We present further evidence that gelation is an arrested phase separation in attractive colloid-polymer mixtures, based on a method combining confocal microscopy experiments with numerical simulations recently established in Lu et al (2008 Nature 453 499). Our results are independent of the form of the interparticle attractive potential and therefore should apply broadly to any attractive particle system with short-ranged, isotropic attractions. We also give additional characterization of the gel states in terms of their structure, inhomogeneous character and local density.
Many common materials display significant nonlinear theological properties. Characterizing these properties can be done with a variety of methods. One such method uses inertio-elastic oscillations, which occur naturally in rotational rheometry as a consequence of a material's elasticity and the inertia of the rheometer. These oscillations have primarily been used to characterize linear viscoelastic properties. In addition to allowing for the imposition of stress-biased oscillations on short time scales, we demonstrate that extending this technique to nonlinear deformations provides accurate measurements of nonlinear material properties. Our experiments are performed on fibrin networks, which are well characterized and have dramatic nonlinear properties that are biologically significant. We compare the tangent moduli measurements of inertio-elastic oscillations with three standard methods of nonlinear rheology: forced oscillations about a prestress, a geometric interpretation of large amplitude oscillatory shears, and an extension of the linear viscoelastic moduli to the nonlinear regime. Inertio-elastic oscillations provide an accurate characterization of fibrin's nonlinear properties, and further, our measurements suggest that inertio-elastic oscillations provide the most straightforward method of distinguishing between nonlinear elasticity and dissipation at any given stress. In fact, we find that inertio-elastic oscillations provide the most accurate measurement of the subdominant loss component of our networks. (C) 2008 The Society of Rheology.
The drying dynamics in three dimensional porous media are studied with confocal microscopy. We observe abrupt air invasions in size from single particle to hundreds of particles. We show that these result from the strong flow from menisci in large pores to menisci in small pores during drying. This flow causes air invasions to start in large menisci and subsequently spread throughout the entire system. We measure the size and structure of the air invasions and show that they are in accord with invasion percolation. By varying the particle size and contact angle we unambiguously demonstrate that capillary pressure dominates the drying process.
Accurately characterizing the non-linear rheological properties of materials remains a challenge. Although there exist a variety of methods to probe non-linear properties, there is no consensus regarding their unique advantages. Inertio-elastic oscillations occur naturally in rotational rheometry as a consequence of a material's elasticity and the inertia of the rheometer's bearing. We demonstrate that extending this technique to non-linear deformations provides accurate measurements of non-linear material properties. Our experiments are performed on biopolymer networks, which are well-characterized, and have dramatic non-linear properties that are biologically significant. We compare these inertio-elastic results to other standard methods of probing non-linear rheology. For fibrin networks, our measurements suggest that inertio-elastic oscillations provide the most straightforward method of distinguishing between non-linear elasticity and dissipation at any given stress. In addition, we show that inertio-elastic oscillations provide the most accurate noise-free generalization of non-linear dissipation for a viscoelastic solid.
Times Cited: 0 15th International Congress on Rheology/80th Annual Meeting of the Society-of-Rheology Aug 03-08, 2008 Monterey, CA Soc Rheol
The actin cross-linker alpha-actinin-4 has been found to be indispensable for the structural and functional integrity of podocytes; deficiency or alteration of this protein due to mutations results in kidney disease. To gain insight into the effect of the cross-linker on cytoskeletal mechanics, we studied the macroscopic rheological properties of actin networks cross-linked with wild-type and mutant alpha-actinin-4. The frequency-dependent viscoelasticity of the networks is characterized by an elastic plateau at intermediate frequencies, and relaxation toward fluid properties at low frequencies. The relaxation frequencies of networks with mutant alpha-actinin-4 are an order of magnitude lower than that with the wild-type, suggesting a slower reaction rate for the dissociation of actin and alpha-actinin for the mutant, consistent with a smaller observed equilibrium dissociation constant. This difference can be attributed to an additional binding site that is exposed as a result of the mutation, and can be interpreted as a difference in binding energy barriers. This is further supported by the Arrhenius-like temperature dependence of the relaxation frequencies.
Cylindrical liquid jets are inherently unstable and eventually break into drops due to the Rayleigh-Plateau instability, characterized by the growth of disturbances that are either convective or absolute in nature. Convective instabilities grow in amplitude as they are swept along by the flow, while absolute instabilities are disturbances that grow at a fixed spatial location. Liquid jets are nearly always convectively unstable. Here we show that two-phase jets can breakup due to an absolute instability that depends on the capillary number of the outer liquid, provided the Weber number of the inner liquid is >O(1). We verify our experimental observations with a linear stability analysis.
