Edery, Y. ; Berg, S. ; Weitz, D. Surfactant Variations in Porous Media Localize Capillary Instabilities during Haines Jumps. Physical Review Letters 2018, 120. Publisher's VersionAbstract
We use confocal microscopy to measure velocity and interfacial tension between a trapped wetting phase with a surfactant and a flowing, invading nonwetting phase in a porous medium. We relate interfacial tension variations at the fluid-fluid interface to surfactant concentration and show that these variations localize the destabilization of capillary forces and lead to rapid local invasion of the nonwetting fluid, resulting in a Haines jump. These spatial variations in surfactant concentration are caused by velocity variations at the fluid-fluid interfaces and lead to localization of the Haines jumps even in otherwise very uniform pore structure and pressure conditions. Our results provide new insight into the nature of Haines jumps, one of the most ubiquitous and important instabilities in flow in porous media.
Zhang, H. ; Zhu, Y. ; Qu, L. ; Wu, H. ; Kong, H. ; Yang, Z. ; Chen, D. ; Mäkilä, E. ; Salonen, J. ; Santos, H. A. ; et al. Gold nanorods conjugated porous silicon nanoparticles encapsulated in calcium alginate nano hydrogels using microemulsion templates. Nano Letters 2018, 18, 1448-1453. Publisher's VersionAbstract

Porous silicon nanoparticles (PSiNPs) and gold nanorods (AuNRs) can be used as biocompatible nanocarriers for delivery of therapeutics but undesired leakage makes them inefficient. By encapsulating the PSiNPs and AuNRs in a hydrogel shell, we create a biocompatible functional nanocarrier that enables sustained release of therapeutics. Here, we report the fabrication of AuNRs-conjugated PSi nanoparticles (AuNRsPSiNPs) through two-step chemical reaction for high-capacity loading of hydrophobic and hydrophilic therapeutics with photothermal property. Furthermore, using water-in-oil microemulsion templates, we encapsulate the AuNRsPSiNPs within a calcium alginate hydrogel nanoshell, creating a versatile biocompatible nanocarrier to codeliver therapeutics for biomedical applications. We find that the functionalized nanohydrogel effectively controls the release rate of the therapeutics while maintaining a high loading efficiency and tunable loading ratios. Notably, combinations of therapeutics coloaded in the functionalized nanohydrogels significantly enhance inhibition of multidrug resistance through synergism and promote faster cancer cell death when combined with photothermal therapy. Moreover, the AuNRs can mediate the conversion of near-infrared laser radiation into heat, increasing the release of therapeutics as well as thermally inducing cell damage to promote faster cancer cell death. Our AuNRsPSiNPs functionalized calcium alginate nanohydrogel holds great promise for photothermal combination therapy and other advanced biomedical applications.

Montessori, A. ; Lauricella, M. ; La Rocca, M. ; Succi, S. ; Stolovicki, E. ; Zibalt, R. ; Weitz, D. Regularized lattice Boltzmann multicomponent models for low capillary and Reynolds microfluidics flows. Computers & Fluids 2018, 167, 33-39. Publisher's VersionAbstract
We present a regularized version of the color gradient lattice Boltzmann (LB) scheme for the simulation of droplet formation in microfluidic devices of experimental relevance. The regularized version is shown to provide computationally efficient access to capillary number regimes relevant to droplet generation via microfluidic devices, such as flow-focusers and the more recent microfluidic step emulsifier devices.
Stolovicki, E. ; Ziblat, R. ; Weitz, D. A. Throughput enhancement of parallel step emulsifier devices by shear-free and efficient nozzle clearance. Lab on a Chip 2018, 18, 132-138. Publisher's VersionAbstract

Step emulsification is an attractive method for production of monodisperse drops. Its main advantage is the ability to parallelize many step emulsifier nozzles to achieve high production rates. However, step emulsification is sensitive to any obstructions at the nozzle exit. At high production rates, drops can accumulate at nozzle exits, disturb the formation of subsequent drops and impair monodispersity. As a result, parallelized step emulsifier devices typically do not work at maximum productivity. Here a design is introduced that parallelizes hundreds of step emulsifier nozzles, and effectively removes drops from the nozzle exits. The drop clearance is achieved by an open collecting channel, and is aided by buoyancy. Importantly, this clearance method avoids the use of a continuous phase flow for drop clearance and hence no shear is applied on the forming drops. The method works well for a wide range of drops, sizing from 30 to 1000 μm at production rates of 0.03 and 10 L per hour and achieved by 400 and 120 parallelized nozzles respectively.

