# Publications

2013
Choi, C. - H. ; Weitz, D. A. ; Lee, C. - S. One step formation of controllable complex emulsions: From functional particles to simultaneous encapsulation of hydrophilic and hydrophobic agents into desired position. Advanced Materials 2013, 25, 2536-2541.
Kim, S. - H. ; Kim, J. W. ; Kim, D. - H. ; Han, S. - H. ; Weitz, D. A. Polymersomes containing a hydrogel network for high stability and controlled release. Small 2013, 9 124-131.Abstract

Capillary microfluidic devices are used to prepare monodisperse polymersomes consisting of a hydrogel core and a bilayer membrane of amphiphilic diblock-copolymers. To make polymersomes, water-in-oil-in-water double-emulsion drops are prepared as templates through single-step emulsification in a capillary microfluidic device. The amphiphile-laden middle oil phase of the double-emulsion drop dewets from the surface of the innermost water drop, which contains hydrogel prepolymers; this dewetting leads to the formation of a bilayer membrane. Subsequently, the oil phase completely separates from the innermost water drop, leaving a polymersome. Upon UV illumination of the polymersome, the prepolymers encapsulated within the interior are crosslinked, forming a hydrogel core. The hydrogel network within the polymersomes facilitates sustained release of the encapsulated materials and increases the stability of the polymersomes through the formation of a scaffold to support the bilayer. In addition, this approach provides a facile method to make monodisperse hydrogel particles directly dispersed in water.

Di Benedetto, F. ; Fasano, V. ; Persano, L. ; Maruccio, C. ; Mele, E. ; Potente, G. ; Weitz, D. A. ; De Lorenzis, L. ; Pisignano, D. Rolling particle lithography by soft polymer microparticles. Soft Matter 2013, 9 2206-2211.Abstract

Elastomeric polymeric microspheres are employed as a direct-writing tool for the continuous delivery of molecular materials. The mechanical properties enabling patterning are investigated and modelled. The proposed approach provides a low cost and versatile lithographic method for transferring features with real-time dynamic control.

Mazutis, L. ; Gilbert, J. ; Ung, L. W. ; Weitz, D. A. ; Griffiths, A. D. ; Heyman, J. A. Single-cell analysis and sorting using droplet-based microfluidics. Nature Protocols 2013, 8 870-891.Abstract

We present a droplet-based microfluidics protocol for high-throughput analysis and sorting of single cells. Compartmentalization of single cells in droplets enables the analysis of proteins released from or secreted by cells, thereby overcoming one of the major limitations of traditional flow cytometry and fluorescence-activated cell sorting. As an example of this approach, we detail a binding assay for detecting antibodies secreted from single mouse hybridoma cells. Secreted antibodies are detected after only 15 min by co-compartmentalizing single mouse hybridoma cells, a fluorescent probe and single beads coated with anti-mouse IgG antibodies in 50-pl droplets. The beads capture the secreted antibodies and, when the captured antibodies bind to the probe, the fluorescence becomes localized on the beads, generating a clearly distinguishable fluorescence signal that enables droplet sorting at similar to 200 Hz as well as cell enrichment. The microfluidic system described is easily adapted for screening other intracellular, cell-surface or secreted proteins and for quantifying catalytic or regulatory activities. In order to screen similar to 1 million cells, the microfluidic operations require 2-6 h; the entire process, including preparation of microfluidic devices and mammalian cells, requires 5-7 d.

Datta, S. S. ; Chiang, H. ; Ramakrishnan, T. S. ; Weitz, D. A. Spatial fluctuations of fluid velocities in flow through a three-dimensional porous medium. Physical Review Letters 2013, 111.Abstract

We use confocal microscopy to directly visualize the spatial fluctuations in fluid flow through a three-dimensional porous medium. We find that the velocity magnitudes and the velocity components both along and transverse to the imposed flow direction are exponentially distributed, even with residual trapping of a second immiscible fluid. Moreover, we find pore-scale correlations in the flow that are determined by the geometry of the medium. Our results suggest that despite the considerable complexity of the pore space, fluid flow through it is not completely random.

DiLauro, A. M. ; Abbaspourrad, A. ; Weitz, D. A. ; Phillips, S. T. Stimuli-responsive core-shell microcapsules with tunable rates of release by using a depolymerizable poly(phthalaldehyde) membrane. Macromolecules 2013, 46, 3309-3313.Abstract

Flow-focusing microfluidic techniques were used to provide access to core shell microcapsules in which the shell is composed of end tapped poly(phthalaldehydes) that depolymerize completely from head-to-tail in response to fluoride. Microcapsules made from these depolymerizable polymers provide an amplified response to the applied chemical signal, where the rate of the response can be tuned both by varying the length of the polymer and the thickness of the shell wall.

Munster, S. ; Jawerth, L. M. ; Leslie, B. A. ; Weitz, J. I. ; Fabry, B. ; Weitz, D. A. Strain history dependence of the nonlinear stress response of fibrin and collagen networks. Proceedings of the National Academy of Sciences of the United States of America 2013, 110, 12197-12202.Abstract

We show that the nonlinear mechanical response of networks formed from un-cross-linked fibrin or collagen type I continually changes in response to repeated large-strain loading. We demonstrate that this dynamic evolution of the mechanical response arises from a shift of a characteristic nonlinear stress-strain relationship to higher strains. Therefore, the imposed loading does not weaken the underlying matrices but instead delays the occurrence of the strain stiffening. Using confocal microscopy, we present direct evidence that this behavior results from persistent lengthening of individual fibers caused by an interplay between fiber stretching and fiber buckling when the networks are repeatedly strained. Moreover, we show that covalent cross-linking of fibrin or collagen inhibits the shift of the nonlinear material response, suggesting that the molecular origin of individual fiber lengthening may be slip of monomers within the fibers. Thus, a fibrous architecture in combination with constituents that exhibit internal plasticity creates a material whose mechanical response adapts to external loading conditions. This design principle may be useful to engineer novel materials with this capability.

