Publications by Year: 2010

Lietor-Santos, J. J. ; Kim, C. ; Lynch, M. L. ; Fernandez-Nieves, A. ; Weitz, D. A. The role of polymer polydispersity in phase separation and gelation in colloid-polymer mixtures. Langmuir 2010, 26, 3174-3178.Abstract
Mixtures of nonadsorbing polymer and colloidal particles exhibit a range of different morphologies depending, on the particle and polymer concentrations and their relative size ratios. These can be very important for technological applications, where gelation can produce a weak solidlike structure that can help reduce phase separation, extending product shelf life. However, industrial products are typically formulated with polydisperse polymers, and the consequences of this on the phase behavior of the mixture are not known. We investigate the role of polymer polydispersity and show that a small amount of larger polymer in a distribution of nominally much smaller polymer can drastically modify the behavior. It can induce formation of a solidlike gel Structure, abetted by the small polymer, but still allow further evolution of the phase separation process, as is seen with a monodisperse distribution of larger polymer. This coarsening ultimately leads to gravitational collapse. We describe the full phase behavior for polydisperse polymer mixtures and account for the origin of the behavior through Measurements of the structure and dynamics and by comparing to the behavior with monodisperse polymers.
Times Cited: 9
Larsen, R. J. ; Kim, J. - W. ; Zukoski, C. F. ; Weitz, D. A. Elasticity of dilatant particle suspensions during flow. Physical Review E 2010, 81.Abstract
Dense suspensions under sufficiently high shear stress can exhibit a dramatic transition to a solidlike state. This is known as extreme shear thickening and is sometimes accompanied by dilatancy. This behavior is contradictory; the material is solidlike but only when flowing. To probe the elasticity of the dilatant state, we measure the transient inertio-elastic oscillations that occur in response to step changes in applied stress. As the volume fraction of solids increases the apparent flow elasticity also increases, and the suspension flows more slowly, approaching an elastic solid, which will support stress statically.
Times Cited: 6 1
Koester, S. ; Lin, Y. - C. ; Herrmann, H. ; Weitz, D. A. Nanomechanics of vimentin intermediate filament networks. Soft Matter 2010, 6 1910-1914.Abstract
Intermediate filaments are one of three classes of fibrous proteins in the cytoskeleton of eukaryotes, the others being actin filaments and microtubules. The dense filamentous networks and bundles provide important mechanical stability for the cell. Here we directly measure both the structure and mechanical properties of an in vitro model system for intermediate filaments reconstituted from purified vimentin protein at 1 mg mL(-1). We show that the mesh size is on the order of 1 mm, a value that is preserved upon addition of divalent ions. These ions act as effective cross-linkers, further stiffening the network.
Times Cited: 4
Kasza, K. E. ; Broedersz, C. P. ; Koenderink, G. H. ; Lin, Y. C. ; Messner, W. ; Millman, E. A. ; Nakamura, F. ; Stossel, T. P. ; MacKintosh, F. C. ; Weitz, D. A. Actin filament length tunes elasticity of flexibly cross-linked actin networks. Biophysical Journal 2010, 99, 1091-1100.Abstract
Networks of the cytoskeletal biopolymer actin cross-linked by the compliant protein filamin form soft gels that stiffen dramatically under shear stress. We demonstrate that the elasticity of these networks shows a strong dependence on the mean length of the actin polymers, unlike networks with small, rigid cross-links. This behavior is in agreement with a model of rigid filaments connected by multiple flexible linkers.
Times Cited: 18
Kanai, T. ; Lee, D. ; Shum, H. C. ; Weitz, D. A. Fabrication of tunable spherical colloidal crystals immobilized in soft hydrogels. Small 2010, 6 807-810. [PDF]
Times Cited: 31
Kanai, T. ; Lee, D. ; Shum, H. C. ; Shah, R. K. ; Weitz, D. A. Gel-immobilized colloidal crystal shell with enhanced thermal sensitivity at photonic wavelengths. Advanced Materials 2010, 22, 4998-+.Abstract
A method for fabricating microcapsules with gel-immobilized colloidal crystal shells using microfluidics is reported. The diffraction color or photonic wavelength of the gel-immobilized colloidal crystal shells can be adjusted by varying the particle concentration before gelation and tuned by external stimuli after preparation. The gel-immobilized colloidal crystal shell has a higher sensitivity and wider range in the stop band wavelength than that displayed by a bulk gel.
