Publications

2004
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
2003
Zebrowski, J. ; Prasad, V. ; Zhang, W. ; Walker, L. M. ; Weitz, D. A. Shake-gels: Shear-induced gelation of laponite-PEO mixtures. Colloids and Surfaces a-Physicochemical and Engineering Aspects 2003, 213, 189-197. Publisher's VersionAbstract
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. (C) 2002 Elsevier Science B.V. All rights reserved.
zebrowski2003.pdf
Tsapis, N. ; Bennett, D. ; O'Driscoll, K. ; Shea, K. ; Lipp, M. M. ; Fu, K. ; Clarke, R. W. ; Deaver, D. ; Yamins, D. ; Wright, J. ; et al. Direct lung delivery of para-aminosalicylic acid by aerosol particles. Tuberculosis 2003, 83, 379-385. Publisher's VersionAbstract
Para-aminosalicylic acid (PAS), a tuberculostatic agent, was formulated into large porous particles for direct delivery into the lungs via inhalation. These particles possess optimized physical properties for deposition throughout the respiratory tract, a drug loading of 95% by weight and physical stability over 4 weeks at elevated temperatures. Upon insufflation in rats, PAS concentrations were measured in plasma, lung lining fluid and homogenized whole lung tissue. Systemic drug concentrations peaked at 15 min, with a maximum plasma concentration of 11+/-1 mug/ml. The concentration in the lung lining fluid was 148 +/- 62 mug/ml at 15 min. Tissue concentrations were 65 +/- 20 mug/ml at 15 min and 3.2 +/- 0.2 mug/ml at 3 h. PAS was cleared within 3 h from the lung lining fluid and plasma but was still present at therapeutic concentrations in the lung tissue. These results suggest that inhalation delivery of PAS can potentially allow for a reduction in total dose delivered while providing for higher local and similar peak systemic drug concentrations as compared to those obtained upon oral PAS dosing. Similar particles could potentially be used for the delivery of additional anti-tuberculosis agents such as rifampicin, aminoglucosides or fluoroquinolones. (C) 2003 Elsevier Ltd. All rights reserved.
tsapis2003.pdf
Rudhardt, D. ; Fernandez-Nieves, A. ; Link, D. R. ; Weitz, D. A. Phase switching of ordered arrays of liquid crystal emulsions. Applied Physics Letters 2003, 82, 2610-2612. Publisher's VersionAbstract
We report a fabrication method for producing interference-based electro-optic phase gratings that switch between diffracting and transparent states. The phase grating consists of a hexagonal-close-packed array of monodisperse emulsion drops of nematic liquid crystal, embedded in a polymer matrix. Monodisperse droplet size allows for fast switching at low electric fields. (C) 2003 American Institute of Physics.
rudhardt2003.pdf
Popp, A. K. ; Valentine, M. T. ; Kaplan, P. D. ; Weitz, D. A. Microscopic origin of light scattering in tissue. Applied Optics 2003, 42, 2871-2880. Publisher's VersionAbstract
A newly designed instrument, the static light-scattering (SLS) microscope, which combines light microscopy with SLS, enables us to characterize local light-scattering patterns of thin tissue sections. Each measurement is performed with an illumination beam of 70-mum diameter. On these length scales, tissue is not homogeneous. Both structural ordering and small heterogeneities contribute to the scattering signal. Raw SLS data consist of a two-dimensional intensity distribution map I(theta, phi), showing the dependence of the scattered intensity I on the scattering angle theta and the azimuthal angle D. In contrast to the majority of experiments and to simulations that consider only the scattering angle, we additionally perform an analysis of the azimuthal dependence I(phi). We estimate different contributions to the azimuthal scattering variation and show that a significant fraction of the azimuthal amplitude is the result of tissue structure. As a demonstration of the importance of the structure-dependent part of the azimuthal signal, we show that this function of the scattered light alone can be used to classify tissue types with surprisingly high specificity and sensitivity. (C) 2003 Optical Society of America.
popp2003.pdf
Prasad, V. ; Trappe, V. ; Dinsmore, A. D. ; Segre, P. N. ; Cipelletti, L. ; Weitz, D. A. Universal features of the fluid to solid transition for attractive colloidal particles. Faraday Discussions 2003, 123, 1-12. Publisher's VersionAbstract
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.
prasad2003.pdf
Pautot, S. ; Frisken, B. J. ; Weitz, D. A. Engineering asymmetric vesicles. PNAS 2003, 100, 10718-10721. Publisher's VersionAbstract
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.