Amstad, E. ; Weitz, D. A. Reply to the 'Comment on "Robust Scalable High Throughput Production of Monodisperse Drops. Lab on a Chip 2017, 17, 2332-2333. Publisher's VersionAbstract

This reply to the comment by Nakajima on our article that appeared in Lab on a Chip (E. Amstad, M. Chemama, M. Eggersdorfer, L. R. Arriaga, M. Brenner and D. A. Weitz, Lab Chip, 2016, 16, 4163–4172) highlights the differences between the microchannel step emulsification devices developed by the Nakajima group and the millipede device reported by us in Lab on a Chip.

Kim, S. - H. ; Kim, J. W. ; Cho, J. - C. ; Weitz, D. A. Correction: Double-emulsion drops with ultra-thin shells for capsule templates. Lab on a Chip 2017, 17, 567. Publisher's VersionAbstract

Correction for ‘Double-emulsion drops with ultra-thin shells for capsule templates’ by Shin-Hyun Kim et al.Lab Chip, 2011, 11, 3162–3166.

In the section “Diameter and shell thickness of double-emulsion drops” there are errors in eqn (2) and in the sentence that begins “In the same fashion, we calculate the thickness of the middle layer of double-emulsion drops which are produced at each values of Q1/Q2 and plot the results in Fig. 3c”. The equation should be

The sentence should read “In the same fashion, we calculate the thickness of the middle layer of double-emulsion drops which are produced at each values of Q2/Q1 and plot the results in Fig. 3c”.

In the caption for Fig. 3c, “Relative thickness of shell to radius of the double-emulsion drops (t/R) as a function of Q1/Q2.” should read “Relative thickness of shell to radius of the double-emulsion drops (t/R) as a function of Q2/Q1.” In addition, the x-axis is incorrectly labelled with “Q1/Q2”. The x-axis should be “Q2/Q1”. A corrected version of Fig. 3c is shown.

Jawerth, L. M. ; Weitz, D. A. Tracking the Structural Deformation of a Sheared Biopolymer Network. In Functional Analysis, Harmonic Analysis, and Image Processing: A Collection of Papers in Honor of Björn Jawerth; 2017; Vol. 693, pp. 255-269. Publisher's VersionAbstract

Biopolymer networks provide mechanical integrity in many important environments in vivo ranging from the cytoskeleton within a cell to the structural support of cells themselves in tissues and tendons. Rheolog-ical studies have shown that they exhibit many unique material properties. Modelling these properties requires a precise knowledge of how the individual filaments in the network deform locally during a global deformation. Here, we present an image processing method to track the three-dimensional motion of a biopolymer network as a simple shear deformation is applied. We track the structure of the network from one shear position to the next by determining the displacement of each branch point using a cross-correlation. To illustrate the use of this algorithm, we apply it to a fluorescently labelled fibrin network.

