Publications by Year: 2012

Vladisavljević, G. T. ; Shum, H. C. ; Weitz, D. A. Control over the Shell Thickness of Core/Shell Drops in Three-Phase Glass Capillary Devices. In UK Colloids 2011: An International Colloid and Surface Science Symposium; 2012; Vol. 139, pp. 115-118. Publisher's VersionAbstract

Monodisperse core/shell drops with aqueous core and poly(dimethylsiloxane) (PDMS) shell of controllable thickness have been produced using a glass microcapillary device that combines co-flow and flow-focusing geometries. The throughput of the droplet generation was high, with droplet generation frequency in the range from 1,000 to 10,000 Hz. The size of the droplets can be tuned by changing the flow rate of the continuous phase. The technique enables control over the shell thickness through adjusting the flow rate ratio of the middle to inner phase. As the flow rate of the middle and inner phase increases, the droplet breakup occurs in the dripping-to-jetting transition regime, with each double emulsion droplet containing two monodisperse internal aqueous droplets. The resultant drops can be used subsequently as templates for monodisperse polymer capsules with a single or multiple inner compartments, as well as functional vesicles such as liposomes, polymersomes and colloidosomes.

Vladisavljević, G. T. ; Henry, J. V. ; Duncanson, W. J. ; Shum, H. C. ; Weitz, D. A. Fabrication of Biodegradable Poly(Lactic Acid) Particles in Flow-Focusing Glass Capillary Devices. In UK Colloids 2011: An International Colloid and Surface Science Symposium; Starov, V. ; Griffiths, P., Ed. Springer Berlin Heidelberg: Berlin, Heidelberg, 2012; Vol. 139, pp. 111–114. Publisher's VersionAbstract
Monodisperse poly(dl-lactic acid) (PLA) particles with a diameter in the range from 12 to 100 $μ$m were fabricated in flow focusing glass capillary devices by evaporation of dichloromethane (DCM) from emulsions at room temperature. The dispersed phase was 5% (w/w) PLA in DCM containing a small amount of Nile red and the continuous phase was 5% (w/w) poly(vinyl alcohol) in reverse osmosis water. Particle diameter was 2.7 times smaller than the size of the emulsion droplet template indicating that the particle porosity was very low. SEM images revealed that the majority of particle pores are in the sub-micron region but in some instances these pores can reach 3 $μ$m in diameter. Droplet diameter was influenced by the flow rates of the two phases and the entry diameter of the collection capillary tube; droplet diameters decreased with increasing values of the flow rate ratio of the dispersed to continuous phase to reach constant minimum values at 40–60% orifice diameter. At flow rate ratios less than 5, jetting can occur, giving rise to large droplets formed by detachment from relatively long jets (\textasciitilde10 times longer than droplet diameter).
Jensen, M. H. ; Morris, E. J. ; Huang, R. ; Rebowski, G. ; Dominguez, R. ; Weitz, D. A. ; Moore, J. R. ; Wang, C. - L. A. The conformational state of actin filaments regulates branching by actin-related protein 2/3 (Arp2/3) complex. Journal of Biological Chemistry 2012, 287, 31447–31453. Publisher's VersionAbstract

Actin is a highly ubiquitous protein in eukaryotic cells that plays a crucial role in cell mechanics and motility. Cell motility is driven by assembling actin as polymerizing actin drives cell protrusions in a process closely involving a host of other actin-binding proteins, notably the actin-related protein 2/3 (Arp2/3) complex, which nucleates actin and forms branched filamentous structures. The Arp2/3 complex preferentially binds specific actin networks at the cell leading edge and forms branched filamentous structures, which drive cell protrusions, but the exact regulatory mechanism behind this process is not well understood. Here we show using in vitro imaging and binding assays that a fragment of the actin-binding protein caldesmon added to polymerizing actin increases the Arp2/3-mediated branching activity, whereas it has no effect on branch formation when binding to aged actin filaments. Because this caldesmon effect is shown to be independent of nucleotide hydrolysis and phosphate release from actin, our results suggest a mechanism by which caldesmon maintains newly polymerized actin in a distinct state that has a higher affinity for the Arp2/3 complex. Our data show that this new state does not affect the level of cooperativity of binding by Arp2/3 complex or its distribution on actin. This presents a novel regulatory mechanism by which caldesmon, and potentially other actin-binding proteins, regulates the interactions of actin with its binding partners.

