol. 19, No. 24. (25 November 2003), pp. 10281-10287. 2008-04-24T19:05:39-00:00 Langmuir 19 24 10281 10287 interface weitz http://www.citeulike.org/user/dchen/article/2713887 <i>Physical Review Letters, Vol. 91, No. 22. (24 November 2003), 224501.</i> Flow and Fracture in Drying Nanoparticle Suspensions ER Dufresne EI Corwin NA Greenblatt J Ashmore DY Wang AD Dinsmore JX Cheng XS Xie JW Hutchinson DA Weitz doi:10.1103/PhysRevLett.91.224501 Physical Review Letters, Vol. 91, No. 22. (24 November 2003), 224501. 2008-04-24T18:38:32-00:00 Physical Review Letters 91 22 224501 American Physical Society dinsmore drying weitz http://www.citeulike.org/user/dchen/article/2713881 <i>Faraday Discuss., 2003</i><br /><br />Attractive colloidal particles can exhibit a fluid to solid phase transition if the magnitude of the attractive interaction is sufficiently large, if the volume fraction is sufficiently high, and if the applied stress is sufficiently small. The nature of this fluid to solid transition is similar for many different colloid systems, and for many different forms of interaction. The jamming phase transition captures the common features of these fluid to solid translations, by unifying the behavior as a function of the particle volume fraction, the energy of interparticle attractions, and the applied stress. This paper describes the applicability of the jamming state diagram, and highlights those regions where the fluid to solid transition is still poorly understood. It also presents new data for gelation of colloidal particles with an attractive depletion interaction, providing more insight into the origin of the fluid to solid transition. Universal features of the fluid to solid transition for attractive colloidal particles Prasad Faraday Discuss., 2003 2008-04-24T18:37:13-00:00 Faraday Discuss., 2003 attractive colloids phase weitz http://www.citeulike.org/user/dchen/article/2713862 <i>Faraday Discuss., 2003</i><br /><br />We use conventional and multispeckle dynamic light scattering to investigate the dynamics of a wide variety of jammed soft materials, including colloidal gels, concentrated emulsions, and concentrated surfactant phases. For all systems, the dynamic structure factor f(q,t) exhibits a two-step decay. The initial decay is due to the thermally activated diffusive motion of the scatterers, as indicated by the q–2 dependence of the characteristic relaxation time, where q is the scattering vector. However, due to the constrained motion of the scatterers in jammed systems, the dynamics are arrested and the initial decay terminates in a plateau. Surprisingly, we find that a final, ultraslow decay leads to the complete relaxation of f(q,t), indicative of rearrangements on length scales as large as several microns or tens of microns. Remarkably, for all systems the same very peculiar form is found for the final relaxation of the dynamic structure factor: f(q,t)exp[–(t/s)p], with p1.5 and sq–1, thus suggesting the generality of this behavior. Additionally, for all samples the final relaxation slows down with age, although the aging behavior is found to be sample dependent. We propose that the unusual ultraslow dynamics are due to the relaxation of internal stresses, built into the sample at the jamming transition, and present simple scaling arguments that support this hypothesis. Universal non-diffusive slow dynamics in aging soft matter Luca Faraday Discuss., 2003 2008-04-24T18:34:09-00:00 Faraday Discuss., 2003 aging weitz http://www.citeulike.org/user/dchen/article/2713647 <i>Science, Vol. 303, No. 5660. (13 February 2004), pp. 968-969.</i><br /><br />10.1126/science.1094581 PHYSICS: Packing in the Spheres David Weitz doi:10.1126/science.1094581 Science, Vol. 303, No. 5660. (13 February 2004), pp. 968-969. 2008-04-24T18:10:33-00:00 Science 303 5660 968 969 review science weitz http://www.citeulike.org/user/dchen/article/2713643 <i>Journal of Fluid Mechanics, Vol. 501, No. -1. (2004), pp. 71-104.