The velocity fluctuations of particles in a low-Reynolds-number fluidized bed have important similarities and differences with the velocity fluctuations in a low-Reynolds-number sedimenting suspension. We show that, like sedimentation, the velocity fluctuations in a fluidized bed are described well by the balance between density fluctuations due to Poisson statistics and Stokes drag. However, unlike sedimentation, the correlation length of the fluctuations in a fluidized bed increases with volume fraction. We argue that this difference arises because the relaxation time of density fluctuations is completely different in the two systems.
The geometric structure of a biopolymer network impacts its mechanical and biological properties. In this paper, we develop an algorithm for extracting the network architecture of three-dimensional (3d) fluorescently labeled collagen gels, building on the initial work of Wu et al., (2003). Using artificially generated images, the network extraction algorithm is then validated for its ability to reconstruct the correct bulk properties of the network, including fiber length, persistence length, cross-link density, and shear modulus.
We present a novel approach for fabricating monodisperse phospholipid vesicles with high encapsulation efficiency using controlled double emulsions as templates. Glass-capillary microfluidics is used to generate monodisperse double emulsion templates. We show that the high uniformity in size and shape of the templates are maintained in the final phospholipid vesicles after a solvent removal step. Our simple and versatile technique is applicable to a wide range of phospholipids.
We describe a versatile technique for fabricating monodisperse polymersomes with biocompatible and biodegradable diblock copolymers for efficient encapsulation of actives. We use double emulsion as a template for the assembly of amphiphilic diblock copolymers into vesicle structures. These polymersomes can be used to encapsulate small hydrophilic solutes. When triggered by an osmotic shock, the polymersomes break and release the solutes, providing a simple and effective release mechanism. The technique can also be applied to diblock copolymers with different hydrophilic-to-hydrophobic block ratios, or mixtures of diblock copolymers and hydrophobic homopolymers. The ability to make polymer vesicles with copolymers of different block ratios and to incorporate different homopolymers into the polymersomes will allow the tuning of polymersome properties for specific technological applications.
Shah, R. K. ; Shum, H. C. ; Rowat, A. C. ; Lee, D. ; Agresti, J. J. ; Utada, A. S. ; Chu, L. - Y. ; Kim, J. - W. ; Fernandez-Nieves, A. ; Martinez, C. J. ; et al.Designer emulsions using microfluidics. Materials Today2008, 11, 18-27.Abstract
We describe new developments for the controlled fabrication of monodisperse emulsions using microfluidics. We use glass capillary devices to generate single, double, and higher order emulsions with exceptional precision. These emulsions can serve as ideal templates for generating well-defined particles and functional vesicles. Polydimethylsiloxane microfluidic devices are also used to generate picoliter-scale water-in-oil emulsions at rates as high as 10 000 drops per second. These emulsions have great potential as individual microvessels in high-throughput screening applications, where each drop serves to encapsulate single cells, genes, or reactants.
We use droplet-based microfluidic techniques to produce monodisperse poly( N-isopropylacrylamide) gel particles in the size range of 10-1000 mu m. Our techniques offer exquisite control over both outer dimensions and inner morphology of the particles. We demonstrate this control by fabricating conventional microgels, microgels with embedded materials and voids, and gel microcapsules with single- and multi-phase cores. These techniques should be applicable for the synthesis of particles and capsules of a variety of chemical compositions and for the generation of higher order "supraparticles'' by directed assembly of colloidal particles in droplets.
The long evolution of vascular plants has resulted in a tremendous variety of natural networks responsible for the evaporatively driven transport of water. Nevertheless, little is known about the physical principles that constrain vascular architecture. Inspired by plant leaves, we used microfluidic devices consisting of simple parallel channel networks in a polymeric material layer, permeable to water, to study the mechanisms of and the limits to evaporation-driven flow. We show that the flow rate through our biomimetic leaves increases linearly with channel density (1/d) until the distance between channels (d) is comparable with the thickness of the polymer layer (5), above which the flow rate saturates. A comparison with the plant vascular networks shows that the same optimization criterion can be used to describe the placement of veins in leaves. These scaling relations for evaporatively driven flow through simple networks reveal basic design principles for the engineering of evaporation-permeation-driven devices, and highlight the role of physical constraints on the biological design or leaves.
Dilute dispersions of fractal particles in hydrocarbon solvents flocculate and form gels with typical scaling of elasticity with particle volume fraction. Surprisingly, these attractive systems display shear thickening in two distinct regimes. At low shear rates, shear thickening is concurrent with the formation of stable vorticity-aligned structures. At high shear rates, shear thickening involves the breakdown of dense particle clusters into smaller aggregates. Pre-shear within the high shear rate shear thickening regime leads to enhanced modulus gels where storage modulus scales as a power law with the pre-shear stress. Shear-rate quenches from shear thickening flows into the quiescent state result in rapid gelation accompanied by slowly decaying internal stresses. Deformation of these mechanically quenched gels is highlighted by the transient formation of highly anisotropic vorticity aligned structures and the relaxation of residual internal stresses.