Liber, S. R. ; Indech, G. ; van der Wee, E. B. ; Butenko, A. V. ; Kodger, T. E. ; Lu, P. J. ; Schofield, A. B. ; Weitz, D. A. ; van Blaaderen, A. ; Sloutskin, E. Axial Confocal Tomography of Capillary-Contained Colloidal Structures. Langmuir 2017, 33, 13343–13349. Publisher's VersionAbstract
Confocal microscopy is widely used for three-dimensional (3D) sample reconstructions. Arguably, the most significant challenge in such reconstructions is posed by the resolution along the optical axis being significantly lower than in the lateral directions. In addition, the imaging rate is lower along the optical axis in most confocal architectures, prohibiting reliable 3D reconstruction of dynamic samples. Here, we demonstrate a very simple, cheap, and generic method of multiangle microscopy, allowing high-resolution high-rate confocal slice collection to be carried out with capillary-contained colloidal samples in a wide range of slice orientations. This method, realizable with any common confocal architecture and recently implemented with macroscopic specimens enclosed in rotatable cylindrical capillaries, allows 3D reconstructions of colloidal structures to be verified by direct experiments and provides a solid testing ground for complex reconstruction algorithms. In this paper, we focus on the implementation of this method for dense nonrotatable colloidal samples, contained in complex-shaped capillaries. Additionally, we discuss strategies to minimize potential pitfalls of this method, such as the artificial appearance of chain-like particle structures.
Chen, D. ; Amstad, E. ; Zhao, C. - X. ; Cai, L. ; Fan, J. ; Chen, Q. ; Hai, M. ; Koehler, S. ; Zhang, H. ; Liang, F. ; et al. Biocompatible Amphiphilic Hydrogel–Solid Dimer Particles as Colloidal Surfactants. ACS Nano 2017, 11, 11978–11985. Publisher's VersionAbstract
Emulsions of two immiscible liquids can slowly coalesce over time when stabilized by surfactant molecules. Pickering emulsions stabilized by colloidal particles can be much more stable. Here, we fabricate biocompatible amphiphilic dimer particles using a hydrogel, a strongly hydrophilic material, and achieve large contrast in the wetting properties of the two bulbs, resulting in enhanced stabilization of emulsions. We generate monodisperse single emulsions of alginate and shellac solution in oil using a flow-focusing microfluidics device. Shellac precipitates from water and forms a solid bulb at the periphery of the droplet when the emulsion is exposed to acid. Molecular interactions result in amphiphilic dimer particles that consist of two joined bulbs: one hydrogel bulb of alginate in water and the other hydrophobic bulb of shellac. Alginate in the hydrogel compartment can be cross-linked using calcium cations to obtain stable particles. Analogous to surfactant molecules at the interface, the resultant amphiphilic particles stand at the water/oil interface with the hydrogel bulb submerged in water and the hydrophobic bulb in oil and are thus able to stabilize both water-in-oil and oil-in-water emulsions, making these amphiphilic hydrogel–solid particles ideal colloidal surfactants for various applications.
Liu, J. ; Wang, N. \ddotu\elseü\fi}; Yu, L. - J. ; Karton, A. ; Li, W. ; Zhang, W. ; Guo, F. ; Hou, L. ; Cheng, Q. ; Jiang, L. ; et al. Bioinspired graphene membrane with temperature tunable channels for water gating and molecular separation. Nat. Commun. 2017, 8 2011. Publisher's VersionAbstract
Smart regulation of substance permeability through porous membranes is highly desirable for membrane applications. Inspired by the stomatal closure feature of plant leaves at relatively high temperature, here we report a nano-gating membrane with a negative temperature-response coefficient that is capable of tunable water gating and precise small molecule separation. The membrane is composed of poly(N-isopropylacrylamide) covalently bound to graphene oxide via free-radical polymerization. By virtue of the temperature tunable lamellar spaces of the graphene oxide nanosheets, the water permeance of the membrane could be reversibly regulated with a high gating ratio. Moreover, the space tunability endows the membrane with the capability of gradually separating multiple molecules of different sizes. This nano-gating membrane expands the scope of temperature-responsive membranes and has great potential applications in smart gating systems and molecular separation.
Guo, M. ; Pegoraro, A. F. ; Mao, A. ; Zhou, E. H. ; Arany, P. R. ; Han, Y. ; Burnette, D. T. ; Jensen, M. H. ; Kasza, K. E. ; Moore, J. R. ; et al. Cell volume change through water efflux impacts cell stiffness and stem cell fate. Proc. Natl. Acad. Sci. U.S.A. 2017, 201705179. Publisher's VersionAbstract
Cell volume is thought to be a well-controlled cellular characteristic, increasing as a cell grows, while macromolecular density is maintained. We report that cell volume can also change in response to external physical cues, leading to water influx/efflux, which causes significant changes in subcellular macromolecular density. This is observed when cells spread out on a substrate: Cells reduce their volume and increase their molecular crowding due to an accompanying water efflux. Exploring this phenomenon further, we removed water from mesenchymal stem cells through osmotic pressure and found this was sufficient to alter their differentiation pathway. Based on these results, we suggest cells chart different differentiation and behavioral pathways by sensing/altering their cytoplasmic volume and density through changes in water influx/efflux.
Hu, Y. ; Mao, A. S. ; Desai, R. M. ; Wang, H. ; Weitz, D. A. ; Mooney, D. J. Controlled self-assembly of alginate microgels by rapidly binding molecule pairs. Lab Chip 2017, 17, 2481–2490. Publisher's VersionAbstract
Controlled self-assembly of cell-encapsulating microscale polymeric hydrogels (microgels) could be advantageous in a variety of tissue engineering and regenerative medicine applications. Here, a method of assembly by chemical modification of alginate polymer with binding pair molecules (BPM) was explored. Alginate was modified with several types of BPM, specifically biotin and streptavidin and click chemistry compounds, and fabricated into 25–30 μm microgels using a microfluidic platform. These microgels were demonstrated to self-assemble under physiological conditions. By combining complementary microgels at a high ratio, size-defined assemblages were created, and the effects of BPM type and assembly method on the number of microgels per assemblage and packing density were determined. Furthermore, a magnetic process was developed to separate assemblages from single microgels, and allow formation of multilayer spheroids. Finally, cells were singly encapsulated into alginate microgels and assembled using BPM-modified alginate, suggesting potential applications in regenerative medicine.
Mao, A. S. ; Shin, J. - W. ; Utech, S. ; Wang, H. ; Uzun, O. ; Li, W. ; Cooper, M. ; Hu, Y. ; Zhang, L. ; Weitz, D. A. ; et al. Deterministic encapsulation of single cells in thin tunable microgels for niche modelling and therapeutic delivery. Nat. Mater. 2017, 16, 236–243. Publisher's VersionAbstract