</i> A model for velocity fluctuations in sedimentation Peter Mucha Shang-You Tee David Weitz Boris Shraiman Michael Brenner Journal of Fluid Mechanics, Vol. 501, No. -1. (2004), pp. 71-104. 2008-04-24T18:08:14-00:00 Journal of Fluid Mechanics 501 -1 71 104 flow gravity weitz http://www.citeulike.org/user/dchen/article/2713489 <i>Physical Review Letters, Vol. 93, No. 4. (23 July 2004), 046001.</i> Shear-Induced Configurations of Confined Colloidal Suspensions Itai Cohen Thomas Mason David Weitz doi:10.1103/PhysRevLett.93.046001 Physical Review Letters, Vol. 93, No. 4. (23 July 2004), 046001. 2008-04-24T16:40:15-00:00 Physical Review Letters 93 4 046001 American Physical Society cohen shear weitz http://www.citeulike.org/user/dchen/article/2710365 <i>Science, Vol. 305, No. 5692. (24 September 2004), pp. 1944-1948.</i><br /><br />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. 10.1126/science.1102186 Visualization of Dislocation Dynamics in Colloidal Crystals Peter Schall Itai Cohen David Weitz Frans Spaepen doi:10.1126/science.1102186 Science, Vol. 305, No. 5692. (24 September 2004), pp. 1944-1948. 2008-04-23T22:01:50-00:00 Science 305 5692 1944 1948 cohen schall science shear weitz http://www.citeulike.org/user/dchen/article/1429816 <i>Trends Biochem Sci, Vol. 29, No. 7. (July 2004), pp. 364-370.</i> Dealing with mechanics: mechanisms of force transduction in cells. PA Janmey DA Weitz doi:10.1016/j.tibs.2004.05.003 Trends Biochem Sci, Vol. 29, No. 7. (July 2004), pp. 364-370. 2007-07-03T03:53:26-00:00 Trends Biochem Sci 0968-0004 29 7 364 370 biology review weitz http://www.citeulike.org/user/dchen/article/2711079 <i>Physics of Fluids, Vol. 16, No. 8. (2004), pp. 2828-2834.</i><br /><br />View This Record in Scopus A new device for the generation of microbubbles José Gordillo Zhengdong Cheng Alfonso M Márquez DA Weitz Physics of Fluids, Vol. 16, No. 8. (2004), pp. 2828-2834. 2008-04-24T00:28:26-00:00 Physics of Fluids 16 8 2828 2834 AIP bubbles microrheology weitz http://www.citeulike.org/user/dchen/article/2711055 <i>Physical Review Letters, Vol. 93, No. 10. (2004), 108302.</i> Limits to Gelation in Colloidal Aggregation S Manley L Cipelletti V Trappe AE Bailey RJ Christianson U Gasser V Prasad PN Segre MP Doherty S Sankaran AL Jankovsky B Shiley J Bowen J Eggers C Kurta T Lorik DA Weitz doi:10.1103/PhysRevLett.93.108302 Physical Review Letters, Vol. 93, No. 10. (2004), 108302. 2008-04-23T23:55:21-00:00 Physical Review Letters 93 10 108302 American Physical Society clusters colloids gel weitz http://www.citeulike.org/user/dchen/article/2710303 <i>J. Am. Chem. Soc., Vol. 126, No. 43. (3 November 2004), pp. 14117-14122.</i><br /><br />Abstract: 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. Self-Assembled Polymer Membrane Capsules Inflated by Osmotic Pressure VD Gordon X Chen JW Hutchinson AR Bausch M Marquez DA Weitz doi:10.1021/ja0474749 J. Am. Chem. Soc., Vol. 126, No. 43. (3 November 2004), pp. 14117-14122. 2008-04-23T21:53:12-00:00 J. Am. Chem. Soc. 126 43 14117 14122 emulsion technique weitz http://www.citeulike.org/user/dchen/article/2710298 <i>Physical Review Letters, Vol. 94, No. 1. (2005)</i><br /><br />Minute concentrations of suspended particles can dramatically alter the behavior of a drying droplet. After a period of isotropic shrinkage, similar to droplets of a pure liquid, these droplets suddenly buckle like an elastic shell. While linear elasticity is able to describe the morphology of the buckled droplets, it fails to predict the onset of buckling. Instead, we find that buckling is coincident with a stress-induced fluid to solid transition in a shell of particles at a droplet's surface, occurring when attractive capillary forces overcome stabilizing electrostatic forces between particles. Onset of Buckling in Drying Droplets of Colloidal Suspensions N Tsapis ER Dufresne SS Sinha CS Riera JW Hutchinson L Mahadevan DA Weitz doi:10.1103/PhysRevLett.94.018302 Physical Review Letters, Vol. 94, No. 1. (2005) 2008-04-23T21:45:07-00:00 