Yoon, K. ; Lee, D. ; Kim, J. W. ; Kim, J. ; Weitz, D. A. Asymmetric functionalization of colloidal dimer particles with gold nanoparticles. Chemical Communications 2012, 48, 9056-9058. Publisher's VersionAbstract
We present a method to asymmetrically functionalize dimer particles with gold nanoparticles. Our method enables the fabrication of asymmetrically functionalized dimer particles on a large scale.
Romanowsky, M. B. ; Abate, A. R. ; Rotem, A. ; Holtze, C. ; Weitz, D. A. High throughput production of single core double emulsions in a parallelized microfluidic device. Lab on a Chip 2012, 12, 802-807. Publisher's VersionAbstract
Double emulsions are useful templates for microcapsules and complex particles, but no method yet exists for making double emulsions with both high uniformity and high throughput. We present a parallel numbering-up design for microfluidic double emulsion devices, which combines the excellent control of microfluidics with throughput suitable for mass production. We demonstrate the design with devices incorporating up to 15 dropmaker units in a two-dimensional or three-dimensional array, producing single-core double emulsion drops at rates over 1 kg day(-1) and with diameter variation less than 6%. This design provides a route to integrating hundreds of dropmakers or more in a single chip, facilitating industrial-scale production rates of many tons per year.
Muluneh, M. ; Weitz, D. A. Direct visualization of three-dimensional crystallization behavior in microgels. Physical Review E 2012, 85, 021405. Publisher's VersionAbstract
We use confocal microscopy to study the three-dimensional (3D) structure of colloidal crystals formed by poly(N-isopropylacrylamide)-co-(acrylic acid) microgels of diameter 1.0-1.5 mu m. The confocal images are tracked to locate particle positions in 3D, which are used to compute pair-correlation functions g(r), bond order parameters, and structure factors s(q). We find that the structure remains fcc for a range of charge, size, and concentration of the particles. When the particles are weakly attractive and are at low concentrations, polycrystalline solids result. In addition, owing to the compressibility of the colloids, the crystals display remarkable structural stability when subjected to external stress.
Menut, P. ; Seiffert, S. ; Sprakel, J. ; Weitz, D. A. Does size matter? Elasticity of compressed suspensions of colloidal- and granular-scale microgels. Soft Matter 2012, 8 156-164. Publisher's VersionAbstract
We investigate the mechanics of dense packing of very small, colloidal-scale, and larger, granular-scale microgel particles. At low particle concentration, thermally induced Brownian motion of the particles is important for the colloidal-scale systems; in contrast, such Brownian motion is irrelevant at particle packing fractions beyond jamming. As a consequence, colloidal and granular systems behave very similarly under these conditions. At sufficiently high compression of the microgel particles, their polymeric nature sets the scale of the osmotic pressure and shear modulus of the whole packing, in direct analogy with macroscopic, continuous polymer gels. This observation suggests that the particulate nature of microgels is inconsequential for their linear elasticity in a highly packed state. In contrast, the particulate nature of the microgels does become essential when the packed suspensions are forced to yield and flow; here, the differences between colloidal-and granular-scale particles are marked.
Martino, C. ; Kim, S. - H. ; Horsfall, L. ; Abbaspourrad, A. ; Rosser, S. J. ; Cooper, J. ; Weitz, D. A. Protein expression, aggregation, and triggered release from polymersomes as artificial cell-like structures. Angewandte Chemie-International Edition 2012, 51, 6416-6420. Publisher's VersionAbstract

Bringing droplets to life: A cytoskeletal protein (red dots, see scheme) is expressed in artificial cells composed of biocompatible polymersomes, which encapsulate expression machinery and amino acid building blocks. Release of the expressed proteins can be triggered by a negative osmotic shock.