Existing techniques to encapsulate cells into microscale hydrogels generally yield high polymer-to-cell ratios and lack control over the hydrogel’s mechanical properties1. Here, we report a microfluidic-based method for encapsulating single cells in an approximately six-micrometre layer of alginate that increases the proportion of cell-containing microgels by a factor of ten, with encapsulation efficiencies over 90%. We show that in vitro cell viability was maintained over a three-day period, that the microgels are mechanically tractable, and that, for microscale cell assemblages of encapsulated marrow stromal cells cultured in microwells, osteogenic differentiation of encapsulated cells depends on gel stiffness and cell density. We also show that intravenous injection of singly encapsulated marrow stromal cells into mice delays clearance kinetics and sustains donor-derived soluble factors in vivo. The encapsulation of single cells in tunable hydrogels should find use in a variety of tissue engineering and regenerative medicine applications.

Chen, D. ; Zhao, C. - X. ; Lagoin, C. ; Hai, M. ; Arriaga, L. R. ; Koehler, S. ; Abbaspourrad, A. ; Weitz, D. A. Dispersing hydrophobic natural colourant β-carotene in shellac particles for enhanced stability and tunable colour. R. Soc. Open Sci. 2017, 4 170919. Publisher's VersionAbstract
Colour is one of the most important visual attributes of food and is directly related to the perception of food quality. The interest in natural colourants, especially β-carotene that not only imparts colour but also has well-documented health benefits, has triggered the research and development of different protocols designed to entrap these hydrophobic natural molecules to improve their stability against oxidation. Here, we report a versatile microfluidic approach that uses single emulsion droplets as templates to prepare microparticles loaded with natural colourants. The solution of β-carotene and shellac in the solvent is emulsified by microfluidics into droplets. Upon solvent diffusion, β-carotene and shellac co-precipitates, forming solid microparticles of β-carotene dispersed in the shellac polymer matrix. We substantially improve the stability of β-carotene that is protected from oxidation by the polymer matrix and achieve different colour appearances by loading particles with different β-carotene concentrations. These particles demonstrate great promise for practical use in natural food colouring.
Thiery, J. ; Rodts, S. ; Weitz, D. A. ; Coussot, P. Drying regimes in homogeneous porous media from macro- to nanoscale. Phys. Rev. Fluids 2017, 2 074201. Publisher's VersionAbstract
Magnetic resonance imaging visualization down to nanometric liquid films in model porous media with pore sizes from micro- to nanometers enables one to fully characterize the physical mechanisms of drying. For pore size larger than a few tens of nanometers, we identify an initial constant drying rate period, probing homogeneous desaturation, followed by a falling drying rate period. This second period is associated with the development of a gradient in saturation underneath the sample free surface that initiates the inward recession of the contact line. During this latter stage, the drying rate varies in accordance with vapor diffusion through the dry porous region, possibly affected by the Knudsen effect for small pore size. However, we show that for sufficiently small pore size and/or saturation the drying rate is increasingly reduced by the Kelvin effect. Subsequently, we demonstrate that this effect governs the kinetics of evaporation in nanopores as a homogeneous desaturation occurs. Eventually, under our experimental conditions, we show that the saturation unceasingly decreases in a homogeneous manner throughout the wet regions of the medium regardless of pore size or drying regime considered. This finding suggests the existence of continuous liquid flow towards the interface of higher evaporation, down to very low saturation or very small pore size. Paradoxically, even if this net flow is unidirectional and capillary driven, it corresponds to a series of diffused local capillary equilibrations over the full height of the sample, which might explain that a simple Darcy's law model does not predict the effect of scaling of the net flow rate on the pore size observed in our tests.