Physical Review Letters 94 1 APS colloids drying weitz http://www.citeulike.org/user/dchen/article/2710294 <i>Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 71, No. 2. (2005)</i><br /><br />Experiments investigating the local viscoelastic properties of a chemically cross-linked polymer are performed on polyacrylamide solutions in the sol and the gel regimes using polystyrene beads of varying sizes and surface chemistry as probes. The thermal motions of the probes are measured to obtain the elastic and viscous moduli of the sample. Probe dynamics are measured using two different dynamic light scattering techniques, diffusing wave spectroscopy (DWS) and quasielastic light scattering (QELS) as well as video-based particle tracking. Diffusing wave spectroscopy probes the short-time dynamics of the scatterers while QELS measures the dynamics at larger times. Video-based particle tracking provides a way to investigate the local environment of the individual probe particles. A combination of all the techniques results in a larger range of frequencies that can be probed compared to conventional bulk measurements while providing local information at the level of individual probes. A modified algebraic form of the generalized Stokes-Einstein equation is used to calculate the frequency-dependent moduli. A comparison of microrheological measurements with bulk rheology exhibits striking similarity, confirming the applicability of microrheology for chemically cross-linked polymeric systems. Microrheology of cross-linked polyacrylamide networks Bivash Dasgupta DA Weitz doi:10.1103/PhysRevE.71.021504 Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 71, No. 2. (2005) 2008-04-23T21:44:04-00:00 Physical Review E (Statistical, Nonlinear, and Soft Matter Physics) 71 2 APS microrheology polymer weitz http://www.citeulike.org/user/dchen/article/2710279 <i>Advanced Materials, Vol. 17, No. 6. (2005), pp. 680-684.</i><br /><br />No abstract. Optically Anisotropic Colloids of Controllable Shape A Fernández-Nieves G Cristobal V Garcés-Chávez G C Spalding K Dholakia D A Weitz doi:10.1002/adma.200401462 Advanced Materials, Vol. 17, No. 6. (2005), pp. 680-684. 2008-04-23T21:34:08-00:00 Advanced Materials 17 6 680 684 colloids liquidcrystal optical technique weitz http://www.citeulike.org/user/dchen/article/2710276 <i>Langmuir, Vol. 21, No. 7. (29 March 2005), pp. 2963-2970.</i><br /><br />Abstract: We construct shells with tunable morphology and mechanical response with colloidal particles that self-assemble at the interface of emulsion droplets. Particles self-assemble to minimize the total interfacial energy, spontaneously forming a particle layer that encapsulates the droplets. We stabilize these layers to form solid shells at the droplet interface by aggregating the particles, connecting the particles with adsorbed polymer, or fusing the particles. These techniques reproducibly yield shells with controllable properties such as elastic moduli and breaking forces. To enable diffusive exchange through the particle shells, we transfer them into solvents that are miscible with the encapsulant. We characterize the mechanical properties of the shells by measuring the response to deformation by calibrated microcantilevers. Self-assembled Shells Composed of Colloidal Particles: Fabrication and Characterization MF Hsu MG Nikolaides AD Dinsmore AR Bausch VD Gordon X Chen JW Hutchinson DA Weitz M Marquez doi:10.1021/la0472394 Langmuir, Vol. 21, No. 7. (29 March 2005), pp. 2963-2970. 2008-04-23T21:31:14-00:00 Langmuir 21 7 2963 2970 colloids dinsmore emulsion technique weitz http://www.citeulike.org/user/dchen/article/2710273 <i>Science, Vol. 308, No. 5721. (22 April 2005), pp. 537-541.