Lu, P. J. ; Giavazzi, F. ; Angelini, T. E. ; Zaccarelli, E. ; Jargstorff, F. ; Schofield, A. B. ; Wilking, J. N. ; Romanowsky, M. B. ; Weitz, D. A. ; Cerbino, R. Characterizing concentrated, multiply scattering, and actively driven fluorescent systems with confocal differential dynamic microscopy. Physical Review Letters 2012, 108, 218103. Publisher's VersionAbstract
We introduce confocal differential dynamic microscopy (ConDDM), a new technique yielding information comparable to that given by light scattering but in dense, opaque, fluorescent samples of micron-sized objects that cannot be probed easily with other existing techniques. We measure the correct wave vector q-dependent structure and hydrodynamic factors of concentrated hard-sphere-like colloids. We characterize concentrated swimming bacteria, observing ballistic motion in the bulk and a new compressed-exponential scaling of dynamics, and determine the velocity distribution; by contrast, near the coverslip, dynamics scale differently, suggesting that bacterial motion near surfaces fundamentally differs from that of freely swimming organisms.
Liu, T. ; Seiffert, S. ; Thiele, J. ; Abate, A. R. ; Weitz, D. A. ; Richtering, W. Non-coalescence of oppositely charged droplets in pH-sensitive emulsions. Proceedings of the National Academy of Sciences of the United States of America 2012, 109, 384-389. Publisher's VersionAbstract
Like charges stabilize emulsions, whereas opposite charges break emulsions. This is the fundamental principle for many industrial and practical processes. Using micrometer-sized pH-sensitive polymeric hydrogel particles as emulsion stabilizers, we prepare emulsions that consist of oppositely charged droplets, which do not coalesce. We observe noncoalescence of oppositely charged droplets in bulk emulsification as well as in microfluidic devices, where oppositely charged droplets are forced to collide within channel junctions. The results demonstrate that electrostatic interactions between droplets do not determine their stability and reveal the unique pH-dependent properties of emulsions stabilized by soft microgel particles. The noncoalescence can be switched to coalescence by neutralizing the microgels, and the emulsion can be broken on demand. This unusual feature of the microgel-stabilized emulsions offers fascinating opportunities for future applications of these systems.
Lindstrom, S. B. ; Kodger, T. E. ; Sprakel, J. ; Weitz, D. A. Structures, stresses, and fluctuations in the delayed failure of colloidal gels. Soft Matter 2012, 8 3657-3664. Publisher's VersionAbstract
Sample-spanning networks of aggregated colloidal particles have a finite stiffness and deform elastically when subjected to a small shear stress. After some period of creep, these gels ultimately suffer catastrophic failure. This delayed yielding is governed by the association and dissociation dynamics of interparticle bonds and the strand structure of the gel. We derive a model which connects the kinetics of the colloids to the erosion of the strand structure and ultimately to macroscopic failure. Importantly, this model relates time-to-failure of the gel to an applied static stress. Model predictions are in quantitative agreement with experiments. It is predicted that the strand structure, characterized by its mesh size and strand coarseness, has a significant impact on delay time. Decreasing the mesh size or increasing the strand thickness makes colloidal gels more resilient to delayed yielding. The quench and flow history of gels modifies their microstructures. Our experiments show that a slow quenching increases the delay time due to the coarsening of the strands; by contrast, preshear reduces the delay time, which we explain by the increased mesh size as a result of shear-induced fracture of strands.
