Carroll, N. J. ; Jensen, K. H. ; Parsa, S. ; Holbrook, M. N. ; Weitz, D. A. Measurement of Flow Velocity and Inference of Liquid Viscosity in a Microfluidic Channel by Fluorescence Photobleaching. Langmuir 2014, 30, 4868-4874. Publisher's Version [PDF]
Datta, S. S. ; Dupin, J. - B. ; Weitz, D. A. Fluid breakup during simultaneous two-phase flow through a three-dimensional porous medium. Physics of Fluids 2014, 26, n/a–n/a. Publisher's Version [PDF]
Chen, L. ; Li, Y. ; Fan, J. ; Bisoyi, H. K. ; Weitz, D. A. ; Li, Q. Photoresponsive Monodisperse Cholesteric Liquid Crystalline Microshells for Tunable Omnidirectional Lasing Enabled by a Visible Light-Driven Chiral Molecular Switch. Advanced Optical Materials 2014, n/a–n/a. Publisher's Version [PDF]
Arriaga, L. R. ; Datta, S. S. ; Kim, S. - H. ; Amstad, E. ; Kodger, T. E. ; Monroy, F. ; Weitz, D. A. Ultrathin Shell Double Emulsion Templated Giant Unilamellar Lipid Vesicles with Controlled Microdomain Formation. Small 2014, 10, 950–956. Publisher's VersionAbstract

A microfluidic approach is reported for the high-throughput, continuous production of giant unilamellar vesicles (GUVs) using water-in-oil-in-water double emulsion drops as templates. Importantly, these emulsion drops have ultrathin shells; this minimizes the amount of residual solvent that remains trapped within the GUV membrane, overcoming a major limitation of typical microfluidic approaches for GUV fabrication. This approach enables the formation of microdomains, characterized by different lipid compositions and structures within the GUV membranes. This work therefore demonstrates a straightforward and versatile approach to GUV fabrication with precise control over the GUV size, lipid composition and the formation of microdomains within the GUV membrane.

Cohen, S. I. A. ; Rajah, L. ; Yoon, C. H. ; Buell, A. K. ; White, D. A. ; Sperling, R. A. ; Vendruscolo, M. ; Terentjev, E. M. ; Dobson, C. M. ; Weitz, D. A. ; et al. Spatial Propagation of Protein Polymerization. Physical Review Letters 2014, 112, 098101. Publisher's Version [PDF]
Jensen*, M.  H. ; Morris*, E.  J. ; Gallant, C.  M. ; Morgan, K.  G. ; Weitz, D.  A. ; Moore, J.  R. Mechanism of Calponin Stabilization of Cross-Linked Actin Networks. Biophysical Journal 2014, 106, 793 - 800. Publisher's Version [PDF]
[*Equal contribution]
Datta, S. S. ; Ramakrishnan, T. S. ; Weitz, D. A. Mobilization of a trapped non-wetting fluid from a three-dimensional porous medium. Physics of Fluids 2014, 26, 022002. Publisher's Version [PDF]
Comunian, T. A. *; Abbaspourrad*, A. ; Favaro-Trindade, C. S. ; Weitz, D. A. Fabrication of solid lipid microcapsules containing ascorbic acid using a microfluidic technique. Food Chemistry 2014, 152, 271 - 275. Publisher's Version [PDF]
[*Equal contribution]
Datta, S. S. *; Abbaspourrad*, A. ; Amstad, E. ; Fan, J. ; Kim, S. - H. ; Romanowsky, M. ; Shum, H. C. ; Sun, B. J. ; Utada, A. S. ; Windbergs, M. ; et al. Double Emulsion Templated Solid Microcapsules: Mechanics And Controlled Release. Advanced Materials 2014, n/a–n/a. Publisher's VersionAbstract
We describe how droplet microfluidics can be used to fabricate solid-shelled microcapsules having precisely controlled release behavior. Glass capillary devices enable the production of monodisperse double emulsion drops, which can then be used as templates for microcapsule formation. The exquisite control afforded by microfluidics can be used to tune the compositions and geometrical characteristics of the microcapsules with exceptional precision. We review the use of this approach to fabricate microcapsules that only release their contents when exposed to a specific stimulus – such as a change in temperature, exposure to light, a change in the chemical environment, or an external stress – only after a prescribed time delay, and at a prescribed rate.
[*Equal contribution]
Röding, M. ; Guo, M. ; Weitz, D. A. ; Rudemo, M. ; Särkkä, A. Identifying directional persistence in intracellular particle motion using Hidden Markov Models. Mathematical Biosciences 2014, 248, 140 - 145. Publisher's Version [PDF]
Herranz-Blanco, B. ; Arriaga, L. R. ; Makila, E. ; Correia, A. ; Shrestha, N. ; Mirza, S. ; Weitz, D. A. ; Salonen, J. ; Hirvonen, J. ; Santos, H. A. Microfluidic assembly of multistage porous silicon-lipid vesicles for controlled drug release. Lab on a Chip 2014, 14, 1083 - 1086. Publisher's VersionAbstract

A reliable microfluidic platform for the generation of stable and monodisperse multistage drug delivery systems is reported. A glass-capillary flow-focusing droplet generation device was used to encapsulate thermally hydrocarbonized porous silicon (PSi) microparticles into the aqueous cores of double emulsion drops, yielding the formation of a multistage PSi-lipid vesicle. This composite system enables a large loading capacity for hydrophobic drugs.