Times Cited: 26
Humphry, K. J. ; Kulkarni, P. M. ; Weitz, D. A. ; Morris, J. F. ; Stone, H. A. Axial and lateral particle ordering in finite Reynolds number channel flows. Physics of Fluids 2010, 22.Abstract
Inertial focusing in a pressure-driven flow refers to the positioning of particles transverse to the mean flow direction that occurs as a consequence of a finite particle Reynolds number. In channels with rectangular cross-sections, and for a range of channel aspect ratios and particle confinement, experimental results are presented to show that both the location and the number of focusing positions depend on the number of particles per unit length along the channel. This axial number density is a function of both the channel cross-section and the particle volume fraction. These results are rationalized using simulations of the particle-laden flow to show the manner in which hydrodynamic interactions set the preferred locations in these confined flows. A criterion is presented for the occurrence of a stepwise transition from one to two or more trains of particles. (c) 2010 American Institute of Physics. [doi:10.1063/1.3478311]
Times Cited: 16
Franke, T. ; Braunmueller, S. ; Schmid, L. ; Wixforth, A. ; Weitz, D. A. Surface acoustic wave actuated cell sorting (SAWACS). Lab on a Chip 2010, 10, 789-794.Abstract
We describe a novel microfluidic cell sorter which operates in continuous flow at high sorting rates. The device is based on a surface acoustic wave cell-sorting scheme and combines many advantages of fluorescence activated cell sorting (FACS) and fluorescence activated droplet sorting ( FADS) in microfluidic channels. It is fully integrated on a PDMS device, and allows fast electronic control of cell diversion. We direct cells by acoustic streaming excited by a surface acoustic wave which deflects the fluid independently of the contrast in material properties of deflected objects and the continuous phase; thus the device underlying principle works without additional enhancement of the sorting by prior labelling of the cells with responsive markers such as magnetic or polarizable beads. Single cells are sorted directly from bulk media at rates as fast as several kHz without prior encapsulation into liquid droplet compartments as in traditional FACS. We have successfully directed HaCaT cells (human keratinocytes), fibroblasts from mice and MV3 melanoma cells. The low shear forces of this sorting method ensure that cells survive after sorting.
Times Cited: 69
Eisenmann, C. ; Kim, C. ; Mattsson, J. ; Weitz, D. A. Shear melting of a colloidal glass. Physical Review Letters 2010, 104.Abstract
We use confocal microscopy to explore shear melting of colloidal glasses, which occurs at strains of similar to 0.08, coinciding with a strongly non-Gaussian step size distribution. For larger strains, the particle mean square displacement increases linearly with strain and the step size distribution becomes Gaussian. The effective diffusion coefficient varies approximately linearly with shear rate, consistent with a modified Stokes-Einstein relationship in which thermal energy is replaced by shear energy and the length scale is set by the size of cooperatively moving regions consisting of similar to 3 particles.
Times Cited: 20
Datta, S. S. ; Shum, H. C. ; Weitz, D. A. Controlled buckling and crumpling of nanoparticle-coated droplets. Langmuir 2010, 26, 18612-18616.Abstract
We introduce a new experimental approach to study the structural transitions of large numbers of nanoparticle-coated droplets as their volume is reduced. We use an emulsion system where the dispersed phase is slightly soluble in the continuous phase. By adding a fixed amount of unsaturated continuous phase, the volume of the droplets can be controllably reduced, causing them to buckle or crumple, thereby becoming nonspherical. The resultant morphologies depend both on the extent of volume reduction and the average droplet size. The buckling and crumpling behavior implies that the droplet surfaces are solid.
Times Cited: 23
Conrad, J. C. ; Wyss, H. M. ; Trappe, V. ; Manley, S. ; Miyazaki, K. ; Kaufman, L. J. ; Schofield, A. B. ; Reichman, D. R. ; Weitz, D. A. Arrested fluid-fluid phase separation in depletion systems: Implications of the characteristic length on gel formation and rheology. Journal of Rheology 2010, 54, 421-438.Abstract
We investigate the structural, dynamical, and rheological properties of colloid-polymer mixtures in a volume fraction range of phi=0.15-0.35. Our systems are density-matched, residual charges are screened, and the polymer-colloid size ratio is similar to 0.37. For these systems, the transition to kinetically arrested states, including disconnected clusters and gels, coincides with the fluid-fluid phase separation boundary. Structural investigations reveal that the characteristic length, L, of the networks is a strong function of the quench depth: for shallow quenches, L is significantly larger than that obtained for deep quenches. By contrast, L is for a given quench depth almost independent of phi; this indicates that the strand thickness increases with phi. The strand thickness determines the linear rheology: the final relaxation time exhibits a strong dependence on phi, whereas the high frequency modulus does not. We present a simple model based on estimates of the strand breaking time and shear modulus that semiquantitatively describes the observed behavior.