Kolinski, J. M. ; Rubinstein, S. M. ; Mandre, S. ; Brenner, M. P. ; Weitz, D. A. ; Mahadevan, L. Skating on a film of air: Drops impacting on a surface. Physical Review Letters 2012, 108, 074503. Publisher's VersionAbstract
The commonly accepted description of drops impacting on a surface typically ignores the essential role of the air that is trapped between the impacting drop and the surface. Here we describe a new imaging modality that is sensitive to the behavior right at the surface. We show that a very thin film of air, only a few tens of nanometers thick, remains trapped between the falling drop and the surface as the drop spreads. The thin film of air serves to lubricate the drop enabling the fluid to skate on the air film laterally outward at surprisingly high velocities, consistent with theoretical predictions. Eventually this thin film of air breaks down as the fluid wets the surface via a spinodal-like mechanism. Our results show that the dynamics of impacting drops are much more complex than previously thought, with a rich array of unexpected phenomena that require rethinking classic paradigms.
Kraft, D. J. ; Ni, R. ; Smallenburg, F. ; Hermes, M. ; Yoon, K. ; Weitz, D. A. ; van Blaaderen, A. ; Groenewold, J. ; Dijkstra, M. ; Kegel, W. K. Surface roughness directed self-assembly of patchy particles into colloidal micelles. Proceedings of the National Academy of Sciences of the United States of America 2012, 109, 10787-10792. Publisher's VersionAbstract
Colloidal particles with site-specific directional interactions, so called "patchy particles", are promising candidates for bottom-up assembly routes towards complex structures with rationally designed properties. Here we present an experimental realization of patchy colloidal particles based on material independent depletion interaction and surface roughness. Curved, smooth patches on rough colloids are shown to be exclusively attractive due to their different overlap volumes. We discuss in detail the case of colloids with one patch that serves as a model for molecular surfactants both with respect to their geometry and their interactions. These one-patch particles assemble into clusters that resemble surfactant micelles with the smooth and attractive sides of the colloids located at the interior. We term these clusters "colloidal micelles". Direct Monte Carlo simulations starting from a homogeneous state give rise to cluster size distributions that are in good agreement with those found in experiments. Important differences with surfactant micelles originate from the colloidal character of our model system and are investigated by simulations and addressed theoretically. Our new "patchy" model system opens up the possibility for self-assembly studies into finite-sized superstructures as well as crystals with as of yet inaccessible structures.
Guo, M. T. ; Rotem, A. ; Heyman, J. A. ; Weitz, D. A. Droplet microfluidics for high-throughput biological assays. Lab on a Chip 2012, 12, 2146-2155. Publisher's VersionAbstract
Droplet microfluidics offers significant advantages for performing high-throughput screens and sensitive assays. Droplets allow sample volumes to be significantly reduced, leading to concomitant reductions in cost. Manipulation and measurement at kilohertz speeds enable up to 10(8) samples to be screened in one day. Compartmentalization in droplets increases assay sensitivity by increasing the effective concentration of rare species and decreasing the time required to reach detection thresholds. Droplet microfluidics combines these powerful features to enable currently inaccessible high-throughput screening applications, including single-cell and single-molecule assays.
Duncanson, W. J. ; Lin, T. ; Abate, A. R. ; Seiffert, S. ; Shah, R. K. ; Weitz, D. A. Microfluidic synthesis of advanced microparticles for encapsulation and controlled release. Lab on a Chip 2012, 12, 2135-2145. Publisher's VersionAbstract
We describe droplet microfluidic strategies used to fabricate advanced microparticles that are useful structures for the encapsulation and release of actives; these strategies can be further developed to produce microparticles for advanced drug delivery applications. Microfluidics enables exquisite control in the fabrication of polymer vesicles and thermosensitive microgels from single and higher-order multiple emulsion templates. The strategies used to create the diversity of microparticle structures described in this review, coupled with the scalability of microfluidics, will enable fabrication of large quantities of novel microparticle structures that have potential uses in controlled drug release applications.
Duncanson, W. J. ; Zieringer, M. ; Wagner, O. ; Wilking, J. N. ; Abbaspourrad, A. ; Haag, R. ; Weitz, D. A. Microfluidic synthesis of monodisperse porous microspheres with size-tunable pores. Soft Matter 2012, 8 10636-10640. Publisher's VersionAbstract
We use a perfluorinated-dendrimer-dye complex that stabilizes microbubbles as a novel pore-forming agent. We use microfluidics to produce monodisperse emulsions containing a polymer matrix material, a model active, and the perfluorinated complex; upon drying, the emulsions form porous microspheres. This porosity causes the encapsulated model active to be released faster than from non-porous microspheres. Moreover, because of the fluorous features of the pores, we can also attach an additional guest molecule to the pores which is released with a profile that is distinct from that of the encapsulated active. These porous microspheres can encapsulate and controllably release multiple actives; this makes them valuable for applications such as drug delivery and imaging.