Zhang, W. ; Seminara, A. ; Suaris, M. ; Brenner, M. P. ; Weitz, D. A. ; Angelini, T. E. Nutrient depletion in Bacillus subtilis biofilms triggers matrix production. New Journal of Physics 2014, 16, 015028. Publisher's VersionAbstract

Many types of bacteria form colonies that grow into physically robust and strongly adhesiveaggregates known as biofilms. A distinguishing characteristic of bacterial biofilms is an extracellular polymeric substance (EPS) matrix that encases the cells and provides physical integrity to the colony. The EPS matrix consists of a large amount of polysaccharide, as well as protein filaments, DNA and degraded cellular materials. The genetic pathways that control the transformation of a colony into a biofilm have been widely studied, and yield a spatiotemporal heterogeneity in EPS production. Spatial gradients in metabolites parallel this heterogeneity in EPS, but nutrient concentration as an underlying physiological initiator of EPS production has not been explored. Here, we study the role of nutrient depletion in EPS production in Bacillus subtilis biofilms. By monitoring simultaneously biofilm size and matrix production, we find that EPS production increases at a critical colony thickness that depends on the initial amount of carbon sources in the medium. Through studies of individual cells in liquid culture we find that EPS production can be triggered at the single-cell level by reducing nutrient concentration. To connect the single-cell assays with conditions in the biofilm, we calculate carbon concentration with a model for the reaction and diffusion of nutrients in the biofilm. This model predicts the relationship between the initial concentration of carbon and the thickness of the colony at the point of internal nutrient deprivation.

Campas, O. ; Mammoto, T. ; Hasso, S. ; Sperling, R. A. ; O'Connell, D. ; Bischof, A. G. ; Maas, R. ; Weitz, D. A. ; Mahadevan, L. ; Ingber, D. E. Quantifying cell-generated mechanical forces within living embryonic tissues. Nat Meth 2014, 11, 183 - 189. Publisher's VersionAbstract

Cell-generated mechanical forces play a critical role during tissue morphogenesis and organ formation in the embryo. Little is known about how these forces shape embryonic organs, mainly because it has not been possible to measure cellular forces within developing three-dimensional (3D) tissues in vivo. We present a method to quantify cell-generated mechanical stresses exerted locally within living embryonic tissues, using fluorescent, cell-sized oil microdroplets with defined mechanical properties and coated with adhesion receptor ligands. After a droplet is introduced between cells in a tissue, local stresses are determined from droplet shape deformations, measured using fluorescence microscopy and computerized image analysis. Using this method, we quantified the anisotropic stresses generated by mammary epithelial cells cultured within 3D aggregates, and we confirmed that these stresses (3.4 nN [mu]m-2) are dependent on myosin II activity and are more than twofold larger than stresses generated by cells of embryonic tooth mesenchyme, either within cultured aggregates or in developing whole mouse mandibles.

Lee, M. ; Collins, J. W. ; Aubrecht, D. M. ; Sperling, R. A. ; Solomon, L. ; Ha, J. - W. ; Yi, G. - R. ; Weitz, D. A. ; Manoharan, V. N. Synchronized reinjection and coalescence of droplets in microfluidics. Lab on a Chip 2014, 14, 509-513.Abstract

Coalescence of two kinds of pre-processed droplets is necessary to perform chemical and biological assays in droplet-based microfluidics. However, a robust technique to accomplish this does not exist. Here we present a microfluidic device to synchronize the reinjection of two different kinds of droplets and coalesce them, using hydrostatic pressure in conjunction with a conventional syringe pump. We use a device consisting of two opposing T-junctions for reinjecting two kinds of droplets and control the flows of the droplets by applying gravity-driven hydrostatic pressure. The hydrostatic-pressure operation facilitates balancing the droplet reinjection rates and allows us to synchronize the reinjection. Furthermore, we present a simple but robust module to coalesce two droplets that sequentially come into the module, regardless of their arrival times. These re-injection and coalescence techniques might be used in lab-on-chip applications requiring droplets with controlled numbers of solid materials, which can be made by coalescing two pre-processed droplets that are formed and sorted in devices.