Habib, N. ; Avraham-Davidi, I. ; Basu, A. ; Burks, T. ; Shekhar, K. ; Hofree, M. ; Choudhury, S. R. ; Aguet, F. ; Gelfand, E. ; Ardlie, K. ; et al. Massively parallel single-nucleus RNA-seq with DroNc-seq. Nat. Methods 2017, 14, 955. Publisher's VersionAbstract
Single-nucleus RNA sequencing (sNuc-seq) profiles RNA from tissues that are preserved or cannot be dissociated, but it does not provide high throughput. Here, we develop DroNc-seq: massively parallel sNuc-seq with droplet technology. We profile 39,111 nuclei from mouse and human archived brain samples to demonstrate sensitive, efficient, and unbiased classification of cell types, paving the way for systematic charting of cell atlases.
Du, J. S. ; Park, J. ; Kim, Q. H. ; Jhe, W. ; Dravid, V. P. ; Yang, D. ; Weitz, D. A. Multistage Transformation and Lattice Fluctuation at AgCl–Ag Interface. J. Phys. Chem. Lett. 2017, 8 5853–5860. Publisher's VersionAbstract
Solid-state transformation between different materials is often accompanied by mechanical expansion and compression due to their volume change and structural evolution at interfaces. However, these two types of dynamics are usually difficult to monitor in the same time. In this work, we use in situ transmission electron microscopy to directly study the reduction transformation at the AgCl–Ag interface. Three stages of lattice fluctuations were identified and correlated to the structural evolution. During the steady state, a quasi-layered growth mode of Ag in both vertical and lateral directions were observed due to the confinement of AgCl lattices. The development of planar defects and depletion of AgCl are respectively associated with lattice compression and relaxation. Topography and structure of decomposing AgCl was further monitored by in situ scanning transmission electron microscopy. Silver species are suggested to originate from both the surface and the interior of AgCl, and be transported to the interface. Such mass transport may have enabled the steady state and lattice compression in this volume-shrinking transformation.
Wang, L. ; Chen, D. ; Gutierrez-Cuevas, K. G. ; Bisoyi, H. K. ; Fan, J. ; Zola, R. S. ; Li, G. ; Urbas, A. M. ; Bunning, T. J. ; Weitz, D. A. ; et al. Optically reconfigurable chiral microspheres of self-organized helical superstructures with handedness inversion. Mater. Horiz. 2017, 4 1190–1195. Publisher's VersionAbstract
Optically reconfigurable monodisperse chiral microspheres of self-organized helical superstructures with dynamic chirality were fabricated via a capillary-based microfluidic technique. Light-driven handedness-invertible transformations between different configurations of microspheres were vividly observed and optically tunable RGB photonic cross-communications among the microspheres were demonstrated.
Weitz, D. A. Perspective on droplet-based single-cell sequencing. Lab Chip 2017, 17, 2539. Publisher's Version weitz2017.pdf
Shi, W. ; Weitz, D. A. Polymer Phase Separation in a Microcapsule Shell. Macromolecules 2017, 50, 7681–7686. Publisher's VersionAbstract
Phase separation has been used for engineering microscale fluids and particles with designed structures. But it is challenging to use phase separation to create complicated microcapsules because phase separation in the shell correlates with applied osmotic pressure and affects capsule stability significantly. Here we employ two biodegradable polymers to study the phase separation in microcapsule shells and its effect on the mechanical stability. The dynamic process reveals that phase separation creates a patchy shell with distinct regions transiently, then transports the discrete domains across the shell, and coalesces them at the surface. The equilibrium structure with balanced osmotic pressure is a Janus shell, where one component forms the shell and the other component dewets on the surface. Under slight osmotic pressure to the shell, phase separation reaches a different Janus shape, which consists of two partial shells of each component. We can in further take advantage of phase separation and osmotic pressure to rupture microcapsules at specific locations. Phase separation in the shell provides a facile approach to create versatile capsule structures and affords a reliable strategy to harness the shell mechanics.