</i><br /><br />Double emulsions are highly structured fluids consisting of emulsion drops that contain smaller droplets inside. Although double emulsions are potentially of commercial value, traditional fabrication by means of two emulsification steps leads to very ill-controlled structuring. Using a microcapillary device, we fabricated double emulsions that contained a single internal droplet in a core-shell geometry. We show that the droplet size can be quantitatively predicted from the flow profiles of the fluids. The double emulsions were used to generate encapsulation structures by manipulating the properties of the fluid that makes up the shell. The high degree of control afforded by this method and the completely separate fluid streams make this a flexible and promising technique. 10.1126/science.1109164 Monodisperse Double Emulsions Generated from a Microcapillary Device AS Utada E Lorenceau DR Link PD Kaplan HA Stone DA Weitz doi:10.1126/science.1109164 Science, Vol. 308, No. 5721. (22 April 2005), pp. 537-541. 2008-04-23T21:28:11-00:00 Science 308 5721 537 541 emulsion microrheology weitz http://www.citeulike.org/user/dchen/article/2710262 <i>Langmuir, Vol. 21, No. 11. (24 May 2005), pp. 4881-4887.</i><br /><br />Abstract: While the important role of electrostatic interactions in aqueous colloidal suspensions is widely known and reasonably well-understood, their relevance to nonpolar suspensions remains mysterious. We measure the interaction potentials of colloidal particles in a nonpolar solvent with reverse micelles. We find surprisingly strong electrostatic interactions characterized by surface potentials, e, from 2.0 to 4.4 kBT and screening lengths, -1, from 0.2 to 1.4 m. Interactions depend on the concentration of reverse micelles and the degree of confinement. Furthermore, when the particles are weakly confined, the values of e and extracted from interaction measurements are consistent with bulk measurements of conductivity and electrophoretic mobility. A simple thermodynamic model, relating the structure of the micelles to the equilibrium ionic strength, is in good agreement with both conductivity and interaction measurements. Since dissociated ions are solubilized by reverse micelles, the entropic incentive to charge a particle surface is qualitatively changed from aqueous systems, and surface entropy plays an important role. Charge Stabilization in Nonpolar Solvents MF Hsu ER Dufresne DA Weitz doi:10.1021/la046751m Langmuir, Vol. 21, No. 11. (24 May 2005), pp. 4881-4887. 2008-04-23T21:21:25-00:00 Langmuir 21 11 4881 4887 micelle weitz http://www.citeulike.org/user/dchen/article/2710257 <i>Physical Review Letters, Vol. 94, No. 21. (2005)</i><br /><br />We present a unified framework for understanding the compaction of colloidal gels under their own weight. The dynamics of the collapse are determined by the value of the gravitational stress g, as compared to the yield stress Y of the network. For g<Y, gels collapse poroelastically, and their rate of compression decays exponentially in time. For g>Y, the network eventually yields, leading to rapid settling. In both cases, the rate of collapse is backflow limited, while its overall magnitude is determined by a balance between gravitational stress and network elastic stress. Gravitational Collapse of Colloidal Gels S Manley JM Skotheim L Mahadevan DA Weitz doi:10.1103/PhysRevLett.94.218302 Physical Review Letters, Vol. 94, No. 21. (2005) 2008-04-23T21:18:48-00:00 Physical Review Letters 94 21 APS colloids gel gravity weitz http://www.citeulike.org/user/dchen/article/2710243 <i>Physical Review Letters, Vol. 95, No. 4. (2005)</i><br /><br />Colloidal silica gels are shown to stiffen with time, as demonstrated by both dynamic light scattering and bulk rheological measurements. Their elastic moduli increase as a power law with time, independent of particle volume fraction; however, static light scattering indicates that there are no large-scale structural changes. We propose that increases in local elasticity arising from bonding between neighboring colloidal particles can account for the strengthening of the network, while preserving network structure. Time-Dependent Strength of Colloidal Gels S Manley Benny Davidovitch Neil Davies L Cipelletti AE Bailey RJ Christianson U Gasser V Prasad PN Segre MP Doherty S Sankaran AL Jankovsky B Shiley J Bowen J Eggers C Kurta T Lorik DA Weitz doi:10.1103/PhysRevLett.95.048302 Physical Review Letters, Vol. 95, No. 4. (2005) 2008-04-23T21:08:32-00:00 Physical Review Letters 95 4 APS colloids gel weitz http://www.citeulike.org/user/dchen/article/2264987 <i>Nat Mater, Vol. 4, No. 7. (July 2005), pp. 557-561.