Amstad, E. ; Datta, S. S. ; Weitz, D. A. The microfluidic post-array device: High throughput production of single emulsion drops. Lab on a Chip 2014, 14, 705-709.Abstract

We present a microfluidic device that enables high throughput production of relatively monodisperse emulsion drops while controlling the average size. The device consists of a two-dimensional array of regularly-spaced posts. Large drops of a highly polydisperse crude emulsion are input into the device and are successively split by the posts, ultimately yielding a finer emulsion consisting of smaller, and much more monodisperse drops. The size distribution of the resultant emulsion depends only weakly on the viscosities of the input fluids and allows fluids of very high viscosities to be used. The average size and polydispersity of the drops depend strongly on the device geometry enabling both control and optimization. We use this device to produce drops of a highly viscous monomer solution and subsequently solidify them into polymeric microparticles. The production rate of these devices is similar to that achieved by membrane emulsification techniques, yet the control over the drop size is superior; thus these post-array microfluidic devices are potentially useful for industrial applications.

The role of vimentin intermediate filaments in cortical and cytoplasmic mechanics
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. [PDF]
Lu, P. J. ; Weitz, D. A. ; Langer, J. S. Colloidal particles: Crystals, glasses, and gels. In Annual Review of Condensed Matter Physics, Vol 4; 2013; Vol. 4, pp. 217-233.Abstract

Colloidal particles are microscopic solid particles suspended in a fluid. Colloids are small enough that thermal energy drives their dynamics and ensures equilibration with the suspending fluid; they are also large enough that their positions and motions can be measured precisely using optical methods, such as light scattering and laser-scanning confocal fluorescence microscopy. Colloidal suspensions are a powerful model system for the study of other phenomena in condensed matter physics, where the collective phase behavior of the solid particles mimics that of other condensed systems. We review three classes of interacting colloidal particles, crystals, glasses, and gels, each of which represents fascinating properties of colloidal particles as well as a model for more general types of materials and their behavior.

Jensen, K. E. ; Pennachio, D. ; Recht, D. ; Weitz, D. A. ; Spaepen, F. Rapid growth of large, defect-free colloidal crystals. Soft Matter 2013, 9 320-328.Abstract

We demonstrate controlled growth of face-centered cubic (FCC), monodisperse hard-sphere colloidal crystals by centrifugation at up to 3000g onto FCC (100) templates. Such rapid deposition rates often result in an amorphous sediment. Surprisingly, however, growth onto (100) templates results only in single crystals with few or no extended defects. By contrast, deposition onto flat, (111), or (110) templates causes rapid disordering to an amorphous sediment if the dimensionless flux (particle volume fraction x Peclet number) exceeds a critical value. This crystalline-to-amorphous crossover results from the degeneracy of possible stacking positions for these orientations. No such degeneracy exists for growth onto (100). After growth, extended defects can nucleate and grow only if the crystal exceeds a critical thickness that depends on the lattice misfit with the template spacing. The experimental observations of the density of misfit dislocations are accounted for by the Frank-van der Merwe theory, adapted for the depth-dependent variation of lattice spacing and elastic constants that results from the gravitational pressure.

Yao, N. Y. ; Broedersz, C. P. ; Depken, M. ; Becker, D. J. ; Pollak, M. R. ; MacKintosh, F. C. ; Weitz, D. A. Stress-enhanced gelation: A dynamic nonlinearity of elasticity. Physical Review Letters 2013, 110.Abstract

A hallmark of biopolymer networks is their sensitivity to stress, reflected by pronounced nonlinear elastic stiffening. Here, we demonstrate a distinct dynamical nonlinearity in biopolymer networks consisting of filamentous actin cross-linked by alpha-actinin-4. Applied stress delays the onset of relaxation and flow, markedly enhancing gelation and extending the regime of solidlike behavior to much lower frequencies. We show that this macroscopic network response can be accounted for at the single molecule level by the increased binding affinity of the cross-linker under load, characteristic of catch-bond-like behavior. DOI: 10.1103/PhysRevLett.110.018103

Datta, S. S. ; Weitz, D. A. Drainage in a model stratified porous medium. Epl 2013, 101.Abstract

We show that when a non-wetting fluid drains a stratified porous medium at sufficiently small capillary numbers Ca, it flows only through the coarsest stratum of the medium; by contrast, above a threshold Ca, the non-wetting fluid is also forced laterally, into part of the adjacent, finer strata. The spatial extent of this partial invasion increases with Ca. We quantitatively understand this behavior by balancing the stratum-scale viscous pressure driving the flow with the capillary pressure required to invade individual pores. Because geological formations are frequently stratified, we anticipate that our results will be relevant to a number of important applications, including understanding oil migration, preventing groundwater contamination, and sub-surface CO2 storage. Copyright (C) EPLA, 2013