Times Cited: 14
Cho, E. C. ; Kim, J. - W. ; Hyun, D. C. ; Jeong, U. ; Weitz, D. A. Regulating volume transitions of highly responsive hydrogel scaffolds by adjusting the network properties of microgel building block colloids. Langmuir 2010, 26, 3854-3859.Abstract
We present a simple method to control the volume change of thermally responsive hydrogel scaffolds, providing a remarkably fast swelling and deswelling response to temperature changes. These scaffolds have 3-dimensional colloidal-network structures which are made from microgel particles while they are above their deswelling transition temperatures. By timing the cross-link density of the microgel particles, we achieve controllable changes of the Volume of the scaffolds in response to temperature. Their fast response rate is determined by the length scale of the unit microgel particles and is not influenced by the properties of the network. The release profile of a model drug (Rhapontin) loaded within the scaffolds can also be regulated by the cross-linking density of the microgel particles. These results offer a new way of fabricating hydrogel scaffolds with tunable matrix geometry and function by adjusting the properties of the unit microgel colloids, without loss or their fast response to temperature change.
Times Cited: 6
Choi, C. - H. ; Lee, J. ; Yoon, K. ; Tripathi, A. ; Stone, H. A. ; Weitz, D. A. ; Lee, C. - S. Surface-tension-induced synthesis of complex particles using confined polymeric fluids. Angewandte Chemie-International Edition 2010, 49, 7748-7752. [PDF]
Times Cited: 19
Angelini, T. E. ; Hannezo, E. ; Trepat, X. ; Fredberg, J. J. ; Weitz, D. A. Cell migration driven by cooperative substrate deformation patterns. Physical Review Letters 2010, 104.Abstract
Most eukaryotic cells sense and respond to the mechanical properties of their surroundings. This can strongly influence their collective behavior in embryonic development, tissue function, and wound healing. We use a deformable substrate to measure collective behavior in cell motion due to substrate mediated cell-cell interactions. We quantify spatial and temporal correlations in migration velocity and substrate deformation, and show that cooperative cell-driven patterns of substrate deformation mediate long-distance mechanical coupling between cells and control collective cell migration.
Times Cited: 43
Agresti, J. J. ; Antipov, E. ; Abate, A. R. ; Ahn, K. ; Rowat, A. C. ; Baret, J. - C. ; Marquez, M. ; Klibanov, A. M. ; Griffiths, A. D. ; Weitz, D. A. Ultrahigh-throughput screening in drop-based microfluidics for directed evolution. Proceedings of the National Academy of Sciences of the United States of America 2010, 107, 4004-4009.Abstract
The explosive growth in our knowledge of genomes, proteomes, and metabolomes is driving ever-increasing fundamental understanding of the biochemistry of life, enabling qualitatively new studies of complex biological systems and their evolution. This knowledge also drives modern biotechnologies, such as molecular engineering and synthetic biology, which have enormous potential to address urgent problems, including developing potent new drugs and providing environmentally friendly energy. Many of these studies, however, are ultimately limited by their need for even-higher-throughput measurements of biochemical reactions. We present a general ultrahigh-throughput screening platform using drop-based microfluidics that overcomes these limitations and revolutionizes both the scale and speed of screening. We use aqueous drops dispersed in oil as picoliter-volume reaction vessels and screen them at rates of thousands per second. To demonstrate its power, we apply the system to directed evolution, identifying new mutants of the enzyme horseradish peroxidase exhibiting catalytic rates more than 10 times faster than their parent, which is already a very efficient enzyme. We exploit the ultrahigh throughput to use an initial purifying selection that removes inactive mutants; we identify similar to 100 variants comparable in activity to the parent from an initial population of similar to 10(7). After a second generation of mutagenesis and high-stringency screening, we identify several significantly improved mutants, some approaching diffusion-limited efficiency. In total, we screen similar to 10(8) individual enzyme reactions in only 10 h, using <150 mu L of total reagent volume; compared to state-of-the-art robotic screening systems, we perform the entire assay with a 1,000-fold increase in speed and a 1-million-fold reduction in cost.