</i> Cytoskeletal remodelling and slow dynamics in the living cell Predrag Bursac Guillaume Lenormand Ben Fabry Madavi Oliver David Weitz Virgile Viasnoff James Butler Jeffrey Fredberg doi:10.1038/nmat1404 Nat Mater, Vol. 4, No. 7. (July 2005), pp. 557-561. 2008-01-21T00:42:06-00:00 Nat Mater 4 7 557 561 biology weitz http://www.citeulike.org/user/dchen/article/2710147 <i>Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 72, No. 4. (2005)</i><br /><br />Calcite crystals often nucleate and grow in solutions of calcium carbonate, and these crystallites can become trapped at the air water interface, where they form unusual structures. The most common is a fractal structure, which can extend over a large fraction of the interface, and whose origin is understood in terms of the aggregation of the particles. Much more rarely, a different and entirely unexpected structure is observed: the particles remain well separated on the interface, forming an ordered phase reminiscent of a two-dimensional colloidal crystal. The structure of the crystallites that form this ordered phase is always observed to be tetrahedral, in contrast to the much more common rhombohedral structure of the crystallites that form the fractal phase. We show that the interparticle interaction potential that leads to this ordered phase is a balance between a long-range attractive interaction and a long-range repulsive interaction. The attraction results from gravity-induced capillary forces, while the repulsion results from a dipole-dipole interaction due to the charged surface of the tetrahedral crystals. The interaction potential is estimated from the thermal motion of the particles, and fits to the theoretically expected values suggest that the effective surface charge on the tetrahedral crystals is ~0.01  charges/nm2. Tetrahedral calcite crystals facilitate self-assembly at the air-water interface SM Hashmi HH Wickman DA Weitz doi:10.1103/PhysRevE.72.041605 Physical Review E (Statistical, Nonlinear, and Soft Matter Physics), Vol. 72, No. 4. (2005) 2008-04-23T20:32:37-00:00 Physical Review E (Statistical, Nonlinear, and Soft Matter Physics) 72 4 APS 2d crystal structure weitz http://www.citeulike.org/user/dchen/article/2710107 <i>Applied Physics Letters, Vol. 88, No. 2. (2006)</i><br /><br />View This Record in Scopus Dielectrophoretic manipulation of drops for high-speed microfluidic sorting devices Keunho Ahn Charles Kerbage Tom Hunt RM Westervelt Darren Link DA Weitz Applied Physics Letters, Vol. 88, No. 2. (2006) 2008-04-23T20:17:36-00:00 Applied Physics Letters 88 2 AIP microrheology weitz http://www.citeulike.org/user/dchen/article/2709851 <i>Physical Review Letters, Vol. 96, No. 2. (2006)</i><br /><br />We show that colloidal particles with attractive interactions induced by a nonadsorbing polymer exhibit a stable phase consisting of a fluid of clusters of particles. This phase persists even in the absence of any long-range repulsion due to charge, contrary to expectations based on simulation and theory. Cluster morphology depends strongly on the range of the interparticle attraction: With a shorter range, clusters are tenuous and branched; with a longer range, they are more compact. Fluids of Clusters in Attractive Colloids Peter Lu Jacinta Conrad Hans Wyss Andrew Schofield David Weitz doi:10.1103/PhysRevLett.96.028306 Physical Review Letters, Vol. 96, No. 2. (2006) 2008-04-23T19:32:58-00:00 Physical Review Letters 96 2 APS cluster colloids weitz http://www.citeulike.org/user/dchen/article/2709840 <i>Physical Review Letters, Vol. 96, No. 