Times Cited: 2 15th International Congress on Rheology/80th Annual Meeting of the Society-of-Rheology Aug 03-08, 2008 Monterey, CA Soc Rheol
Vorticity aligned cylindrical flocs of carbon black particles are formed in steady flow at low shear rates and, strikingly, appear as transient structures in the flow response of gels produced by the quenching of high rate shear thickening flows.
Dilute oil dispersions of fractal carbon black particles with attractive van der Waals interactions display continuous shear thickening followed by shear thinning at high shear rates. The shear thickening transition occurs at (gamma) over dot(c)approximate to 10(2)-10(3) s(-1) and is driven by hydrodynamic breakup of clusters. Pre-shearing dispersions at shear rates (gamma) over dot>(gamma) over dot(c) produces enhanced-modulus gels where G'similar to sigma(1.5-2)(pre-shear) and is directly proportional to the residual stress in the gel measured at a fixed sample age. The observed data can be accounted for using a simple scaling model for the breakup of fractal clusters under shear stress.
We describe a robust method for determining morphological properties of. lamentous biopolymer networks, such as collagen or other connective tissue matrices, from confocal microscopy image stacks. Morphological properties including pore size distributions and percolation thresholds are important for transport processes, e. g., particle diffusion or cell migration through the extracellular matrix. The method is applied to fluorescently labeled fiber networks prepared from rat-tail tendon and calf-skin collagen, at concentrations of 1.2, 1.6, and 2.4 mg/ml. The collagen fibers form an entangled and branched network. The medial axes, or skeletons, representing the collagen fibers are extracted from the image stack by threshold intensity segmentation and distance-ordered homotopic thinning. The size of the fiuid pores as defined by the radii of largest spheres that fit into the cavities between the collagen fibers is derived from Euclidean distance maps and maximal covering radius transforms of the fluid phase. The size of the largest sphere that can traverse the fluid phase between the collagen fibers across the entire probe, called the percolation threshold, was computed for both horizontal and vertical directions. We demonstrate that by representing the fibers as the medial axis the derived morphological network properties are both robust against changes of the value of the segmentation threshold intensity and robust to problems associated with the point-spread function of the imaging system. We also provide empirical support for a recent claim that the percolation threshold of a fiber network is close to the fiber diameter for which the Euler index of the networks becomes zero.
Nanoscale or colloidal particles are important in many realms of science and technology. They can dramatically change the properties of materials, imparting solid-like behaviour to a wide variety of complex fluids(1,2). This behaviour arises when particles aggregate to form mesoscopic clusters and networks. The essential component leading to aggregation is an interparticle attraction, which can be generated by many physical and chemical mechanisms. In the limit of irreversible aggregation, infinitely strong interparticle bonds lead to diffusion-limited cluster aggregation(3) (DLCA). This is understood as a purely kinetic phenomenon that can form solid-like gels at arbitrarily low particle volume fraction(4,5). Far more important technologically are systems with weaker attractions, where gel formation requires higher volume fractions. Numerous scenarios for gelation have been proposed, including DLCA(6), kinetic or dynamic arrest(4,7-10), phase separation(5,6,11-16), percolation(4,12,17,18) and jamming(8). No consensus has emerged and, despite its ubiquity and significance, gelation is far from understood-even the location of the gelation phase boundary is not agreed on(5). Here we report experiments showing that gelation of spherical particles with isotropic, short-range attractions is initiated by spinodal decomposition; this thermodynamic instability triggers the formation of density fluctuations, leading to spanning clusters that dynamically arrest to create a gel. This simple picture of gelation does not depend on microscopic system-specific details, and should thus apply broadly to any particle system with short- range attractions. Our results suggest that gelation-often considered a purely kinetic phenomenon(4,8-10)-is in fact a direct consequence of equilibrium liquid gas phase separation(5,13-15). Without exception, we observe gelation in all of our samples predicted by theory and simulation to phaseseparate; this suggests that it is phase separation, not percolation(12), that corresponds to gelation in models for attractive spheres.
Nanoparticle colloidosomes, shown in the SEM image, are generated by using water-in-oil-in-water double emulsions as templates. Hydrophobic silica nanoparticles that are dispersed in the oil phase stabilize the double emulsions, and subsequently become the shell of the colloidosomes upon removal of the organic solvent as shown in the figure.
We use microfluidic devices to encapsulate, incubate, and manipulate individual cells in picoliter aqueous drops in a carrier fluid at rates of up to several hundred Hz. We use a modular approach with individual devices for each function, thereby significantly increasing the robustness of our system and making it highly flexible and adaptable to a variety of cell-based assays. The small volumes of the drops enables the concentrations of secreted molecules to rapidly attain detectable levels. We show that single hybridoma cells in 33 pL drops secrete detectable concentrations of antibodies in only 6 h and remain fully viable. These devices hold the promise of developing microfluidic cell cytometers and cell sorters with much greater functionality, allowing assays to be performed on individual cells in their own microenvironment prior to analysis and sorting.