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Sediments of soft spheres arranged by effective density


Colloidal sedimentation has been studied for decades from both thermodynamic and dynamic perspectives. In the present work, binary mixtures of colloidal spheres are observed to separate spontaneously into two distinct layers on sedimentation. Both layers have a high volume fraction and contain distinct compositions of particles. Although predicting these compositions using settling dynamics is challenging, here we show that the compositions are readily predicted thermodynamically by minimizing the gravitational energy of the system. As the random packing fraction of a mixture of spheres exceeds that of monodisperse spheres of either type, the mixture produces a denser suspension that forms the bottom phase. Experimentally, the use of charged particles and low-ionic-strength solutions provides interparticle repulsions that keep the packed particles mobile, avoiding a glassy state that would prevent particles from reaching their equilibrium configuration. We extend this work beyond binary systems, showing similar separated layers for a five-component mixture of particles.

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Figure 1: Particle recovery of the 2.0 and 3.0 μm PSL particle sediment and particle recovery of the 3.0 μm PSL and the 3.0 μm silica sediment (1:1 by volume initial mixture).
Figure 2: Theoretical predictions and experimental results for effective density sorting.
Figure 3: Confocal microscopy images of a sediment formed by an initial mixture of 1:1 volume ratio of 4.0 μm red-fluorescence and 2.0 μm yellow–green-fluorescence PSL particles and evidence of particle mobility in the packed state.
Figure 4: Layers of sorted particles.

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The authors thank the Department of Energy (grant no DE-FG02-07ER46414) for funding this project. We also thank Penn State’s Huck Institute of Life Sciences Center for Qualitative Cell Analysis for their assistance and use of their Olympus Fluoview 1000 confocal laser scanning microscope, and M. Boland and D. Palm for their contribution to the initial stages of the research.

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C.G.S. carried out many of the experiments, with help from J.J.M. All authors contributed to understanding the theoretical aspect of the work. C.G.S. and D.V. carried out the initial modelling calculations, which were later refined by all three authors. All authors co-wrote the paper. The work was initiated and guided by D.V.

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Correspondence to Joseph J. McDermott or Darrell Velegol.

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The authors declare no competing financial interests.

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Serrano, C., McDermott, J. & Velegol, D. Sediments of soft spheres arranged by effective density. Nature Mater 10, 716–721 (2011).

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