8. (2006)</i><br /><br />Networks of filamentous actin cross-linked with the actin-binding protein filamin A exhibit remarkable strain stiffening leading to an increase in differential elastic modulus by several orders of magnitude over the linear value. The variation of the frequency dependence of the differential elastic and loss moduli as a function of prestress is consistent with that observed in living cells, suggesting that cell elasticity is always measured in the nonlinear regime, and that prestress is an essential control parameter. Stress-Dependent Elasticity of Composite Actin Networks as a Model for Cell Behavior ML Gardel F Nakamura J Hartwig JC Crocker TP Stossel DA Weitz doi:10.1103/PhysRevLett.96.088102 Physical Review Letters, Vol. 96, No. 8. (2006) 2008-04-23T19:30:02-00:00 Physical Review Letters 96 8 APS biology elasticity weitz http://www.citeulike.org/user/dchen/article/553502 <i>Nature, Vol. 440, No. 7082., pp. 319-323.</i> Visualizing dislocation nucleation by indenting colloidal crystals Peter Schall Itai Cohen David Weitz Frans Spaepen doi:10.1038/nature04557 Nature, Vol. 440, No. 7082., pp. 319-323. 2006-03-15T23:36:35-00:00 Nature 0028-0836 440 7082 319 323 Nature Publishing Group defect schall shear weitz http://www.citeulike.org/user/dchen/article/2709807 <i>(2006)</i><br /><br />We report the fabrication of three-dimensional lattices of bipolar nematic liquid crystal droplets. The electro-optic properties of these materials consist of transitions between opaque, Bragg diffracting, and transparent states. These occur continuously at moderate electric (E) fields through bipolar axis rotation of the nematic droplets. An E field applied normal to the hexagonally close packed planes results in a Bragg diffraction pattern that is polarization independent. Application of in-plane fields allows switching between diffracting and transmitting states that depends on the polarization of the incident light. ©2006 American Institute of Physics Polarization dependent Bragg diffraction and electro-optic switching of three-dimensional assemblies of nematic liquid crystal droplets Fernández-Nieves (2006) 2008-04-23T19:12:50-00:00 liquidcrystal weitz http://www.citeulike.org/user/dchen/article/2264824 <i>Physical Review Letters, Vol. 96, No. 11. (2006)</i><br /><br />We exploit the power of microrheology to measure the viscoelasticity of entangled F-actin solutions at different length scales from 1 to 100 µm over a wide frequency range. We compare the behavior of single probe-particle motion to that of the correlated motion of two particles. By varying the average length of the filaments, we identify fluctuations that dissipate diffusively over the filament length. These provide an important relaxation mechanism of the elasticity between 0.1 and 30 rad/sec. Microrheology Probes Length Scale Dependent Rheology J Liu ML Gardel K Kroy E Frey BD Hoffman JC Crocker AR Bausch DA Weitz doi:10.1103/PhysRevLett.96.118104 Physical Review Letters, Vol. 96, No. 11. (2006) 2008-01-20T23:36:22-00:00 Physical Review Letters 96 11 APS polymer semiflexiable weitz http://www.citeulike.org/user/dchen/article/2709789 <i>Langmuir, Vol. 22, No. 10. (9 May 2006), pp. 4457-4461.</i><br /><br />Abstract: We investigate the formation of polymer vesicles, or polymersomes, of polystyrene-block-poly(ethylene oxide) diblock copolymers using double emulsion droplets of controlled architecture as templates. To engineer the structure of the polymersomes, it is important to consider the concentration of diblock copolymer in the middle phase of the double emulsion. We describe how the presence of excess polymer can induce a transition from complete wetting to partial wetting of the middle phase, resulting in polymer shells with inhomogeneous thicknesses. Dewetting Instability during the Formation of Polymersomes from Block-Copolymer-Stabilized Double Emulsions RC Hayward AS Utada N Dan DA Weitz doi:10.1021/la060094b Langmuir, Vol. 22, No. 10. (9 May 2006), pp. 4457-4461. 