Duncanson, W. J. ; Abbaspourrad, A. ; Shum, H. C. ; Kim, S. - H. ; Adams, L. L. A. ; Weitz, D. A. Monodisperse gas-filled microparticles from reactions in double emulsions. Langmuir 2012, 28, 6742-6745. Publisher's VersionAbstract
We present a strategy for preparing size-controlled gas-filled microparticles using two aqueous components that chemically react to produce the gas. We use a dual-bore microfluidic device to isolate the reactants of two gas-producing reactions until they are encapsulated in the outer droplet. The reactants in the monodisperse droplets merge and produce the gas bubbles, which are stabilized with a surfactant and form the core of the microparticles. The number and size of the generated gas bubbles are governed by the gas-forming reaction used. Our versatile strategy can be applied to a wide range of gas-producing reactions.
Datta, S. S. ; Kim, S. - H. ; Paulose, J. ; Abbaspourrad, A. ; Nelson, D. R. ; Weitz, D. A. Delayed buckling and guided folding of inhomogeneous capsules. Physical Review Letters 2012, 109, 134302. Publisher's VersionAbstract
Colloidal capsules can sustain an external osmotic pressure; however, for a sufficiently large pressure, they will ultimately buckle. This process can be strongly influenced by structural inhomogeneities in the capsule shells. We explore how the time delay before the onset of buckling decreases as the shells are made more inhomogeneous; this behavior can be quantitatively understood by coupling shell theory with Darcy's law. In addition, we show that the shell inhomogeneity can dramatically change the folding pathway taken by a capsule after it buckles.
Buhr, D. L. ; Acca, F. E. ; Holland, E. G. ; Johnson, K. ; Maksymiuk, G. M. ; Vaill, A. ; Kay, B. K. ; Weitz, D. A. ; Weiner, M. P. ; Kiss, M. M. Use of micro-emulsion technology for the directed evolution of antibodies. Methods 2012, 58, 28-33. Publisher's VersionAbstract
Affinity reagents, such as antibodies, are needed to study protein expression patterns, sub-cellular localization, and post-translational modifications in complex mixtures and tissues. Phage Emulsion, Secretion, and Capture (ESCape) is a novel micro-emulsion technology that utilizes water-in-oil (W/O) emulsions for the identification and isolation of cells secreting phage particles that display desirable antibodies. Using this method, a large library of antibody-displaying phage will bind to beads in individual compartments. Rather than using biopanning on a large mixed population, phage micro-emulsion technology allows us to individually query clonal populations of amplified phage against the antigen. The use of emulsions to generate microdroplets has the promise of accelerating phage selection experiments by permitting fine discrimination of kinetic parameters for binding to targets. In this study, we demonstrate the ability of phage micro-emulsion technology to distinguish two scFvs with a 300-fold difference in binding affinities (100 nM and 300 pM, respectively). In addition, we describe the application of phage microemulsion technology for the selection of scFvs that are resistant to elevated temperatures. (C) 2012 Published by Elsevier Inc.
Amstad, E. ; Kim, S. - H. ; Weitz, D. A. Photo- and thermoresponsive polymersomes for triggered release. Angewandte Chemie-International Edition 2012, 51, 12499-12503. Publisher's VersionAbstract

Microfluidics: Thermo‐ and photoresponsive polymersomes are assembled using capillary microfluidic devices. Encapsulants can be selectively released from the thermoresponsive polymersomes if they are incubated at and above temperatures of 40 °C, whereas the photoresponsive polymersomes selectively release encapsulants if illuminated with laser light (see picture; NP=nanoparticle).