pautot2003.pdf
Pautot, S. ; Frisken, B. J. ; Weitz, D. A. Production of unilamellar vesicles using an inverted emulsion. Langmuir 2003, 19, 2870-2879. Publisher's VersionAbstract
We investigate a method for the controlled assembly of unilamellar vesicles consisting of bilayers assembled one leaflet at a time. We use water-in-oil emulsions stabilized by the material for the inner leaflet and produce vesicles by passing the water droplets through a second oil-water interface, where they become coated with the outer leaflet. We have used this technique to form vesicles from lipids, mixed lipid and surfactant systems, and diblock copolymers. The stability of lipid-stabilized emulsions limits the range of sizes that can be produced and the vesicle yield; nevertheless, there are several advantages with this emulsion-based technique: It is possible to make unilamellar vesicles with sizes ranging from 100 nm to 1 mum. Moreover, the process allows for efficient encapsulation and ensures that the contents of the vesicles remain isolated from the continuous aqueous phase. To illustrate possible applications of this technique, we demonstrate the use of vesicles as microreactors where we polymerize actin through the addition of magnesium and show that the polymerization kinetics are unaffected by the encapsulation.
pautot2003.pdf
Pautot, S. ; Frisken, B. J. ; Cheng, J. X. ; Xie, X. S. ; Weitz, D. A. Spontaneous formation of lipid structures at oil/water lipid interfaces. Langmuir 2003, 19, 10281-10287. Publisher's VersionAbstract
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 semierystalline 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.
pautot2003.pdf
Nikolaides, M. G. ; Bausch, A. R. ; Hsu, M. F. ; Dinsmore, A. D. ; Brenner, M. P. ; Gay, C. ; Weitz, D. A. Like-charged particles at liquid interfaces - Reply. Nature 2003, 424, 1014. Publisher's VersionAbstract

Our experiment provides a clear measure of the interactions between charged particles at fluid–fluid interfaces and demonstrates that there can be a long-range attractive interaction between such particles.

nikolaides2003.pdf
Kilfoil, M. L. ; Pashkovski, E. E. ; Masters, J. A. ; Weitz, D. A. Dynamics of weakly aggregated colloidal particles. Philosophical Transactions of the Royal Society of London Series A 2003, 361, 753-764. Publisher's VersionAbstract
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.
kilfoil2003.pdf
Hyatt, C. J. ; Mironov, S. F. ; Wellner, M. ; Berenfeld, O. ; Popp, A. K. ; Weitz, D. A. ; Jalife, J. ; Pertsov, A. M. Synthesis of voltage-sensitive fluorescence signals from three-dimensional myocardial activation patterns. Biophysical Journal 2003, 85, 2673-2683. Publisher's VersionAbstract
Voltage-sensitive fluorescent dyes are commonly used to measure cardiac electrical activity. Recent studies indicate, however, that optical action potentials (OAPs) recorded from the myocardial surface originate from a widely distributed volume beneath the surface and may contain useful information regarding intramural activation. The first step toward obtaining this information is to predict OAPs from known patterns of three-dimensional (3-D) electrical activity. To achieve this goal, we developed a two-stage model in which the output of a 3-D ionic model of electrical excitation serves as the input to an optical model of light scattering and absorption inside heart tissue. The two-stage model permits unique optical signatures to be obtained for given 3-D patterns of electrical activity for direct comparison with experimental data, thus yielding information about intramural electrical activity. To illustrate applications of the model, we simulated surface fluorescence signals produced by 3-D electrical activity during epicardial and endocardial pacing. We discovered that OAP upstroke morphology was highly sensitive to the transmural component of wave front velocity and could be used to predict wave front orientation with respect to the surface. These findings demonstrate the potential of the model for obtaining useful 3-D information about intramural propagation.
hyatt2003.pdf
Gordon, V. D. ; Valentine, M. T. ; Gardel, M. L. ; Andor-Ardo, D. ; Dennison, S. ; Bogdanov, A. A. ; Weitz, D. A. ; Deisboeck, T. S. Measuring the mechanical stress induced by an expanding multicellular tumor system: A case study. Experimental Cell Research 2003, 289, 58-66. Publisher's VersionAbstract
Rapid volumetric growth and extensive invasion into brain parenchyma are hallmarks of malignant neuroepithelial tumors in vivo. Little is known, however, about the mechanical impact of the growing brain tumor on its microenvironment. To better understand the environmental mechanical response, we used multiparticle tracking methods to probe the environment of a dynamically expanding, multicellular brain tumor spheroid that grew for 6 days in a three-dimensional Matrigel-based in vitro assay containing 1.0-mum latex beads. These beads act as reference markers for the gel, allowing us to image the spatial displacement of the tumor environment using high-resolution time-lapse video microscopy. The results show that the volumetrically expanding tumor spheroid pushes the gel outward and that this tumor-generated pressure propagates to a distance greater than the initial radius of the tumor spheroid. Intriguingly, beads near the tips of invasive cells are displaced inward, toward the advancing invasive cells. Furthermore, this localized cell traction correlates with a marked increase in total invasion area over the observation period. This case study presents evidence that an expanding microscopic tumor system exerts both significant mechanical pressure and significant traction on its microenvironment. (C) 2003 Elsevier Science (USA). All rights reserved.