2008-04-23T19:03:05-00:00 Langmuir 22 10 4457 4461 emulsion technique weitz http://www.citeulike.org/user/dchen/article/1676566 <i>Physical Review Letters, Vol. 96, No. 18. (2006)</i><br /><br />We directly probe the microscopic structure, connectivity, and elasticity of colloidal gels using confocal microscopy. We show that the gel is a random network of one-dimensional chains of particles. By measuring thermal fluctuations, we determine the effective spring constant between pairs of particles as a function of separation; this is in agreement with the theory for fractal chains. Long-range attractions between particles lead to freely rotating bonds, and the gel is stabilized by multiple connections among the chains. By contrast, short-range attractions lead to bonds that resist bending, with dramatically suppressed formation of loops of particles. Microscopic Structure and Elasticity of Weakly Aggregated Colloidal Gels AD Dinsmore V Prasad IY Wong DA Weitz doi:10.1103/PhysRevLett.96.185502 Physical Review Letters, Vol. 96, No. 18. (2006) 2007-09-19T16:41:52-00:00 Physical Review Letters 96 18 APS colloids dinsmore elasticity gel journalclub weitz http://www.citeulike.org/user/dchen/article/2709675 <i>Physical Review Letters, Vol. 96, No. 19. (2006)</i><br /><br />We have studied the effect of shear on the stability of suspensions made of non-Brownian solid particles. We demonstrate the existence of an irreversible transition where the solid particles aggregate at remarkably low volume fractions (0.1). This shear-induced aggregation is dramatic and exhibits a very sudden change in the viscosity, which increases sharply after a shear-dependent induction time. We show that this induction time is related exponentially to the shear rate, reflecting the importance of the hydrodynamic forces in reducing the repulsive energy barrier that prevents the particles from aggregating. Irreversible Shear-Activated Aggregation in Non-Brownian Suspensions J Guery E Bertrand C Rouzeau P Levitz DA Weitz J Bibette doi:10.1103/PhysRevLett.96.198301 Physical Review Letters, Vol. 96, No. 19. (2006) 2008-04-23T18:01:46-00:00 Physical Review Letters 96 19 APS hydrodynamics shear weitz http://www.citeulike.org/user/dchen/article/954119 <i>J. Cell Biol., Vol. 173, No. 5. (5 June 2006), pp. 733-741.</i><br /><br />Cytoskeletal microtubules have been proposed to influence cell shape and mechanics based on their ability to resist large-scale compressive forces exerted by the surrounding contractile cytoskeleton. Consistent with this, cytoplasmic microtubules are often highly curved and appear buckled because of compressive loads. However, the results of in vitro studies suggest that microtubules should buckle at much larger length scales, withstanding only exceedingly small compressive forces. This discrepancy calls into question the structural role of microtubules, and highlights our lack of quantitative knowledge of the magnitude of the forces they experience and can withstand in living cells. We show that intracellular microtubules do bear large-scale compressive loads from a variety of physiological forces, but their buckling wavelength is reduced significantly because of mechanical coupling to the surrounding elastic cytoskeleton. We quantitatively explain this behavior, and show that this coupling dramatically increases the compressive forces that microtubules can sustain, suggesting they can make a more significant structural contribution to the mechanical behavior of the cell than previously thought possible. 10.1083/jcb.200601060 Microtubules can bear enhanced compressive loads in living cells because of lateral reinforcement Clifford Brangwynne Frederick Mackintosh Sanjay Kumar Nicholas Geisse Jennifer Talbot L Mahadevan Kevin Parker Donald Ingber David Weitz doi:10.1083/jcb.200601060 J. Cell Biol., Vol. 173, No. 5. (5 June 2006), pp. 733-741. 2006-11-20T22:19:20-00:00 J. Cell Biol. 173 5 733 741 biology microrheology weitz