Times Cited: 173
Abate, A. R. ; Thiele, J. ; Weinhart, M. ; Weitz, D. A. Patterning microfluidic device wettability using flow confinement. Lab on a Chip 2010, 10, 1774-1776.Abstract
We present a simple method to spatially pattern the surface properties of microfluidic devices using flow confinement. Our technique allows surface patterning with micron-scale resolution. To demonstrate its effectiveness, we use it to pattern wettability to form W/O/W and O/W/O double emulsions.
Times Cited: 32
Abate, A. R. ; Hung, T. ; Mary, P. ; Agresti, J. J. ; Weitz, D. A. High-throughput injection with microfluidics using picoinjectors. Proceedings of the National Academy of Sciences of the United States of America 2010, 107, 19163-19166.Abstract
Adding reagents to drops is one of the most important functions in droplet-based microfluidic systems; however, a robust technique to accomplish this does not exist. Here, we introduce the picoinjector, a robust device to add controlled volumes of reagent using electro-microfluidics at kilohertz rates. It can also perform multiple injections for serial and combinatorial additions.
Times Cited: 57
Abate, A. R. ; Agresti, J. J. ; Weitz, D. A. Microfluidic sorting with high-speed single-layer membrane valves. Applied Physics Letters 2010, 96.Abstract
Sorting is one of the most important applications of microfluidic devices; however, current sorters place specific requirements on the density, size, and electrical properties of the objects to be sorted, limiting applicability. We present widely applicable microfluidic sorting. We use high-speed single-layer membrane valves to control flows in a bifurcating channel junction, to direct the paths of objects. This allows sorting at hundreds of hertz. Moreover, since the sorting action is mechanical, it is very widely applicable-to drops, particles, and even living cells. (C) 2010 American Institute of Physics. [doi:10.1063/1.3431281]
Times Cited: 23
Schwertz, H. ; Koester, S. ; Kahr, W. H. A. ; Michetti, N. ; Kraemer, B. F. ; Weitz, D. A. ; Blaylock, R. C. ; Kraiss, L. W. ; Greinacher, A. ; Zimmerman, G. A. ; et al. Anucleate platelets generate progeny. Blood 2010, 115, 3801-3809.Abstract

Platelets are classified as terminally differentiated cells that are incapable of cellular division. However, we observe that anucleate human platelets, either maintained in suspension culture or captured in microdrops, give rise to new cell bodies packed with respiring mitochondria and alpha-granules. Platelet progeny formation also occurs in whole blood cultures. Newly formed platelets are structurally indistinguishable from normal platelets, are able to adhere and spread on extracellular matrix, and display normal signal-dependent expression of surface P-selectin and annexin V. Platelet progeny formation is accompanied by increases in biomass, cellular protein levels, and protein synthesis in expanding populations. Platelet numbers also increase during ex vivo storage. These observations indicate that platelets have a previously unrecognized capacity for producing functional progeny, which involves a form of cell division that does not require a nucleus. Because this new function of platelets occurs outside of the bone marrow milieu, it raises the possibility that thrombopoiesis continues in the bloodstream. (Blood. 2010; 115(18):3801-3809)


Times Cited: 35

Jawerth, L. M. ; Muenster, S. ; Vader, D. A. ; Fabry, B. ; Weitz, D. A. A blind spot in confocal reflection microscopy: The dependence of fiber brightness on fiber orientation in imaging biopolymer networks. Biophysical Journal 2010, 98, L1-L3.Abstract

We investigate the dependence of fiber brightness on three-dimensional fiber orientation when imaging biopolymer networks with confocal reflection microscopy (CRM) and confocal fluorescence microscopy (CFM). We compare image data of fluorescently labeled type I collagen networks concurrently acquired using each imaging modality. For CRM, fiber brightness decreases for more vertically oriented fibers, leaving fibers above similar to 50 degrees from the imaging plane entirely undetected. As a result, the three-dimensional network structure appears aligned with the imaging plane. In contrast, CFM data exhibit little variation of fiber brightness with fiber angle, thus revealing an isotropic collagen network. Consequently, we find that CFM detects almost twice as many fibers as are visible with CRM, thereby yielding more complete structural information for three-dimensional fiber networks. We offer a simple explanation that predicts the detected fiber brightness as a function of fiber orientation in CRM.


Times Cited: 9