Granular packings of elongated faceted particles deposited under gravity

We report experimental and theoretical results of the effect that particle shape has on the packing properties of granular materials. We have systematically measured the particle angular distribution, the cluster size distribution and the stress profiles of ensembles of faceted elongated particles deposited in a bidimensional box. Stress transmission through this granular system has been numerically simulated using a two-dimensional model of irregular particles. For grains of maximum symmetry (squares), the stress propagation localizes and forms chain-like forces analogous to those observed for granular materials composed of spheres. For thick layers of grains, a pressure saturation is observed for deposit depths beyond a characteristic length. This scenario correlates with packing morphology and can be understood in terms of stochastic models of aggregation and random multiplicative processes. As grains elongate and lose their symmetry, stress propagation is strongly affected. Lateral force transmission becomes less favored than vertical transfer, and hence, an increase in the pressure develops with depth, hindering force saturation

The coordination number of granular cylinders

We report the first experimental measurements of the contact number distribution between randomly packed granular right cylinders (rods and disks) as a function of aspect ratio. The average coordination number $\langle z \rangle$ for cylinders varies smoothly with aspect ratio rising from $\langle z \rangle \sim 6$ for aspect ratios close to one to $\langle z \rangle \sim 10$ for large aspect ratio. Additionally, our measurements demonstrate the validity of the random contact model for compacted piles of long granular cylinders

Microfluidic Production of Droplet Pairs

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Effect of particle shape on the random packing density of amorphous solids

The packing density of a particulate solid strongly depends on the shape of the particles that are jammed at random close packing (RCP). To investigate the effect of particle shape on the RCP density of an amorphous solid, we studied jammed packings of binary mixtures of a-thermal or granular spherocylinders by means of mechanical contraction computer simulations. We showed that the packing density of a jammed solid can be optimized by using slightly elongated particles. Starting from the Bernal random sphere packing, the RCP density first raises to a maximum for nearly spherical rod-like particles and only beyond this maximum it monotonically decreases with the particle aspect ratio. We demonstrated that the density maximum appears not only for monodisperse systems but it is a universal feature of mixtures of randomly packed nonspherical particles. The position of the density maximum is also universal – the optimal packing is found when one of the components in the binary mixture has the unique particle aspect ratio 0.5, irrespectively of the mixture composition. In the limit of large particle size disparity in a bidisperse jammed system, we revealed a universal scaling for the total packing density as a function of the aspect ratio of one component, regardless of the shape of the second component. An amorphous solid composed of a binary mixture of spherical and non-spherical, slightly elongated particles

Droplet-Based Microfluidic Platforms for the Encapsulation and Screening of Mammalian Cells and Multicellular Organisms

High-throughput, cell-based assays require small sample volumes to reduce assay costs and to allow for rapid sample manipulation. However, further miniaturization of conventional microtiter plate technology is problematic due to evaporation and capillary action. To overcome these limitations, we describe droplet-based microfluidic platforms in which cells are grown in aqueous microcompartments separated by an inert perfluorocarbon carrier oil. Synthesis of biocompatible surfactants and identification of gas-permeable storage systems allowed human cells, and even a multicellular organism (C. elegans), to survive and proliferate within the microcompartments for several days. Microcompartments containing single cells could be reinjected into a microfluidic device after incubation to measure expression of a reporter gene. This should open the way for high-throughput, cell-based screening that can use >1000-fold smaller assay volumes and has approximately 500x higher throughput than conventional microtiter plate assays

Chemistry & Biology Article Droplet-Based Microfluidic Platforms for the Encapsulation and Screening of Mammalian Cells and Multicellular Organisms

High-throughput, cell-based assays require small sample volumes to reduce assay costs and to allow for rapid sample manipulation. However, further miniaturization of conventional microtiter plate technology is problematic due to evaporation and capillary action. To overcome these limitations, we describe droplet-based microfluidic platforms in which cells are grown in aqueous microcompartments separated by an inert perfluorocarbon carrier oil. Synthesis of biocompatible surfactants and identification of gas-permeable storage systems allowed human cells, and even a multicellular organism (C. elegans), to survive and proliferate within the microcompartments for several days. Microcompartments containing single cells could be reinjected into a microfluidic device after incubation to measure expression of a reporter gene. This should open the way for high-throughput, cell-based screening that can use>1000-fold smaller assay volumes and has 5003 higher throughput than conventional microtiter plate assays