Abbaspourrad, A. ; Carroll, N. J. ; Kim, S. - H. ; Weitz, D. A. Surface functionalized hydrophobic porous particles toward water treatment application. Advanced Materials 2013, 25, 3215-3221.Abstract

A microfluidic-based approach for the fabrication of organic contaminants absorbing core-shell particles is demonstrated. The hydrophobic porous core absorbs oil while the hydrophilic surface enables the particles to be well-dispersed in aqueous solutions. These particles can uptake oil from aqueous solution saturated with oil or via direct contact with oil blobs as depicted in the figure.

Guo, M. ; Ehrlicher, A. J. ; Mahammad, S. ; Fabich, H. ; Jensen, M. H. ; Moore, J. R. ; Fredberg, J. J. ; Goldman, R. D. ; Weitz, D. A. The role of vimentin intermediate filaments in cortical and cytoplasmic mechanics. Biophysical Journal 2013, 105, 1562-1568.Abstract

The mechanical properties of a cell determine many aspects of its behavior, and these mechanics are largely determined by the cytoskeleton. Although the contribution of actin filaments and microtubules to the mechanics of cells has been investigated in great detail, relatively little is known about the contribution of the third major cytoskeletal component, intermediate filaments (IFs). To determine the role of vimentin IF (VIF) in modulating intracellular and cortical mechanics, we carried out studies using mouse embryonic fibroblasts (mEFs) derived from wild-type or vimentin(-/-) mice. The VIFs contribute little to cortical stiffness but are critical for regulating intracellular mechanics. Active microrheology measurements using optical tweezers in living cells reveal that the presence of VIFs doubles the value of the cytoplasmic shear modulus to similar to 10 Pa. The higher levels of cytoplasmic stiffness appear to stabilize organelles in the cell, as measured by tracking endogenous vesicle movement. These studies show that VIFs both increase the mechanical integrity of cells and localize intracellular components.

Krummel, A. T. ; Datta, S. S. ; Muenster, S. ; Weitz, D. A. Visualizing multiphase flow and trapped fluid configurations in a model three-dimensional porous medium. Aiche Journal 2013, 59, 1022-1029.Abstract

We report an approach to fully visualize the flow of two immiscible fluids through a model three-dimensional (3-D) porous medium at pore-scale resolution. Using confocal microscopy, we directly image the drainage of the medium by the nonwetting oil and subsequent imbibition by the wetting fluid. During imbibition, the wetting fluid pinches off threads of oil in the narrow crevices of the medium, forming disconnected oil ganglia. Some of these ganglia remain trapped within the medium. By resolving the full 3-D structure of the trapped ganglia, we show that the typical ganglion size, as well as the total amount of residual oil, decreases as the capillary number Ca increases; this behavior reflects the competition between the viscous pressure in the wetting fluid and the capillary pressure required to force oil through the pores of the medium. This work thus shows how pore-scale fluid dynamics influence the trapped fluid configurations in multiphase flow through 3-D porous media. (C) 2013 American Institute of Chemical Engineers AIChE J, 59: 1022-1029, 2013

Deng, N. - N. ; Wang, W. ; Ju, X. - J. ; Xie, R. ; Weitz, D. A. ; Chu, L. - Y. Wetting-induced formation of controllable monodisperse multiple emulsions in microfluidics. Lab on a Chip 2013, 13, 4047-4052.Abstract

Multiple emulsions, which are widely applied in a myriad of fields because of their unique ability to encapsulate and protect active ingredients, are typically produced by sequential drop-formations and drop-encapsulations using shear-induced emulsification. Here we report a qualitatively novel method of creating highly controlled multiple emulsions from lower-order emulsions. By carefully controlling the interfacial energies, we adjust the spreading coefficients between different phases to cause drops of one fluid to completely engulf other drops of immiscible fluids; as a result multiple emulsions are directly formed by simply putting preformed lower-order emulsion drops together. Our approach has highly controllable flexibility. We demonstrate this in preparation of both double and triple emulsions with a controlled number of inner drops and precisely adjusted shell thicknesses including ultra-thin shells. Moreover, this controllable drop-engulfing-drop approach has a high potential in further investigations and applications of microfluidics. Importantly, this innovative approach opens a window to exploit new phenomena occurring in fluids at the microscale level, which is of great significance for developing novel microfluidics.

Times Cited: 0

Muenster, S. ; Jawerth, L. M. ; Fabry, B. ; Weitz, D. A. Structure and mechanics of fibrin clots formed under mechanical perturbation. Journal of Thrombosis and Haemostasis 2013, 11, 557-560.

Times Cited: 0

2012
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; 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.

[*Equal contribution]

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.Abstract
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.Abstract
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.
Times Cited: 25
Muluneh, M. ; Weitz, D. A. Direct visualization of three-dimensional crystallization behavior in microgels. Physical Review E 2012, 85.Abstract
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.
Times Cited: 3 1
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.Abstract
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.
Times Cited: 7
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.
Times Cited: 11
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.Abstract
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.
Times Cited: 10