gordon2003.pdf
Gasser, U. ; Schofield, A. ; Weitz, D. A. Local order in a supercooled colloidal fluid observed by confocal microscopy. Journal of Physics: Condensed Matter 2003, 15, S375-S380. Publisher's VersionAbstract
The local order in a supercooled monodisperse colloidal fluid is studied by direct imaging of the particles with a laser scanning confocal microscope. The local structure is analysed with a bond order parameter method, which allows one to discern simple structures that are relevant in this system. As expected for samples that crystallize eventually, a large fraction of the particles are found to sit in surroundings with dominant face-centred cubic or hexagonally close-packed character. Evidence for local structures that contain fragments of icosahedra is found, and, moreover, the icosahedral character increases with volume fraction phi, which indicates that it might play an important role at volume fractions near the glass transition.
gasser2003.pdf
Gardel, M. L. ; Valentine, M. T. ; Crocker, J. C. ; Bausch, A. R. ; Weitz, D. A. Microrheology of entangled F-actin solutions. Physical Review Letters 2003, 91, 158302. Publisher's VersionAbstract
We measure the viscoelasticity of entangled F-actin over length scales between 1 and 100 mum using one- and two-particle microrheology, and directly identify two distinct microscopic contributions to the elasticity. Filament entanglements lead to a frequency-independent elastic modulus over an extended frequency range of 0.01-30 rad/sec; this is probed with one-particle microrheology. Longitudinal fluctuations of the filaments increase the elastic modulus between 0.1 and 30 rad/sec at length scales up to the filament persistence length; this is probed by two-particle microrheology.
gardel2003.pdf
Dufresne, E. R. ; Corwin, E. I. ; Greenblatt, N. A. ; Ashmore, J. ; Wang, D. Y. ; Dinsmore, A. D. ; Cheng, J. X. ; Xie, X. S. ; Hutchinson, J. W. ; Weitz, D. A. Flow and fracture in drying nanoparticle suspensions. Physical Review Letters 2003, 91, 224501. Publisher's VersionAbstract
Drying aqueous suspensions of monodisperse silica nanoparticles can fracture in remarkable patterns. As the material solidifies, evenly spaced cracks invade from the drying surface, with individual cracks undergoing intermittent motion. We show that the growth of cracks is limited by the advancement of the compaction front, which is governed by a balance of evaporation and flow of fluid at the drying surface. Surprisingly, the macroscopic dynamics of drying show signatures of molecular-scale fluid effects.
dufresne2003.pdf
Cipelletti, L. ; Ramos, L. ; Manley, S. ; Pitard, E. ; Weitz, D. A. ; Pashkovski, E. E. ; Johansson, M. Universal non-diffusive slow dynamics in aging soft matter. Faraday Discussions 2003, 123, 237-251. Publisher's VersionAbstract
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) similar to exp [ (t /tau(s))(p)], with p approximate to 1.5 and t(s) similar to q(-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.
cipelletti2003.pdf
Cheng, J. X. ; Pautot, S. ; Weitz, D. A. ; Xie, X. S. Ordering of water molecules between phospholipid bilayers visualized by coherent anti-Stokes Raman scattering microscopy. PNAS 2003, 100, 9826-9830. Publisher's VersionAbstract
We demonstrate ordered orientation of the hydration water at the surface of phospholipid bilayers by use of coherent anti-Stokes Raman scattering (CARS) microscopy, a highly sensitive vibrational imaging method recently developed. We investigated negatively charged POPS (1-palmitoyl-2-oleoyl-sn-glycero-3-phospho-L-serine) and neutral POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) multilamellar onions dispersed in deuterated dodecane. The imaging contrast based on the CARS signal from the H2O stretching vibration shows a clear dependence on the excitation field polarization. Our results provide direct experimental evidence that water molecules close to the phospholipid bilayer surface are ordered with the symmetry axis along the direction normal to the bilayer. Moreover, the amount of ordered water molecules depends on the lipid polar group. The spectral profile for the interlamellar water shows that the water molecules bound to the bilayer surface are less hydrogen-bonded and exhibit a higher vibrational frequency than bulk water.
cheng2003.pdf

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